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REPLY TO
ATTN OF;
GP
Aic\J-^ Ipi^ (I
NATIONAL AERONAUTICS AND SPACE ADMINISTRATION
Washington, D.C. 20546
APR 1 0 1974
TO; KSl/Scientif ic & Technical Information Division
Attn; Miss Winnie M. Morgan
FROM: GP/Office of Assistant General
Counsel for Patent Matters
SUBJECT; Announcement of NASA-Owned U.S. Patents in STAR
In accordance with the procedures agreed upon by Code GP
and Code KSI, the attached NASA-owned U.S. Patent is being
forwarded for abstracting and announcement in NASA STAR.
The following information is provided;
U.S. patent No.
Government or
Corporate Employee
Supplementary Corporate
Source (if applicable)
NASA Patent case No.
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NOTE - If this patent covers an invention made by a corporate
employee of a NASA Contractor, the following is applicable;
VES NO / /
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Pursuant to Section 305(a) of the National Ae
Space Act, the name of the Administrator of
the first page of the patent; however, the n
inventor (author) appears at the heading of ^luilttSA|^)^C('|J,^f
the Specification, following the words " . . .wi(,^
an invention of ..."
3
Of;
V V-
Bonnie L. Woerner
Enclosure
United. States Patent [in 3,800,224
Fletcher et al. [45] Mar. 26, 1974
[54] automatic frequency CONTROL FOR
F. M. TRANSMITTER
[76] Inventors: James C. Fletcher, Administrator of
the National Aeronautics and Space
Administration with respect to an
invention of; Martial A. Honnell,
Auburn, Ala.
[22] Filed; Feb. 20, 1973
[21] Appl. No.; 333,912
[52] U.S. Cl 325/148, 178/7.1
[51] Int. Cl H04b 1/04
[58] Field of Search 179/69.5 TV; 325/45, 148,
325/159, 184, 187; 178/5.8 AF, 7.1, 7.2
[56] References Cited
UNITED STATES PATENTS
3,137,816 6/1964 McLin et al 325/148
Primary Examiner — Albert J. Mayer
Attorney, Agent, or Firm — L. D. Wofford; G. J. Porter;
J. R. Manning
[57] ABSTRACT
An automatic frequency control circuit for an F.M.
television transmitter in which the frequency of the
transmitter is sampled during what is termed the
“back porch” portion of the horizontal synchronizing
pulse which occurs during the retrace interval, the fre-
quency sample compared with the frequency of a ref-
erence oscillator, and a correction applied to the fre-
quency of the transmitter during this portion of the re-
trace interval.
7 Claims, 2 Drawing Figures
(NASA^Gase-HFS-21540-1 ). AUTOMATIC
FBEQOENCY CONTEOL FOR FM TRANSMITTER
Patent (NASA) 7 p CSCL 17B
N74- 197901
Onclas
00/07 34646
PATENTEDMR261974
3.800.224
MET 1 OF 2
PAFENTE0HAR26 1974
SHEET 2 Of 2
3.800.224
FIG, 2
3,800,224
1
AUTOMATIC FREQUENCY CONTROL FOR F. M.
TRANSMITTER
ORIGIN OF THE INVENTION
The invention described herein was made in the per-
formance of work under a NASA contract and is sub-
ject to the provisions of Section 305 of the National
Aeronautics and Space Act of 1958, Public Law
85-568 (72 Stat. 435; 42 U.S.C. 2457).
BACKGROUND OF THE INVENTION
This invention relates to automatic frequency control
systems for radio transmitters and particularly to a sys-
tem adapted to control the frequency of a frequency
modulation, F.M., transmitter employed to transmit
television or other signals having recurring periods of
fixed frequency operation.
GENERAL DESCRIPTION OF THE PRIOR ART
Commercial television transmitters generally employ
amplitude modulation to apply the picture information
to the carrier and since in such systems the carrier sig-
nal is generated by a very accurately controlled crystal
oscillator, carrier frequency control is not a substantial
problem. On the other hand there are applications, par-
ticularly in microwave television links and in industrial,
space and military fields wherein it is sometimes desir-
able to employ frequency modulation for the transmis-
sion of video signals. In such cases, since the frequency
determing oscillator must necessarily be variable in fre-
quency and thus not inherently stable, there exists the
problem of controlling the frequency of the F.M. car-
rier in some manner which will assure that a given mod-
ulation signal will always produce the same output fre-
quency. The problem becomes particularly difficult for
television signals and where transmission frequencies
are in or above the ultra-high-frequency range.
The well-known techniques used to control the fre-
quency of frequency modulated transmitters modu-
lated with audio frequency signals compare the average
frequency of the transmitter with a stable frequency
produced by a crystal oscillator. A correction signal de-
rived from the difference between the average fre-
quency of the transmitter and that of the crystal oscilla-
tor is used to correct the frequency of the transmitter.
This can be done because the average value of the
modulating audio frequency signal is zero, that is, the
audio frequency signal does not contain a DC compo-
nent.
A television, or video, signal contains a varying DC
component proportional to the brightness information
in the scene televised. The difficulty of transmitting the
DC component and, at the same time, maintaining the
frequency of the transmitter within its assigned channel
has, in the prior art, led to the design of FM television
transmitters without direct crystal control of the fre-
quency.
These transmitters used open-loop systems with volt-
age-controlled oscillators carefully temperature-
compensated for the required frequency stability. This
compensation process is slow, and the open-loop sys-
tem provides no protection against frequency drift
caused by aging effects. Another scheme used is to up-
convert the output signal of the frequency modulated
oscillator by mixing it with a crystal-controlled high-
frequency signal. This system reduces the percent error
2
in the output frequency, but it is subject to aging drifts
and presents modulation linearity problems when wide
frequency deviation is required.
j SUMMARY OF THE INVENTION
Accordingly, it is the object of this invention to pro-
vide an automatic frequency control capable of very
closely regulating the frequency of an F.M. television
transmitter employing a stable reference source.
10 In accordance with this invention the frequency of a
transmitter which generates a signal having a known
recurring reference frequency, such as a video signal,
is sampled during the recurring frequency portion of
the signal. This signal, or as converted to a selected
15 lower frequency signal, is fed to a discriminator to-
gether with a fixed reference frequency from a crystal-
controlled oscillator with respect to which the transmit-
ter is to be controlled in frequency. The transmitter sig-
nal and fixed reference signal are alternately processed
20 by the discriminator by an electronic switch which
causes the transmitter signal to be sampled during peri-
ods of the recurring reference frequency signal and the
fixed reference signal to be sampled between such peri-
ods. During the discrimination of one of the two sig-
25 nals, the discriminator output is stored. Thereafter, and
during the discrimination of the other signal, the dis-
criminator output is connected in series with the previ-
ously stored output in such a manner that a voltage rep-
resentative of the difference of the discriminator out-
20 put is obtained. This difference is representative of the
error in transmitter frequency and is applied back to
the transmitter during the reference frequency period
and employed to correct the frequency of the transmit-
ter. The result is that the frequency of the transmitter
25 is held to a very small percentage of error.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is an electrical block diagram of an embodi-
ment of the invention.
FIG. 2 is a series of waveