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NATIONAL AERONAUTICS AND SPACE ADMINISTRATION
MSC INTERNAL NOTE NO. EB-R-68-11
18 OCTOBER 1968
AN FM OPT I /VI I ZAT I ON STUDY
FOR THE
APOLLO UNIFIED S-BAND
TELECOMMUNICATIONS SYSTEM
• • a s • • «a
• ■ • ••••«
By F. J Loch
. J*- ■»/,■■■ ^ *T ■=• M
h y/££?-l y y • •> 5 »*•■ V ■•-■■■. I
* ^ *
N70 -35 793
ACCESSION N
(NASA CR OR TMX OR AD NUMBER)
MSC FORM 2026A (IUN 66)
INFORMATION SYSTEMS DIVISION
MANNED SPACECRAFT CENTER
HOUSTON, TEXAS
Ic.* 5 : - , »-•>> V‘ ■"* ^ u < vl> ' .
(CATEGORY)
Reproduced by
•> NATIONAL TECHNICAL 1
INFORMATION SERVICE
^ Spnngfiold, Va~ Z2I5h " * *
Kg
N70-35793
AN FM OPTIMIZATION STUDY FOR THE APOLLO UNIFIED S-
BAND TELECOMMUNICATIONS SYSTEM
anned Spacecraft Center
ouston, Texas
This document has been approved for public release and sale
MSC INTERNAL NOTE NO. EB-R-68-11
AN FM OPTIMIZATION STUDY FOR THE
APOLLO UNIFIED S-BAND TELECOMMUNICATIONS SYSTEM
Prepared by
F
Applied Analysis Section
Approved by
$ 3 . 0 . Qwjt
G . D Arndt
Head, Applied Analysis Section
14 *
R W Moorehead
Chief, Systems Analysis Branch
Approved for /^Tj)) /j /
Distribution f
P H Vavra
Chief, Information Systems Division
NATIONAL AERONAUTICS AND SPACE ADMINISTRATION
MANNED SPACECRAFT CENTER
INFORMATION SYSTEMS DIVISION
SYSTEMS ANALYSIS BRANCH
18 OCTOBER 1968
A CKNOWLEDGEMENTS
The author wishes to thank Mr William M Conrad of the Philco-Ford
Corporation s Education and Technical Services Division j for his
valuable contributions to the EM optimization study . Mr Conrad is
credited with the circuit design of the click detection and elimi-
nator threshold extension device discussed in this document Mr
Conrad also made valuable contributions to the test program that
resulted in the data and photographs presented in this document
-li-
TABLE OF CONTENTS
Section
1 INTRODUCTION
1 1 Purpose
1.2 Theoretical And Experimental Approach
2 SUMMARY
3 FM THRESHOLD
3 1 General ... ...
3 2 Click Event . . . .
3.3 FM Demodulator Performance With
Click Noise
3 4 Click Symmetry
4 THRESHOLD EXTENSION TECHNIQUES
4.1 General . .
4 2 Click Detection . . .
4 3 Click Elimination
4 4 Threshold Extension Device
5 EXPERIMENTAL RESULTS
5
1
General
5
2
Signal-Plus -Noise Waveform Analysis
5
3
Signal-to-Noise Ratio Tests
5
3 1
CSM FM Modes 1 and 2 .
5
3 2
CSM FM Mode 4
5
4
Word Intelligibility Tests
5
5
Television Picture Quality Analysis
Appendix
A BIBLIOGRAPHY
B ABBREVIATIONS AND SYMBOLS
Page
. 1-1
. 1-1
2-1
3-1
3-5
. 3-15
. 3-27
4-1
4-1
4-7
. 4-10
5-1
5-2
5-11
5-11
5-15
5-21
. 5-26
A-l
B-l
-in-
LIST OF FIGURES
Figure Page
3-1 Graphical Determination of FM Threshold For
A Typical Demodulator SNR Transfer Curve . .3-2
3-2 FM demodulator Output Noise Characteristics . . . 3-3
3-3 Phasor Representation of A, X, Y, R and N .... 3-6
3-4 Phasor Representation of Noise, Carrier, and
Resultant R, Near FM Threshold 3-7
3-5 Relationship Between ±2ir Phase Excursions and
Resulting Impulses of Area 2m 3-9
3-6 First Order Click Waveform 3-10
3-7 First Order Click Waveform . ... 3-10
3-8 Second Order Click Waveform . 3-11
3-9 Second Order Click Waveform 3-11
3-10 Third Order Click Waveform 3-12
3-11 Third Order Click Waveform 3-12
3-12 Fourth Order Click Waveform ... 3-13
3-13 ' Fourth Order Click Waveform 3-13
3-14 Effect of Carrier Offset on Click Noise
Distribution . . 3-28
3- 15 Output Noise Power Increase vs Center Frequency
Offset 3-29
4- 1 Demodulated Signal Plus Noise Waveform With and
Without Postdetection Filtering (f m = 10 KHz). . 4-2
4-2 Demodulated Signal Plus Noise Waveform With and
Without Postdetection Filtering (f m = 10 KHz). . 4-2
4-3 Demodulated Signal Plus Noise Waveform With and
Without Postdetection Filtering (fjjj = 50 KHz). . 4-3
4-4 Demodulated Signal Plus Noise Waveform With and
Without Postdetection Filtering (f m = 50 KHz). . 4-3
-IV-
LIST OF FIGURES (CONT'D)
Figure Page
4-5 Demodulated Signal Plus Noise Waveform With and
Without Postdetection Filtering (f m = 100 KHz) . 4-4
4-6 Demodulated Signal Plus Noise Waveform With and
Without Postdetection Filtering (f m = 100 KHz) . 4-4
4-7 Click Detector Block Diagram 4-6
4-8 Click Eliminator Block Diagram 4-8
4- 9 Click Detection and Elimination Threshold
Extension Device Configuration .... 4-9
5- 1 Signal Plus Noise Waveform Analysis Test
Configuration 5-3
5-2 Signal Plus Noise Waveform Analysis 5-4
5-3 Signal Plus Noise Waveform Analysis 5-4
5-4 Signal Plus Noise Waveform Analysis . . 5-5
5-5 Signal Plus Noise Waveform Analysis 5-5
5-6 Signal Plus Noise Waveform Analysis 5-6
5-7 Signal Plus Noise Waveform Analysis 5-6
5-8 Signal Plus Noise Waveform Analysis 5-7
5-9 Signal Plus Noise Waveform Analysis 5-7
5-10 Signal Plus Noise Waveform Analysis 5-8
5-11 Signal Plus Noise Waveform Analysis . 5-8
5-12 Signal Plus Noise Waveform Analysis 5-9
5-13 Signal Plus Noise Waveform Analysis 5-9
5-14 Test Configuration For CSM Modes 1 and 2 Signal-
to-Noise Ratio Tests With and Without Threshold
Extension Device 5-12
5-15 Output SNR vs Input SNR For 1 1 CSM Playback Voice
Mode 5-13
-v-
LIST OF FIGURES (CONT’D)
Figure Page
S-16 Output SNR vs Input SNR For 32.1 CSM Playback Voice
Mode ... 5-14
5-17 Output SNR Improvement vs Input SNR For 1 1 and 32 1
CSM Playback Voice 5-16
5-18 Configuration For CSM Mode 4 SNR Tests 5-17
5-19 Output SNR vs Input SNR For CSM FM Mode 4 5-18
5-20 Output SNR Improvement vs Input SNR For CSM FM
Mode 4 With Click Elimination . . 5-20
5-21 Configuration For CSM FM Mode 1 Word Intelligibility
Tests With and Without Threshold Extension
Device 5-22
5-22 Percent Word Intelligibility vs Input SNR For 1 1
CSM Playback Voice Mode S-23
5-23 Configuration For CSM FM Mode 2 Word Intelligibility
• With and Without Threshold Extension Device . 5-24
5-24 Percent Word Intelligibility vs Input SNR For CSM
32 1 Playback Voice Mode 5-25
5-25 Configuration For CSM Mode 4 Television Picture
Quality Tests 5-27
5-26 CSM Mode 4 TV Picture Quality Tests 5-28
5-27 CSM Mode 4 TV Picture Quality Tests 5-28
5-28 CSM Mode 4 TV Picture Quality Tests 5-29
5-29 CSM Mode 4 TV Picture Quality Tests 5-29
5-30 CSM Mode 4 TV Picture Quality Tests 5-30
5-31 CSM Mode 4 TV Picture Quality Tests 5-30
5-32 CSM Mode 4 TV Picture Quality Tests 5-31
5-33 CSM Mode 4 TV Picture Quality Tests 5-31
-vi-
LIST OF FIGURES (CONT'D)
Figure
5-34
CSM
Mode
4
TV
Picture
5-35
CSM
Mode
4
TV
Picture
5-36
CSM
Mode
4
TV
Picture
5-37
CSM
Mode
4
TV
Picture
5-38
CSM
Mode
4
TV
Picture
5-39
CSM
Mode
4
TV
Picture
5-40
CSM
Mode
4
TV
Picture
5-41
CSM
Mode
4
TV
Picture
Page
Quality Tests 5-33
Quality Tests 5-33
Quality Tests 5-34
Quality Tests 5-34
Quality Tests 5-35
Quality Tests 5-35
Quality Tests 5-36
Quality Tests 5-36
-v 11 -
LIST OF TABLES
Table Page
1- 1 Marginal Apollo Down- Link FM Modes 1-2
2- 1 Threshold Extension Device Performance Summary. . .2-3
3- 1 Duration and Amplitude Characteristics of Nth
Order Click Waveforms 3-14
5-1 CSM Modes Used For Threshold Extension Device
Test 5-1
-Vlll-
SECTION 1
INTRODUCTION
1 1 PURPOSE
An analysis of the Apollo Unified. S-Band (USB) communications
system^, performed by the Information Systems Division, Manned
Spacecraft Center, has indicated that certain down- link FM modes
exhibit negative circuit margins at lunar distance for worst case
conditions A negative margin implies that the operational re-
quirements will not be met for the affected modes The three FM
modes that were found to have negative margins are listed m
Table 1-1 with their respective channel characteristics
As a result of these findings, an intensive study of optimization
techniques was conducted by ISD/MSC for the purpose of improving
the performance of the marginal FM modes This optimization study
resulted m the development of an FM threshold extension device
that could be used to provide improved performance for the Apollo
Manned Spaceflight Network (MSFN) carrier frequency demodulator
By extending the threshold operation of the MSFN demodulator, it
is possible to obtain better circuit margins for the affected FM
modes
The purpose of this document is to present the results of the FM
optimization study which resulted m the development of the thresh-
old extension device This document also presents and evaluates
test data representing the performance of the device when it is
used m conjunction with an MSFN receiver
1 2 THEORETICAL AND EXPERIMENTAL APPROACH
An analysis of the FM threshold phenomena based on the earlier
work of S. 0. Rice^ is presented. The results of this analysis
show that the presence of impulse noise m the output of a de-
modulator is a primary factor causing threshold m an FM system
1
C. Dawson, A Performance Analysts of the Apollo Unified S-Band
Communications System for a Typical Lunar Mission (MSC Internal
Note MSC-EB-R-67-1, August 1967)
2
S 0 Rice, "Noise in FM Receivers," Proceedings 3 Symposium on
Time Series Analyses 3 ed M Rosenblatt (New York, 1963)
1-1
MODE
CSM MODE 1
CSM MODE 2
SERVICE(S)
SPECIFIED
CARRIER FREQUENCY
DEVIATION (Af) (KHZ)
WORST NOMINAL BEST
1 1 PLAYBACK 60 100 120
OF VOICE
1 1 PLAYBACK
CSM 51 2 KBPS 510 600 69 0
TLM
32 1 PLAYBACK 60 100 120
OF VOICE
32 1 PLAYBACK
CSM 1 6 KBPS 510 600 690
TLM
0 9 10 11
CSM MODE 4
TV
WORST CASE
MODE-AS-A-WHOLE
CIRCUIT MARGIN (DB)
215,000 NM
MODULATION
FREQUENCY
300-3000 HZ
-0 7
1 024 MHZ
(subcarrier:)
9600-96,000 HZ
-0 7
1 024 MHZ
(SUBCARRIER)
-0 7
0-500 KHZ
1 2 THEORETICAL AND EXPERIMENTAL APPROACH (Cont'd)
The threshold extension technique is based on the elimination
of noise impulses m the demodulator output An amplitude de-
tection technique is used to obtain information for the noise
elimination circuits that operate on a delayed output of the
demodulator.
1-3
SECTION 2
SUMMARY
The performance of an FM demodulator can be defined m terms of
the threshold phenomena, which determines the minimum acceptable
input signal- to-noise ratio for the system The presence of high
amplitude noise impulses (click noise) m the demodulator output
is a primary factor contributing to the occurrence of threshold
m an FM system Certain distinguishing characteristics of click
noise provide a basis for practical techniques that can be im-
plemented at the output of an FM demodulator to improve its thresh-
old performance
A threshold extension device has been developed that operates on
the principle of click detection and elimination The device was
designed to be compatible with the MSFN ground station receiver
and to improve the performance of the Apollo Unified S-Band
Communications System
Previous work m the area of FM demodulator improvement has been
concentrated on the design of specialized demodulation schemes that
provide optimum performance for one set of channel parameters
The Apollo down- link FM modes, however, use several channels having
different characteristics m terms of required channel bandwidth,
maximum frequency deviation, and range of modulation frequencies
The click noise eliminator offers an advantage m flexibility
since it is not a demodulator but, rather, a device which can be
implemented at the output of any FM discriminator to provide im-
proved system performance The presence of unsymmetrical click
noise m the demodulator output, resulting from offsets m carrier
frequency, does not affect the performance of the device
Photographs of demodulated Apollo television pictures comparing
the unprocessed demodulator output with the click-eliminated
output are presented to illustrate the effectiveness of the
threshold extension technique
2-1
v2 SUMMARY (Cont'd)
Additional data obtained from tests of the Apollo playback voice
inodes reflect the output ,signal- to -noise ratio improvement m the
form of increased voice intelligibility scores Improvements of
15 percent to 20 percent m word intelligibility scores are obtained
when the device is used at the output of the Apollo MSFN receiver.
Experimental results using the Apollo television and playback voice
modes have shown that the click elimination device provides an
improvement m output signal-to-noise ratio of 6 dB to 10 dB
This improvement m output signal-to-noise ratio corresponds to
a 1.5 dB to a 2.0 dB extension of threshold
The performance characteristics of the threshold extension device
are summarized m Table 2-1.
The values listed m Table 2-1 are based on the improvement ob-
tained when the device was used at the output of an Apollo MSFN
phase lock loop (PLL) demodulator. The threshold extension obtained
was m addition to the normally expected improvement of a PLL
demodulator over a standard FM discriminator. The device was also
tested with a frequency modulation feedback (FMFB) discriminator,
and similar values of threshold extension were obtained Therefore,
the threshold extension technique is capable of improving the
performance of any FM demodulation scheme that is degraded by the
presence of click noise m the output
A breadboard version of the threshold extension device was used
for the tests referenced m this document Work is presently under-
way to complete a refined click detection and elimination device
that uses integrated circuits to improve performance. It is ex-
pected that the finalized version of the threshold extension device
will provide additional SNR improvement
The following steps must be followed to insure optimum performance
of the click elimination device
A. The threshold level of the click detector must be set as
close as possible to the modulation peaks to insure maximum
click detection efficiency.
B The click-eliminator cutoff time must equal the click dura-
tion The click duration is determined by the bandwidth of
the preclick-detection filter. An optimum low pass band-
width of 2 MHz was used for the Apollo modes.
2-2
TABLE 2-1
THRESHOLD EXTENSION DEVICE PERFORMANCE SUMMARY
CSM MODE
SERVICE
MAXIMUM
OUTPUT SNR
IMPROVEMENT
CDB)
THRESHOLD
EXTENSION
CDB)
INPUT SNR
FOR MAXIMUM
IMPROVEMENT
EFFECTIVE
INPUT SNR
OPERATING
RANGE
COMMENTS
1
1 1 PLAYBACK
VOICE
10 DB
2 0 DB
3.3 DB
0 TO 10 DB
MAXIMUM WORD IN-
TELLIGIBILITY
IMPROVEMENT OF
25% WAS OBTAINED
2
32 1 PLAYBACK
VOICE
7 5 db
1 5 db
4.0 DB
0 TO 10 DB
MAXIMUM WORD IN-
TELLIGIBILITY
IMPROVEMENT OF 4%
WAS LIMITED BY IN"
HERENT RECORDING-
PLAYBACK DISTORTION.
4
TELEVISION
7 5 db
1 5 db
TO
2 0 DB
3 0 DB
0 TO 10 DB
THRESHOLD EXTENSION
IS OBSERVED BY COM-
PARING OUTPUT VIDEO
PICTURES WITH AND
WITHOUT CLICK
ELIMINATION
2 SUMMARY (Cont'd)
C The beginning of the click-eliminator cutoff must coincide
exactly with the beginning of the click in the demodulator
output
D The recovery time for the click detection and elimination
circuits must be made as small as possible to insure effi-
cient operation at high click rates
2-4
SECTION 3
FM THRESHOLD
3 1 GENERAL
By definition, the criteria for evaluating the performance of an
FM demodulator is based on the ability of the device to provide
a linear relationship between the output and input signal-to-noise
ratios The useful operating range of all FM demodulators, however,
is limited by the fact that this relationship, or transfer charac-
teristic, becomes non-linear below a certain value of input signal-
to-noise ratios This value of input SNR is called the "point of
threshold" for the demodulator
Since it is difficult m most cases to determine the exact value
of input SNR that divides the linear and non- linear regions of the
performance curve, it is necessary to define a reasonable criteria
for determining the threshold point
One accepted definition of FM threshold is based on the graphical
determination of the specific input SNR value whose corresponding
output SNR occurs exactly 1 dB below an extension of the linear
portion of the transfer curve This method of defining FM thresh-
old is illustrated m Figure 3-1 and will be used consistently
throughout this document
The occurrence of threshold m an FM system can also be defined in
terms of the demodulator output noise characteristics In general,
when the demodulator is operating with values of input SNR greater
than 10 dB, the output noise spectrum is parabolic, and the amplitude
distribution is Gaussian As the input SNR decreases ,. the output noise
voltage is punctuated with occasional high amplitude noise spikes
having either positive or negative polarity These noise impulses
become more frequent as the input SNR is reduced to values below
10 dB For the region of input SNR's between 0 dB and 10 dB, the
output noise power increases at such a rate that the slope of the
SNR transfer curve becomes much more severe than the slope of the
linear portion This implies that a small change m input SNR
results m a relatively large change m output SNR as illustrated
m Figure 3-1 The difference m the relative contribution of
Gaussian noise and click noise can be seen by noting their indi-
vidual spectral characteristics m Figure 3-2
3-1
OUTPUT SNR (dB)
6 8 10 12
INPUT SNR (dB)
Figure 5-1 Graphical Determination of FM Threshold for
a Typical Demodulator SNR Transfer Drive
GC 10 177 (A) 10
3-2
G(f)
(a) BASEBAND NOISE SPECTRUM ABOVE THRESHOLD
(b) CLICK NOISE SPECTRUM BELOW THRESHOLD
GC 101771 A; W
Figure 3-2 FM Demodulator Output
Noise Characteristics
3-3
3.1 GENERAL (Cont'd)
The energy contained in the individual noise spikes (called click
noise) at the output of an FM demodulator contributes significantly
to the total output noise power. In addition, the impulsive nature
of click noise causes it to be much more degrading to the demodulated
signal than the Gaussian output noise. Although click noise is not
the only phenomena which causes FM threshold, it is definitely a
primary factor contributing to the occurrence of threshold in an
FM system.
3.2 CLICK EVENT
Consider an unmodulated signal at the input of an ideal FM
discriminator where the carrier is represented as G(t).
G(t) = A cos w c t ( 1 )
The input noise to the discriminator can be represented in
quadrature form by the following expression:
N(t) = X(t) cos m^t + Y(t) sin w^t (2)
where X(t) and Y(t) are independent random variables. Therefore,
the total input carrier plus noise is
G (t) + N (t) = [a + X (t ) ] cos to t + Y(t) sin a> t
c c
(3)
which can also be expressed as follows
G (t) + N (t) = R cos tu t + <J>(t)]
(4)
where R is defined as the magnitude of the resultant carrier plus
noise waveform, w c is the carrier frequency, and <{> is the phase
of the resultant R with respect to the carrier. The FM demodulator
detects the frequency of the signal by differentiating the phase
of the received signal plus noise. The phase of the signal -plus-noise
waveform can be obtained from equation (3) .
<f>
tan
Y
A+X
(5)
3-5
3.2 CLICK EVENT (Cont'd)
For high signal- to-noise ratios, we can assume that A >> X.
Therefore,
- 1 Y Y
<P = tan - = - (6)
The following phasor relationship exists between A, X, Y, R, N
and <J>, as shown in Figure 3-3.
Figure 3-3 Phasor Representation of A, X, Y, R, and N
X and Y are Gaussian distributed random variables whose fluctuation
causes the angle <j> to change accordingly.
The click event takes place at the input of the FM discriminator
as an interaction between the randomly varying noise envelope and
the carrier amplitude that results in a sudden 2ir phase excursion
of the carrier-plus -noise vectoral resultant, R.
Both plus or minus 2 t r phase excursions can occur with equal proba-
bility, which results in a corresponding positive or negative noise
spike in the demodulator output.
Using the phasor representation of Figure 3-4, the click event
can be defined in terms of a phase excursion of ±2tt radians by
the angle <f> . The probability of such an excursion increases as
the input SNR to the discriminator decreases below a value of
10 dB. This figure shows that, for low values of input SNR, a
small change in the phase angle between the noise and carrier wave-
forms can result in a relatively large change in the resultant phase
angle .
3-6
Figure 3-4 Phasor Representation of Noise,
Carrier, and Resultant R, Near
FM Threshold
L D JO 1 79 ( A '
3.2 CLICK EVENT (Cont'd)
Since the output of an ideal discriminator is proportional to
d<J>/dt, the 2 tt phase excursion causes a noise spike of area 2ir
to occur at the output as shown in Figure 3-5.
It is also possible for the angle <f> to experience a phase excursion
of 4tt, 6tt, or even 8ir radians. For phase steps exceeding 2n radians,
the resulting output noise spike will have a proportionally greater
duration prior to postdetection filtering. Figures 3-6 through
3-13 show the click waveform before and after postdetection filter-
ing. Several photographs are provided to illustrate each of the
higher order clicks since each unfiltered waveform is unique. Fig-
ures 3-6 and 3-7 represent clicks resulting from a ±2ir phase ex-
cursion. These noise spikes will be referred to as first order
clicks since they are a result of the minimum click-producing phase
excursions. Some higher order clicks are also present in these
figures, but the clicks referenced in the figure titles are shown
in the extreme left portion of the photographs. Figures 3-8 and
3-9 represent second order clicks (4tt radians) . The duration of
the second order clicks is approximately twice that of the first
order clicks at the unfiltered output of the demodulator. The
noise contribution of the second order clicks is correspondingly
greater than that of the first order clicks.
The difference between first and second order clicks can easily
be distinguished at the output of the demodulator postdetection
filter by observing the relative amplitude of the spikes. The cut-
off frequency of the low pass filter determines the duration of
the output noise spikes so that both first and second order clicks
have the same duration at the filter output. Therefore, the dif-
ference in duration between the first and second order clicks at
the unfiltered output is translated to an amplitude difference in
the filtered output. Table 3-1 summarizes the relative amplitude
and duration relationship between the different orders of filtered
and unfiltered click waveforms.
Figures 3-10 through 3-13 represent third and fourth order click
waveforms at the demodulator output. These higher order clicks
occur only for very low values of input SNR, and for practical
considerations, their contribution to the total output noise power
can be neglected. In most cases the degradation of the demodulator
output below threshold will primarily be caused by the occurrence
of first order clicks.
3-8
GC10177CA) 9
Figure 3-5 Relationship Between ±2tt Phase Excursions
and Resulting Impulses of Area 2tt
3-9
Figure 3-6 First Order Click Waveform
UPPER TRACE: FILTERED
LOWER TRACE: UNFILTERED
HORIZONTAL SCALE: 1 ^SEC/CM
Figure 3-7 First Order Click Waveform
UPPER TRACE: FILTERED
LOWER TRACE: UNFILTERED
HORIZONTAL SCALE: 1 *iSEC/CM
G C 10 177 (A) —
3-10
Figure 3-8 Second Order Click Waveform
UPPER TRACE: FILTERED
LOWER TRACE: UNFILTERED
HORIZONTAL SCALE: 1 m SEC/CM
Figure 3-9 Second Order Click Waveform
UPPER TRACE: FILTERED
LOWER TRACE: UNFILTERED
HORIZONTAL SCALE: 1 /iSEC/CM
GC 10 177 (A) — 19
3-11
Figure 3-10 Third Order Click Waveform
UPPER TRACE:* FILTERED
LOWER TRACE: UNFILTERED
HORIZONTAL SCALE: 1 /xSEC/CM
Figure 3-11 Third Order Click Waveform
UPPER TRACE: FILTERED
LOWER TRACE: UNFILTERED
HORIZONTAL SCALE: 1 fiSEC/CM
GC10t77(A)- 18
3-12
Figure 3-12 Fourth Order Click Waveform
UPPER TRACE: FILTERED
LOWER TRACE: UNFILTERED
HORIZONTAL SCALE: 1 ^SEC/CM
Figure 3-13 Fourth Order Click Waveform
UPPER TRACE: FILTERED
LOWER TRACE: UNFILTERED
HORIZONTAL SCALE: 1 m SEC/CM
GC 10 177 (A) — IG
3-13
TABLE 3-1
DURATION AND AMPLITUDE
CHARACTERISTICS OF Nth ORDER CLICK V/AVEFORMS
CLICK ORDER
UNFILTERED
DURATION
CIO- 6 SEC)
FILTERED
RELATIVE AMPLITUDE
CCM)
FIRST
0 5
0 5
SECOND
1 0
1 0
THIRD
1 5
1 5
FOURTH
2 0
2 0
3-14
3.3 FM DEMODULATOR PERFORMANCE WITH CLICK NOISE
The click event was described m Paragraph 32m terms of the
interaction between an unmodulated carrier and a quadrature-carrier
representation of the input narrow-band noise However, m order
to discuss the effect of click noise on the performance of an FM
demodulator, it is necessary to consider a modulated input signal.
An FM signal can be represented by S(t) m the following form
S(t) = A cos [w c t + <J>(t) ] ,
( 7 )
where A is a constant representing the amplitude of the signal,
o) c = 2 it f c , which is the carrier angular frequency, and <j>(t) is
the modulation term
The modulation information can be utilized to determine the in-
stantaneous frequency of the carrier m the following manner
= io c + ^ = w c + Atom(t) (8)
where is the instantaneous frequency of the carrier, to c is the
carrier rest frequency when modulation is not present, and Am is
the carrier frequency deviation, and m(t) is the modulating signal
waveform
The magnitude of the frequency deviation Am is determined by the
amplitude of the modulating waveform m(t) and by the sensitivity
of the transmitter modulator
For sinusoidal modulation, equation (8) can be written as follows
to = to + Atocos to t (9)
ic m
3-15
3 3 FM DEMODULATOR PERFORMANCE WITH CLICK NOISE (Cont'd)
Since
a + d<£
Aw cos to t = —
m dt
Then
<f> = / Aw cos w t dt
J m
Therefore, equation (7) can be rewritten as
S(t) = A cos [v + / Aw cos w^t dtj
F Aw 1
w t + — sm w t
c w m
L m -1
= A cos
Aw
Cio)
(ii)
By definition, — is designated as the modulation index, £
w
m
The final form of equation (1) is
S(t) = A cos j^w^t + g sin w m tj (12)
The input SNR to the demodulator is determined by considering a
rectangular predetection bandpass filter having a bandwidth
BW Ip - 2(Af * f m )
(13)
where Af is the peak frequency deviation and f m is the
modulating frequency
The input signal power can be found from equation (12) to be
(14)
3-16
3.3 FM DEMODULATOR PERFORMANCE WITH CLICK NOISE (Cont'd)
and the input noise power is
N = KT BW Ip , (15)
-23 -1
where K is Boltzman's constant (1 38 X 10 Watts-sec °K ),
T is the effective system temperature m degrees Kelvin, and BW^p
is defined by equation (13)
Therefore, the input SNR can be found by dividing equation (14) by
equation (15)
S i /N i 2KT BW
IF
(16)
The output signal power at high levels of input SNR is independent
of the input carrier amplitude A and is a function of the frequency
deviation Af
Aw 2 _ Tr (2-rrAf) 2
d ~ ' h
(17)
The output noise power of the discriminator for the unmodulated case
can be determined by considering the noise disturbance about the
carrier frequency w c The input noise can be represented m quadrature
form by the following expression
N = X(t) cos (0 t = Y(t) sm w t (18)
0)1 c c
where X(t) and Y(t) represent the in-phase and quadrature components
of noise and are defined by a summation of Gaussian random variables
having zero mean.
X(t) = Y(t) = 0
(19)
and
X 2 (t) = Y 2 (t) = N 2
v v (DC
( 20 )
3-17
3 3 FM DEMODULATOR PERFORMANCE WITH CLICK NOISE (Cont'd)
3
By definition ,
- 0 ) ) t + y sin(nu - w )t1 (21)
c 'n o c J
- w )t - y cos(nio - w )tl (22)
c. n o c J
where x n and y n are Gaussian random variables with zero mean
The total expression for the instantaneous carrier plus noise can be
written as follows
Total input voltage = A cos w t + X(t) cos co t + Y(t) sm co t
c c c
= [A+X(t)] cos co t + Y(t) sm <o t (23)
c c
The phase error caused by the noise disturbance about the carrier
frequency co is represented by <|>
C 6
<i>
e
tan
Y (t) „ Y(t)
A+X(t) A
(24)
This expression is similar to equation (6) for the angle <J>
Similarly, the frequency error, or noise disturbance about the
carrier frequency to , at the output of the discriminator is repre-
sented by
' e
e e
~ Y(t)
Substituting equation (22) for
co
- “5 It S [* n
n=l L
Y(t) into equation (25), we get
- co )t - y cos (nto - to )t
c J ; n v o c l
(25)
(26)
John J Downing, Modulation Systems and Noise
1964), p . 52
(Englewood Cliffs,
3-18
3.3 FM DEMODULATOR PERFORMANCE WITH CLICK NOISE (Cont'd)
n-1
x (nco - co )cos (n<o -w_)t+y (no) -a.)sm(nw -w_)t (27)
o c o c' ' n o c J o c ' ■
2tr
« = - 7 —
e A
00
[x n Cn£ o -f c ) cos(nw o -t») c )t+y n Cnf 0 -f c )sm(nw o -a) )tj (28)
n=l L
The average output noise power is proportional to a)
w 2 = ~ V[x 2 (nf -£ ) Z cos 2 (nw -w )t+y 2 (nf -£ ) 2 sm 2 (nco -u ) t
e . 2 Z— . L n o c v oc n v oc o c
A n =i
o c 'no c
+ 2x y (nf -f ) L cos (nw -w )t sm (nw -w t
n' n o c o c o c
“
(29)
Since the mean value of the cross product term,
2
2x y (nf -£ ) cos fnu -w )t sm (nw -w )t,
n 7 n o c o c ^ o c 7 7
is zero
0)
2 4tt
2 2
/ (n£ -£ ) 2 x 2 cos 2 (nw -w )t+y ^sm (nto -w )t
e . 2 L-J L o c 7 n o c 7 7 n v oc
A n =l L
(30)
but, x = y = 0
n n
(31)
2 2 KT
anci n / n T
(32)
Where KT = equivalent noise spectral density and r is the period of
noise under consideration
3-19
3.3 FM DEMODULATOR PERFORMANCE WITH CLICK NOISE (Cont'd)
Therefore ,
o > e 2 = — “T^ n£ o" £ c ' 12 [ cos2 (nw o -u) c ]t+sin 2 Cnw o -u c )t J
Since {"cos (nw -w )t+sm 2 (nu) -co )tl = 1 we have,
L o c o c J ’
7- £ E
A n=l
By making the period t relatively long, the summation can be
changed to an integral m equation (34) .
f +f
2 r c m
co 2 = ~ I KT(f -f J 2 df
e A 2 J o c o
f -f
c m
where f is the maximum modulating frequency (the cutoff
frequency of the postdetection low pass filter, BW )
o
2 „ 2
w = 4 tt
e
(s)
(f -£ V
u o cr
f = f +f
o cm
f » f -f
o cm
f +f -f
c m c
f -f -f
c m c
f 3 +f 3
m m
87r 2 fm 3 KT
3-20
3 3 FM DEMODULATOR PERFORMANCE WITH CLICK NOISE (Cont‘d)
Finally, we get
N
o
K n w
D e
2KT (£ ) 3 4tt 2 K^
m' D
3A
( 37 )
The output SNR can be found by dividing equation (17) by equation
(37).
S /N
o o
3A f 2 A 2
4KTf
m
(38)
The output SNR can be expressed m terms of the input SNR by com-
bining equation (16) and equation (38)
5 /N = / S /N \
o o 1 l 1/
3Af 2 BW
IF
2f
m
(39)
By assuming that the cutoff frequency of the postdetection filter
is the same as the maximum modulating frequency f m we can rewrite
equation (39) as follox\rs
3Af 2 BW Ip
2BW 3
O
(40)
where BW is the bandwidth of the postdetection low pass filter
3-21
3 3 FM DEMODULATOR PERFORMANCE WITH CLICK NOISE (Cont'd)
Remembering that the modulation index 3 is defined as
_ Aw 2-?rAf _ Af
w 2irf BW
m o
we can express equation (39) another way
S /N
o
o
3B 2 BW
2BW
o
IF
(41)
Equations (39), (40), and (41) express the SNR transfer character-
istic of an FM discriminator operating m the linear region above
threshold This region corresponds to the portion of the curve
(see Figure 3-1) for values of input SNR greater than 10 dB.
The output noise represented by equation (37) has a Gaussian
distribution and a parabolic spectrum This noise determines the
behavior of the SNR transfer curve m the linear region above
threshold The occurrence of click noise m the output, however,
causes the transfer curve to deviate from its linearity, and the
FM threshold effect results Therefore, equations (39) , (40) and
(41) are valid only at values of input SNR that are large enough
(generally > 10 dB) such that noise clicks do not appear m the
output
In order to describe the performance of an FM discriminator for
values of input SNR below 10 dB , it is necessary to consider the
addition of click noise to the Gaussian component m the output
The noise power contribution of click noise increases as the
number of clicks per second increases
3-22
3.3 FM DEMODULATOR PERFORMANCE WITH CLICK NOISE (Cont'd)
Following the analysis of S 0 Rice4, the output noise power
below threshold can be found by adding the contributions of the
Gaussian and click noise as shown below
N" = N + N
o o c
( 42 )
where Nq is the total output noise power, N Q is the Gaussian noise
contribution, and is the click noise contribution
Rearranging equation (39), we get
N
o
2S f 3
o m
(y Ni )3Af 2 BW IF
(43)
Substituting equation (17) into equation (43), we have
K 47r 2 Af 2 f 3
D m
° (V» x )3tf 2B »i F
„ , 2_ 3
K n 4ir f
D m
(44)
The noise power contribution of click noise N has been found by
Rice to be 8^ 2 (N + + N')f m c
• N C ■ K D 8,r2(N+ +
(45)
+ —
where N and N represent the number of positive and negative clicks
per second, respectively
4
Rice,
PP
375-424
3-23
3 3 FM DEMODULATOR PERFORMANCE WITH CLICK NOISE (Cont'd)
Rice further shows that the expression for the click rate N+ is
given by
N
+
(
1-erf
( 46 )
where r is defined as the radius of gyration of the power spectrum
w e (f) about its axis of symmetry, f = £ c
r
_1
2ir
(27 j) 2 J (f-f c ) 2 to(f )df
*_0
1/2
/
0)(f )df
(47)
r
27T
tt 2 0) BW_ 3
o IF
3w BW__
o IF
1/2
BW
IF
2\/3
(48)
Substituting equation (48) into equation (46) we obtain
BW t „ / , \
N + = ( 1-erf Vs /N ] (49)
4vr \ 1 x ;
The click rate given by equation (49) is valid only for an un-
modulated carrier Assuming that the number of positive and
negative clicks are equal, the_n equation (45) can be written as
N = K T ,167r 2 (N + )f
c D m
(50)
3-24
3 3 FM DEMODULATOR PERFORMANCE WITH CLICK NOISE (Cont ' d)
Substituting equation (49) into equation (50) we get
K 4it BW / \
N = — — [ l-er£ VS /N If
c vr \ 1 1 / m
Therefore, the total average output noise power can be written as
4 ' n ' 2 f ^K-, K f 4 tt 2 BW / , \
■ V» e ■ mZfty ( l -^) (
vr
From equation (17) we get.
K_(2irAf)
S = -J 4 —
o 2
Dividing equation (53) by equation (52), we obtain the following
expression for the SNR transfer characteristic of an FM discriminator
S /N" =
o o
4 *V r, 3 bw if 2 ( :
3BW if /VM vr f 2
27T 2 Af 2
3BW if 2 ( S x /N x
1- erf VS /N
i l
Substituting BW^ in equation (54) for f and rearranging, we obtain
Af BW.,
(v n i)
S /N ^ = L _ J
00 Vr/BW IF \ 2 /s i /N i Vl-e r fVs7N i \
Equation (55) can be expressed m the following form by substituting
$ for Af.
3-25
3 3 FM DEMODULATOR PERFORMANCE WITH CLICK NOISE (Cont'd)
( 56 )
Plotting equation (56) gives results similar to the curve shown
in Figure 3-1 The deviation from the linear portion of the curve
is due to the click noise contribution as expressed by equation (51)
For values of input SNR greater than 10 dB, the click noise term
becomes negligible, and equation (56) reduces to the form of equa-
tion (41) .
It should be noted that the click noise term expressed by equation
(51)’ is valid only for an unmodulated carrier at the input to the
discriminator For a modulated signal, the click rate increases
substantially so that the portion of the curve below threshold in
Figure 3-1 exhibits a more severe slope
The previous analysis indicates that the threshold effect m an
FM system is primarily determined by the click rate in the demod-
ulator output This implies that a substantial extension of thresh-
old could be accomplished by reducing the click rate at the output
for a given input SNR Experimental results have verified that,
indeed, the threshold performance of an FM demodulator can be im-
proved by reducing click noise The following sections describe
a threshold extension technique that is based on the detection and
elimination of click noise m the demodulator output
3-26
3 4 CLICK SYMMETRY
For a properly aligned FM demodulator, both the positive and negative
noise spikes m the output below threshold are present m equal
quantity However, certain conditions can exist that cause the
output noise voltage to be unsymmetrical as represented by the pre-
dominance of either positive or negative clicks
The most common cause of unsymmetrical click noise is an input
carrier that is not centered properly m the passband of the
demodulator predetection filter. Either a frequency drift m
the transmitter or an unaligned predetection filter can result m
offsets that cause unsymmetrical clicks The effect of unsymmetri-
cal clicks is to increase the output noise power and, hence, degrade
the performance of the demodulator Figure 3-14 illustrates the
effect of input frequency offsets on the output noise power, while
Figure 3-15 represents measured data that shows the output noise
power increase as a function of carrier offsets
The problem of unsymmetrical click noise is significant since
the CSM FM transmitter can experience offsets of ± 500 KHz from
center frequency which adversely affects the performance of the
FM channels
The amount of degradation is a function of frequency offset, input
SNR, and the shape of the filter response In most cases a degra-
dation of 1 dB to 2 dB was measured m tests of the CSM modes
using offsets of - 500 KHz The problem of frequency offsets can
be solved by implementing Automatic Frequency Control (AFC) in the
MSFN receiver so that the IF frequency is always centered m the pre-
detection filter passband It is also necessary to use a predetec-
tion filter that has a symmetrical frequency response
3-27
-28
MINIMIZED
OUTPUT NOISE POWER
INCREASED FROM MINIMUM
SIGNAL OFFSET ABOVE CENTER
FREQUENCY IN IF BANDWIDTH
(C)
2Af
t
OUTPUT NOISE VOLTAGE
PREDOMINANTLY POSITIVE
IMPULSES
OUTPUT NOISE POWER
INCREASED FROM MINIMUM
Figure 3-14 Effect of Carrier Offset on Click Noise Distribution
GCI0I77(A>-E
OUTPUT NOISE POWER INCREASE (dBm)
-1.6 -1.4 -1.2 -1.0 -.8 -.6 -.4 -.2 0 +.2 +.4 +.6 +.8 +1.0 +1.2 +1.4 +1.6
OFFSET FROM f (MHz)
0 GC 10 1 77(A) — 6
Figure 3-15 Output Noise Power Increase vs Center Frequency Offset
SECTION 4
THRESHOLD EXTENSION TECHNIQUES
4 . 1 GENERAL
The discussion of FM threshold in the preceding section recognized
the presence of high amplitude impulse noise (click noise) in the
demodulator output as a primary factor contributing to the occur-
rence of threshold in an FM system.
Certain distinguishing characteristics of click noise provide a
basis for practical techniques which can be implemented at the
output of an FM demodulator to extend the threshold performance
of the system. The threshold extension technique that will be
described in this section is based on the accomplishment of the
following two steps:
A. Detection of the click producing noise impulses in the
demodulator output by distinguishing them from the demodu-
lated signal and low-level Gaussian noise.
B. Utilization of the detected click noise information to per-
form a click elimination operation on a delayed version of
the demodulator output.
4.2 CLICK DETECTION
The problem of click detection can be simplified by observing that,
in most cases, the peak amplitude of the click-producing impulse
noise is greater than that of both the modulation and Gaussian
noise in the unfiltered demodulator output. Figures 4-1 through
4-6 are presented to illustrate this point.
The upper waveform in each photograph represents the signal plus
noise output of a 500 KHz low-pass postdetection filter whereas
the lower sweep in each figure represents the signal plus noise
present in the unfiltered demodulator output. The sweeps in each
photograph are aligned such that the relationship between unfiltered
4-1
f = 10 KHz
m
Af = 1 MHz
BW = 500 KHz
o
Figure 4-1 Demodulated Signal Plus Noise Waveform With
and Without Postdetection Filtering (f m = 10 KHz)
UPPER TRACE: FILTERED
LOWER TRACE: UNFILTERED
HORIZONTAL SCALE: 20 ^SEC/CM
f = 10 KHz
m
Af = 1 MHz
BW = 500 KHz
o
Figure 4-2 Demodulated Signal Plus Noise Waveform with
and Without Postdetection Filtering (f m = 10 KHz)
UPPER TRACE: FILTERED
LOWER TRACE: UNFILTERED
HORIZONTAL SCALE: 20 ^SEC/CM
GCIO 177 (A)— 14
4-2
f = 50 KHz
m
Af = 1 MHz
BW = 500 KHz
o
Figure 4-3 Demodulated Signal Plus Noise Waveform With
and Without Postdetection Filtering (f m = 50 KHz)
UPPER TRACE: FILTERED
LOWER TRACE: UNFILTERED
HORIZONTAL SCALE: 10 m SEC/CM
f = 50 KHz
m
Af = 1 MHz
BW = 500 KHz
o
Figure 4-4 Demodulated Signal Plus Noise Waveform With
and Without Postdetection Filtering (f m = 50 KHz)
UPPER TRACE: FILTERED
LOWER TRACE: UNFILTERED
HORIZONTAL SCALE: 10 m SEC/CM
G C 10 177 (AJ — 12
4-3
Figure 4-5 Demodulated Signal Plus Noise Waveform With
and Without Postdetection Filtering (f m =100 KHz)
UPPER TRACE: FILTERED
LOWER TRACE: UNFILTERED
HORIZONTAL SCALE: 5 mSEC/CM
Vs
* ^
‘ W
i I !
f = 100 KHz
m
Af = 1 MHz
BW q = 500 KHz
•//'I W
Figure 4-6
Demodulated Signal Plus Noise Waveform With
and Without Postdetection Filtering (f m = 100 KHz)
UPPER TRACE: FILTERED
LOWER TRACE: UNFILTERED
HORIZONTAL SCALE: 5 ^SEC/CM
GC 10 177 (A) — 13
4-4
4 2 CLICK DETECTION (Cont'd)
and filtered waveforms may be observed for a particular click event.
It should be noted that the 500 KHz postdetection filter acts to
attenuate (as well as to broaden) the click waveform.
It is important to note that a click occurs on the 500 KHz low-
pass filtered output only when the peak amplitude of the corres-
ponding noise impulse m the unfiltered output exceeds the average
peak amplitude of the modulation plus Gaussian noise There are,
however, a certain number of high amplitude noise impulses present
on the unfiltered waveforms shown m Figures 4-1 through 4-6 that
reach the amplitude of the click-producing noise spikes but do not
result m a corresponding click on the unfiltered output waveform
These noise spikes belong to the Gaussian portion of the unfiltered
output noise whose energy is concentrated primarily m frequencies
above the 500 KHz cutoff of the low-pass postdetection filter
The parabolic spectrum of these occasional high amplitude excur-
sions of the Gaussian noise do not produce clicks on the filtered
output waveform since most of the energy is concentrated m fre-
quencies above the cutoff frequency of the low-pass postdetection
filter .
The probability of detecting these non-click-producing noise
impulses can be minimized by implementing a pre-click detection
filter to attenuate the higher frequency Gaussian noise components
The relative high amplitude characteristic of the click-producing
impulse noise will be preserved if the cutoff frequency of the
filter is high compared with the 500 KHz postdetection filter A
2 MHz low-pass filter was used for this purpose as shown m Figure
4-7
Experimental results have shown that amplitude detection of the
unfiltered click noise m the demodulator output is a simple and
efficient means for obtaining the desired information indicating
the occurrence of a click The actual detection process is accom-
plished with a pair of conventional Schmidt triggers It is neces-
sary to use two triggers since both positive and negative clicks
must be independently detected
These devices are configured to detect noise spikes that exceed a
predetermined positive or negative voltage level The reference
(trigger) voltage level is selected so that only the click-producing
noise spikes will be detected Figure 4-7 shows the system config-
uration for the click detection process
The output of the amplitude level detector is used to trigger a
pulse generator which provides gating pulses for the click elimi-
nation circuitry
4-5
FM
DEMODULATOR
TO CLICK
ELIMINATOR
Figure 4-7 Click Detector Block Diagram
BD 10 178 (A)
4.3 CLICK ELIMINATION
The detected click information is used to perform a click elimi-
nation operation on a delayed version of the demodulator output
The elimination process is accomplished by first passing the delayed
demodulator output through an amplifier whose output can be gated
off for a predetermined time.
Gating pulses from the click detection system are then supplied
to the amplifier so that it is turned off at the beginning of a
click event The turnoff time of the amplifier is preset to coin-
cide with the duration of the click
A "holding" circuit is used m conjunction with the amplifier to
provide a constant output voltage from the system during the time
that the amplifier is biased off The output voltage of the "holding"
circuit corresponds to the amplitude of the demodulator output just
before the beginning of the click The net effect of the click elim-
ination circuitry is to provide an estimate of the modulation as a
substitute for the high amplitude noise spike m the demodulated
output A block diagram of the click elimination system is shown
m Figure 4-8
Sance the click elimination process is performed prior to postde-
tection filtering, the turnoff time, or click duration, is small
compared with the average modulation frequency The 500 KHz post-
detection filter provides a smoothing effect on the click-eliminated
output, which enhances the performance of the device
A block diagram of the complete threshold extension device is shown
m Figure 4-9
4-7
i
CO
FROM CLICK
DETECTOR
OUTPUT
Figure 4-8 Click Eliminator Block Diagram
BQIO I7S(A)-2
Figure 4-9 Click Detection and Elimination Threshold Extension Device Configuration
4 4 THRESHOLD EXTENSION DEVICE
The following steps summarize the operation of the threshold
extension device
A The output of an FM demodulator, which consists of signal plus
noise, is split into two separate channels The first channel,
A, is passed through a pre-click detector filter and is then fed
to a series of circuits that detect the presence of high amplitude
impulse noise. This is the noise that is a primary factor contri-
buting to the degraded performance of an FM system.
B The output of the impulse noise detection circuits consists
of a series of positive pulses that are fed to a gated
amplifier
C The input to the gated amplifier is channel B of the de-
modulator output Channel B is identical to channel A
except that it is time delayed by a preset value
D The gating pulses from the noise detection circuits are
used to turn off channel B whenever a noise impulse occurs
This creates a "hole" m the channel B output that is
smoothed over by additional circuitry which provides an
estimate of the modulation during the turnoff time
E. The output of the gating amplifier is fed to a low-pass
filter that acts to provide additional smoothing to the
output modulation during turnoff time
F The resulting output consists of signal plus low-level
noise with fewer high amplitude impulses to degrade the
signal- to-noise ratio
4-10
SECTION 5
EXPERIMENTAL RESULTS
5 1 GENERAL
The performance of the click elimination device has been evaluated
m terms of its ability to improve the demodulated output of an
Apollo, MSFN, phase-locked loop carrier frequency demodulator Sev-
eral tests have been conducted with input signals having parameters
representative of the Apollo Block II downlink television and play-
back voice modes The signal characteristics of these modes are
listed m Table 5-1
TABLE 5-1
CSM MODES USED FOR THRESHOLD EXTENSION DEVICE TEST
MODE
BASEBAND
SERVICE
Af
f '
m
BW 0
CSM FM
1 1 PLAYBACK
100 KHZ
3 KHZ
70 KHZ
MODE 1
VOICE
CSM FM
32 1 PLAYBACK
100 KHZ
70 KHZ
70 KHZ
MODE 2
VOICE
CSM FM
TV
1.0 MHZ
409 KHZ
500 KHZ
MODE 4
The performance evaluation tests can be outlined as follows
A Signal plus noise waveform analysis
B Signal- to-noise ratio tests
1 CSM Modes 1 and 2
2 CSM Mode 4
C. Voice intelligibility tests
D. TV picture quality tests
5-1
5 2 SIGNAL-PLUS -NOISE WAVEFORM ANALYSIS
A qualitative estimate of the click eliminator performance was
obtained by observing the effect of the device on the output noise
and signal-plus-noise waveforms as displayed on an oscilloscope
The test configuration is shown m Figure 5-1 The results of
this test are shown m Figures 5-2 through 5-13
The upper trace m each figure represents the unprocessed demod-
ulator output whereas the lower trace shows the click eliminated
output
Figures 5-2 and 5-3 show the effectiveness of the click elimination
process when the demodulator output consists of noise without
modulation The device eliminates 100 percent of the noise spikes
for an input signal- to-noise ratio of 3 dB as shown m Figure 5-2
For an input SNR of 1 dB, the elimination process is more than
90 percent efficient. In both cases the postdetection filter
was 70 KHz, which corresponds to the CSM Modes 1 and 2 demodulator
configuration
Figures 5-6 through 5-13 show the improvement that is obtained
when both sinusoidal signal plus noise are present at the output of
the demodulator. The modulation frequencies and deviations (Af)
used for these photographs are compatible with the Apollo CSM FM
modes listed m Table 5-1
Several combinations of modulation frequency and output bandwidth
were used to demonstrate the spike noise elimination process for
various ratios of click duration to modulation frequency
Figures 5-4 and 5-5 illustrate the click elimination process for a
modulation frequency of 30 KHz, a peak frequency deviation of 1 MHz
and a postdetection low-pass filter bandwidth of 500 KHz
For Figures 5-6 and 5-7, the modulation frequency is 1 KHz, the
peak frequency deviation is 100 KHz, and the postdetection low-pass
filter bandwidth is 70 KHz
5-2
B D 10 178 (A) — 4
Figure 5-1
Signal Plus Noise Waveform Analysis Test Configuration
WITHOUT CLICK
ELIMINATION
*r**y w 'r
WITH CLICK
ELIMINATION
Figure 5-2 Signal Plus Noise Waveform Analysis
BW 0 = 70 KHz
SNR... = 3dB
IN
NO MODULATION
WITHOUT CLICK
ELIMINATION
WITH CLICK
ELIMINATION
Figure 5-3 Signal Plus Noise Waveform Analysis
BW 0 = 70 KHz
SNR... = ldB
IN
NO MODULATION
GC 10 1 77 (A) - 38
1
1
1
1
1
wwvv
WITHOUT CLICK
ELIMINATION
■
1
wvwv
WITH CLICK
ELIMINATION
Figure 5-4 Signal Plus Noise Waveform Analysis
R f = 30 KHz
m
Af = 1 MHz
. BW 0 = 500 KHz
1 SNR |N =2dB
1
1
1
WVWx?
WITHOUT CLICK
ELIMINATION
1
1
VvWA
WITH CLICK
ELIMINATION
1
1
1
Figure 5-5 Signal Plus Noise Waveform Analysis
f = 30 KHz
m
Af = 1 MHz
BW 0 = 500 KHz
SNR = ldB
GC 10 177 (A) — 37
1 0
5-5
WITHOUT CLICK
ELIMINATION
WITH CLICK
ELIMINATION
Figure 5-6 Signal Plus Noise Waveform Analysis
f = 1 KHz
m
Af = 100 KHz
BW 0 = 70 KHz
SNR |N = 4dB
WITHOUT CLICK
ELIMINATION
WITH CLICK
ELIMINATION
Figure 5-7 Signal Plus Noise Waveform Analysis
t = 1 KHz
m
Af = 100 KHz
BW 0 = 70 KHz
SNR m = 3dB
G C 10 1 77 (A) — 36
5-6
WITHOUT CLICK
ELIMINATION
WITH CLICK
ELIMINATION
Figure 5-8 Signal Plus Noise Waveform Analysis
f = 30 KHz
m
Af = 100 KHz
BW 0 = 70 KHz
SNR|N = 3 dB
WITHOUT CLICK
ELIMINATION
WITH CLICK
ELIMINATION
Figure 5-9 Signal Plus Noise Waveform Analysis
t = 30 KHz
m
Af = 100 KHz
BWo= 70 KHz
SNR |n = 2 dB
GC tO 1 77 (A)— 35
5-7
WITHOUT CLICK
ELIMINATION
WITH CLICK
ELIMINATION
Figure 5-10 Signal Plus Noise Waveform Analysis
f = 30 KHz
m
Af = 100 KHz
BWo= 70 KHz
SNR |N = 2 dB
WITHOUT CLICK
ELIMINATION
WITH CLICK
ELIMINATION
Figure 5-11 Signal Plus Noise
f = 30 KHz
m
Af = 100 KHz
BW 0 = 70 KHz
SNR (N = 1 dB
Waveform Analysis
GC 10 177 (A) — 34
5-8
WITHOUT CLICK
ELIMINATION
WITH CLICK
ELIMINATION
Figure 5-12 Signal Plus Noise Waveform Analysis
t = 30 KHz
m
Af = 100 KHz
BW 0 = 70 KHz
SNR )N = 1 d B
WITHOUT CLICK
ELIMINATION
WITH CLICK
ELIMINATION
Figure 5-13 Signal Plus Noise Waveform Analysis
f = 30 KHz
m
Af = 100 KHz
BW 0 = 70 KHz
SNR 1N = ldB
G CI0 177 (A) — 33
5-9
5.2 SIGNAL-PLUS-NOISE WAVEFORM ANALYSIS (Cont'd)
These figures represent the case where the click duration is
small compared to the modulation period. The individual noise
spikes do not significantly distort the overall shape of the
signal because of their relatively short duration. However, the
contribution of the clicks to the output noise power is still con-
siderable because of their relatively high amplitude.
An important observation to be made from Figures 5-4 through 5-7
is that the click eliminator is capable of removing both low-level
(those spikes having a peak amplitude less than the maximum modu-
lation amplitude) and high-amplitude spikes from the demodulated
waveform. This observation verifies the validity of the criteria
for amplitude detection of click noise in the unfiltered demodula-
tor output .
In Figures 5-8 through 5-13, the modulation frequency is 30 KHz;
the peak frequency deviation is 100 KHz; and the postdetection
low-pass filter bandwidth is 70 KHz. These figures represent the
case where the click duration is approximately equal to half the
modulation period. This means that a single noise spike will cause
considerable distortion to the shape of the demodulated signal
waveform .
Figures 5-8 through 5-13 illustrate the ability of the click elim-
inator to provide a modulation estimate as a substitute for the
distortion caused by the occurrence of a noise spike on the output
waveform. It should be noted that, in most cases, the noise spikes
occur when the modulation amplitude is at a maximum.
5-10
5.3 SIGNAL -TO -NOISE RATIO TESTS
5.3.1 CSM FM Modes 1 and 2
A quantitative evaluation of the click eliminator's ability to
improve the performance of an FM channel can be obtained by de-
termining the threshold extension that results from the suppres-
sion of spike noise. The threshold extension can be determined
by measuring the output signal- to-noise ratio of an FM demodulator
before and after the click elimination process.
These tests were performed with a simulated Apollo Block II CSM-to-
MSFN playback voice channel in the Electronics Systems Compatibility
Laboratory (ESCL) of ISD/MSC. The ESCL contains an Apollo MS FN
ground station receiver that was used in conjunction with a simu-
lated RF path and CSM transponder as shown in Figure 5-14. The
click elimination device was inserted between the output of the
carrier frequency demodulator and the 71-KHz low-pass postdetection
filter .
The output s ignal - to-noisc ratio was measured with and without
the click eliminator in the system for a wide range of received
signal levels. Figure 5-15 represents the measured data for the
1:1 playback voice mode test. The threshold point is approximately
-90.6 dBm for the unprocessed channel and -92.7 dBm for the click
eliminated channel. The difference between these two values of
total received RF power represents the effective threshold exten-
sion obtained with the click elimination device. From Figure 5-15
it can be determined that a 2-dB extension of threshold was obtained
for the Apollo CSM Mode 1. However, the improvement in output SNR
is significantly greater than indicated by the 2-dB threshold ex-
tension. A maximum improvement of 8 dB to 10 dB is obtained for
certain values of total received power that are below the threshold
points found in Figure 5-15. The significance of this SNR improve-
ment will be shown in the results of the word intelligibility tests.
Figure 5-16 represents the measured data for the 32:1 playback
voice mode test. The threshold point is approximately -89.0 dBm
for the unprocessed channel and -90.4 for the click eliminated
channel. A threshold extension of approximately 1.5 dB was achieved
for this mode, while the maximum output SNR improvement was 10 dB.
A 1-KHz tone was used as modulation for the CSM Mode 2 tests.
5-11
CSM
*NOTE: The test configuration MSFN receiver does not
include a paramp and, therefore, has a noise
figure of approximately 10 dB.
Figure 5-14 Test Configuration for CSM Modes 1 and
2 Signal- to-Noise Ratio Tests With and
Without Threshold Extension Device
5-12
OUTPUT SNR (dB)
INPUT SNR (dB)
Figure 5-15 Output SNR vs Input SNR for 1:1
CSM Playback Voice Mode
5-13
OUTPUT SNR(dB)
toi to
55
a:\cz
TOTAL RECEIVED POWER (dBm)
3 4 5 6 7 8 9 10 11 12 13 14 15
INPUT SNR (dB)
G CIO 1 77 (A) — 5
Figure 5-16 Output SNR vs Input SNR for
32:1 CSM Playback Voice Mode
5-14
5.3.1 CSM FM Modes 1 and 2 (Cont’d)
It should be noted that in both Figure 5-15 and 5-16 the measured
data does not agree with the theoretical one-to-one output vs input
SNR relationship expected for large values of input SNR. Instead,
the measured curves flatten such that the output SNR remains almost
constant for values of input SNR greater than 12 dB. This limiting
effect is due to the presence of three scientific subcarriers which
are part of CSM FM Modes 1 and 2. The subcarrier frequencies are
close enough to the playback voice bandpass to limit the output SNR
at large values of input SNR.
Straight lines have been drawn tangent to the measured curve with
a slope of 1 for the purpose of determining the threshold point
for Figure 5-15 and 5-16. The threshold point was then determined
graphically according to the procedure discussed in Section 3.1.
A plot of the output SNR improvement versus input SNR is shown in
Figure 5-17. The maximum improvement occurs for values of input
SNR between 3 dB and 5 dB for both modes. The click eliminator im-
provement increases linearly as the input SNR decreases from approx-
imately 9 dB to 5 dB. For values of input SNR below 3 dB , the
improvement drops off rapidly due to saturation of the click noise
detection circuits. It is possible that this saturation effect
could actually degrade the output performance of the demodulator.
This would occur when the number of clicks per second is such that
the output of the click eliminator is turned off most of the time.
5.3.2 CSM FM Mode 4
The system configuration for the CSM television mode test is
shown in Figure 5-18. A 300-KHz tone was used to modulate the
50-MHz FM test set. A threshold improvement of 1.5 dB was obtained
as shown in Figure 5-19. The maximum improvement in output SNR was
found to be approximately 8 dB. It should be noted that the shape
of the plot representing the performance of the demodulator with
click elimination is considerably more linear than the curve repre
senting the performance of the same demodulator without click elim-
ination.
5-15
-1 0123456789 10
INPUT SNR(dB)
Figure 5-17 Output SNR improvement vs Input SNR
for 1:1 and 32:1 CSM Playback Voice
5-16
GC10»77(A)- 1
LI-
CD
TEST POINT: SNR 0|JT
BD!0I78(A)-S
Figure 5-18 Configuration for CSM Mode 4 SNR Tests
INPUT SNR(dB)
Figure 5-19 Output SNR vs Input
SNR for CSM FM Mode 4
5-18
5.3.2 CSM FM Mode 4 (Cont'd)
The implication of this observation is that the rapid deterioration
of the output signal which is normally associated with the operation
of an FM demodulator below threshold can be significantly reduced
by the application of click noise elimination techniques. There-
fore, an input SNR of 3 dB would probably result in a "useless"
output signal for a demodulator without click elimination whereas
the corresponding output signal from the demodulator using click
elimination might be useful.
A plot of output SNR improvement versus input SNR for CSM Mode 4
is shown in Figure 5-20. The maximum improvement is obtained for
values of input SNR between 1 and 3 dB.
5-19
Figure 5-20 Output SNR Improvement vs Input SNR
for CSM FM Mode 4 With Click Elimination
o C 10 177 (A) -8
5-20
5.4 WORD INTELLIGIBILITY TESTS
Experimental results from the signal- to-noise ratio tests indicate
that a threshold improvement of 1.5 dB to 2 dB was obtained for
CSM Modes 1 and 2, while the maximum output SNR improvement for
the modes was 8 dB to 10 dB .
A more realistic evaluation of the click elimination performance,
however, can be obtained by determining the improvement in output
word intelligibility. Several tests were conducted, using the
configuration shown in Figure 5-21, for the 1:1 playback voice
mode. The pre-recorded voice tapes used to modulate the CSM FM
transmitter were composed of 150 words separated into three groups.
Each group of 50 words was spoken by a different person. A
different word list tape was used for each test run representing
a specific value of total received power, and the demodulated
information was recorded for later evaluation of the output word
intelligibility.
The results of the word intelligibility tests are shown in
Figure 5-22. The maximum improvement in word intelligibility
was approximately 20 percent for an input SNR of 4.3 dB. Referring
to Figure 5-17, this value of input SNR lies within the region of
maximum improvement (between 3 dB and 5 dB input SNR) for this
particular mode.
The results of the word intelligibility tests for the 32:1 play-
back mode are shown in Figure 5-24. The improvement obtained for
this mode is considerably less than that obtained for the 1:1
playback mode. A maximum improvement of only 4 percent in word
intelligibility occurred for an input SNR of 6.3 dB . The differ-
ence between the improvement observed for these two modes is re-
lated to the distortion inherent in the spacecraft playback system.
It should be noted that even at relatively high values of input
SNR the intelligibility is limited to 81 percent for the 32:1
playback mode, whereas 91 percent intelligibility is obtained for
the 1:1 playback mode for the same input SNR. The 32:1 playback
voice tests were performed with the test configuration shown in
Figure 5-23.
5-21
CSM
Figure 5-21 Configuration for CSM FM Mode 1 Word
Intelligibility Test With and Without
Threshold Extension Device
5-22
BD10178(A) 9
INPUT SNR (dB)
Figure 5-22 Percent Word Intelligibility vs Input
SNR for 1:1 CSM Playback Voice Mode
5-23
GC10177(AJ — 3
CSM
Figure 5-23 Configuration for CSM FM Mode 2 Word
Intelligibility Tests With and Without
Threshold Extension Device
5-24
B D 10 1 78 ( A) — 7
Figure 5-24
Percent Word Intelligibility vs Input SNR
for CSM 32:1 Playback Voice Mode
GCI0177(A) 7
5-25
5.5 TELEVISION PICTURE QUALITY ANALYSIS
The test configuration shown in Figure 5-25 was used to obtain
a series of photographs representing the demodulated video signal
output of an Apollo type FM demodulator. The demodulated signal
was displayed on the CRT of a slow scan monitor operating in the
CSM-LM Block II, 10 framc-per- second mode. The photographs were
taken with equal exposure times of 1/10 second and with a constant
camera aperture. Therefore, each picture represents one complete
frame of information.
A grey scale signal was used to modulate the FM test set with a
peak frequency deviation of 1.0 MHz. Figure 5-26 shows the de-
modulated signal displayed on the slow scan monitor for a 20-dB
input SNR. This picture is used primarily as a calibration refer-
ence to which the following data will be compared.
Figure 5-27 shows the demodulated output for a 10-dB input SNR.
The presence of Gaussian noise results in a "fuzzy" picture as
compared with Figure 5-26. The absence of click noise in these
pictures indicates that the demodulator is operating above thresh-
old.
The presence of click noise, which appears as white or black spots
on the photograph, is first noticed in Figure 5-28 for an input
SNR of 8 dB. Although only two or three clicks can be found in
this figure, one of them has resulted in a momentary loss of line
synchronization as indicated by the torn segment of the vertical
white bar. The synchronization circuitry in the MSFN slow scan
monitor is particularly sensitive to the presence of click noise.
Figure 5-29 shows the demodulated output for an 8 dB input SNR
with the click eliminator in the system. Notice that the click
noise has been eliminated and that the line synchronization has been
preserved.
As the input SNR is decreased to 7 dB, the demodulator approaches
threshold, and the number of clicks, as well as synchronization
perturbations, increases accordingly.
Figures 5-30 and 5-31 show the demodulated signal before and after
the click elimination process for an input SNR of 7 dB . Figures
5-32 and 5-33 provide similar results for an input SNR of 6 dB
(the click eliminator has been able to dispose of almost every
click for values of input SNR down to 6 dB. For SNR's below
5-26
CSM
Figure 5-25 Configuration for CSM Mode 4
Television Picture Quality Tests
5-27
BDIOI 78 (A) -
Af = 1.0 MHz
BW |F = 4.9 MHz
BW = 500 KHz
o
Figure 5-26 CSM Mode 4: TV Picture Quality Tests
SNR |N = 20 dB REFERENCE PICTURE
Af = 1.0 MHz
BW |p = 4.9 MHz
BW = 500 KHz
o
Figure 5-27 CSM Mode 4: TV Picture Quality Tests
SNR... = 10 dB WITHOUT CLICK ELIMINATION
GCI0177 (A) — 32
5-28
Figure 5-28 CSM Mode 4: TV Picture Quality Tests
SNR m = 8 dB WITHOUT CLICK ELIMINATION
IN
Figure 5-29 CSM Mode 4: TV Picture Quality Tests
SNR... = 8 dB WITH CLICK ELIMINATION
IN
1.0 MHz
= 4.9 MHz
= 500 KHz
1.0 MHz
= 4.9 MHz
: 500 KHz
GC 10177 (A) - 31
5-29
Af = 1.0 MHz
BW |p = 4.9 MHz
BW = 500 KHz
o
Figure 5-30 CSM Mode 4: TV Picture Quality Tests
SNR |N = 7 dB WITHOUT CLICK ELIMINATION
Af = 1.0 MHz
BW |p = 4.9 MHz
BW = 500 KHz
o
Figure 5-31 CSM Mode 4: TV Picture Quality Tests
SNR |N = 7 dB WITH CLICK ELIMINATION
GC 10 177(A)- 30
5-30
Af = 1.0 MHz
BW |p = 4.9 MHz
BW = 500 KHz
o
Figure 5-32 CSM Mode 4: TV Picture Quality Tests
SNR |N = 6 dB WITHOUT CLICK ELIMINATION
Af = 1.0 MHz
BW |p = 4.9 MHz
BW q = 500 KHz
Figure 5-33 CSM Mode 4: TV Picture Quality Tests
SNR |N = 6 dB WITH CLICK ELIMINATION
GC 10 177 (A) — 29
5-31
5.5 TELEVISION PICTURE QUALITY ANALYSIS (Cont'd)
6 dB, the efficiency of the device decreases as a result of
circuit saturation. The ability of the device to eliminate noise
spikes is primarily related to the click rate and the recovery
time of the detection circuitry.
In Figure 5-34 the number of click-produced white spots (before
elimination) is considerably greater than the number found in
previous pictures, which indicates that the demodulator is begin-
ning to operate below threshold.
Figure 5-35 shows the effect of click elimination on the output
signal for a 5-dB input SNR. The click rate is such that approx-
imately 10 percent of the impulse noise perturbations remain
after the elimination process. However, the improvement in picture
quality due to the click elimination process is still significant.
The effectiveness of the modulation insertion circuitry incorporated
in the device can be seen by comparing Figures 5-36 and 5-37. In
Figure 5-36 the demodulated signal information is almost completely
masked by the perfusion of impulse noise. The line synchronization
is also perturbed for most of the frame duration. Figure 5-37
however, displays a demodulated signal that is considerably more
intelligible than that of the previous figure. The number of missed
line synchronizations is also drastically reduced.
A similar evaluation of the modulation preservation action can be
obtained by comparing Figures 5-38 and 5-39 for an input SNR of
3 dB. The grey scale pattern is not visible in Figure 5-38 due
to the presence of click noise whereas in Figure 5-39 it is possible
to distinguish the individual vertical bars from the remaining noise.
The improved line synchronization also contributes to the overall
image enhancement obtained with the click eliminator.
Figures 5-40 and 5-41 represent the output of the system for an
input SNR of 2 dB. The grey scale modulation is completely masked
by the impulse noise as shown in Figure 5-40. The click eliminated
output shown in Figure 5-41 contains considerably less noise, but
the grey scale vertical bars are still not intelligble. It appears,
therefore, that the modulation insertion operation has become inef-
fective for this low value of input SNR. This observation is correct
since previous results have shown that the high click rate which
occurs at low input SNR's causes the click eliminator to turn off
the output of the demodulator at a rate approaching the modulation
frequency .
5-32
Af = 1.0 MHz
BW |p = 4.9 MHz
BW 0 = 500 KHz
Figure 5-34 CSM Mode 4: TV Picture Quality Tests
SNR |N = 5 dB WITHOUT CLICK ELIMINATION
Af = 1.0 MHz
BW |p = 4.9 MHz
BW 0 = 500 KHz
Figure 5-35 CSM Mode 4: TV Picture Quality Tests
SNR |N = 5 dB WITH CLICK ELIMINATION
GC 10 177(A)— 28
5-33
Af = 1.0 MHz
BW (F = 4.3 MHz
BW q = 500 KHz
Figure 5-36 CSM Mode 4: TV Picture Quality Tests
SNR... = 4 dB WITHOUT CLICK ELIMINATION
Af = 1.0 MHz
BW |p = 4.3 MHz
BW q = 500 KHz
Figure 5-37 CSM Mode 4: TV Picture Quality Tests
SNR| N =4 dB WITH CLICK ELIMINATION
GC10177CA) — 27
5-34
Af = 1.0 MHz
Figure 5-38 CSM Mode 4: TV Picture Quality Tests
SNR |N = 3 dB WITHOUT CLICK ELIMINATION
Af = 1.0 MHz
BW |p = 4.9 MHz
BW rt = 500 KHz
o
Figure 5-39 CSM Mode 4: TV Picture Quality Tests
SNR |N = 3 dB WITH CLICK ELIMINATION
GC 10 177 (A) —26
5-35
Af = 1.0 MHz
BW |p = 4.9 MHz
BW = 500 KHz
o
Figure 5-40 CSM Mode 4: TV Picture Quality Tests
SNR |N = 2 dB WITHOUT CLICK ELIMINATION
Af = 1.0 MHz
BW |p = 4.9 MHz
BW = 500 KHz
o
Figure 5-41 CSM Mode 4: TV Picture Quality Tests
SNR |N =2dB WITH CLICK ELIMINATION
GC 10 177 (A) — 25
5-36
S 5 TELEVISION PICTURE QUALITY ANALYSIS (Cont'd)
This implies that the modulation tracking circuitry m the device
does not have sufficient time to obtain an accurate estimate of
the modulation amplitude between successive click events This
situation is not so much a limitation of the threshold extension
device as it is a consequence of having a click rate approach the
modulation frequency m the demodulator output
5-37
APPENDIX A
BIBLIOGRAPHY
Clarke, K , R. Pickholtz, and D Schilling, A Space Communications
Study--Final Report — September 19 6 6- September 1967 Prepared
for NASA Electronics Research Center under NASA Grant NGR-33-
006.020.
Clarke, K , K, and D. T Hess, "Frequency Locked Loop FM Demodula-
tor," IEEE Transactions on Communications Technology } Vol COM-
15, No, 4 August 1967
Dawson, C, A Performance Analysis of the Apollo Unified S-Band
Communications System for a Typical Lunar Mission. MSC Internal
Note MSC-EB-R- 67- 1 , August 1967,
Downing, J. Modulation Systems and Noise Englewood Cliffs, New
Jersey Prentice-Hall, Inc , 1964
Enloe , L H "Decreasing the Threshold m FM by Frequency Feedback",
Proceedings of IRE , Vol 50, January 1962.
Hess, D T , "Cycle Slipping m First Order Phase Locked Loop",
IEEE Transactions on Communications Technology 3 April, 1968
Rice, S 0 , "Noise m FM Receivers " Proceedings 3 Symposium on
Time Series Analysis M Rosenblatt, Editor New York John
Wiley § Sons, Inc , 1963.
Schwartz, M , W Bennet, and S. Stem , Communications Systems
and Techniques New York McGraw Hill, 1966
A-l
APPENDIX B
ABBREVIATIONS AND SYMBOLS
A
Carrier Amplitude
AMP
Amplifier
ATTEN
Attenuator
BW
Bandwidth.
w
1— 1
*ri
Predetection (input) Bandwidth
BW 0
Postdection (output) Bandwidth
cm
Centimeter
CSM
Command/Service Module
dB
Decibel
dBm
Decibel, referenced to 1 milliwatt
DEMOD
Demodulator
DSE
Data Storage Equipment
ESCL
Electronic Systems Compatibility Laboratory
FM
Frequency Modulation
fm
Modulation Frequency m Cycles per Second
FMFB
Frequency Modulation Feedback Discriminator
f 0
Reference or Center Frequency m Cycles per
Second
GEN
Generator
G(t)
Unmodulated Carrier
Hz
Hertz
IF
Intermediate Frequency
ISD
Information Systems Division
ISO AMP
Isolation Amplifier
K
Boltzman's Constant (1 38 x 10"23 Watts-sec
0 K _1 )
B-l
KHz
K D
LM
MHz
MSC
MSFN
m(t)
mv
N +
N“
*c
Hi
NM
N 0
No
N ( t)
OSC
PA
PLL
PMP
R
r
RCVR
RF
RMS
sec
Kilohertz
Discriminator Constant
Lunar Module
Megahertz
Manned Spacecraft Center
Manned Space Flight Network
Modulating Signal
Millivolt
Positive Click Rate
Negative Click Rate
Output Click Noise Power
Input Noise Power
Nautical Miles
Output Gaussian Noise Power
Total Output Noise Power
Discriminator input Noise
Oscillator
Power Amplifier
Phase Lock Loop
Pre-Modulation Processor
Resultant Carrier Plus Noise Magnitude
Radius of Gyration of the Power Spectrum about
its Axis of Symmetry
Receiver
Radio Frequency
Root-Mean-Square
Second
Input Signal Power
B-2
SNR
snr in
SNR
OUT
T
TRIPLX
TV
USB
XMIT
X(t), Y(t)
3
A£
A<j)
Ah)
psec
T
4 >
<b
T e
h) c
(0
m
h) r
s
Signal-to-Noise Ratio
Input (Predetection) Signal-to-Noise Ratio
Output (Postdetection) Signal-to-Noise Ratio
Output Signal Power
System Temperature
Triplexer
%
Television
Unified S-Band
*
Transmitter
Independent Random Variables, Representing the
Magnitude of the In-Phase and Quadrative Phase
Components of N(t).
Modulation Index
Frequency Deviation
Phase Difference between the Resultant Signal plus
Noise Vector and the Carrier Vector at t^ and
Radian Frequency Deviation
Microsecond
Time Interval
Phase of the Resultant Signal-plus-Noise Vector with
Respect to the Carrier Vector
Phase Error caused by the Noise Disturbance about
the Carrier Frequency.
Radian Carrier Frequency
Radian Frequency Error caused by the Noise Disturb-
ance about the Carrier Frequency.
Modulation Frequency m Radians per Second
Refereace or Center Frequency m Radians per Second
B-3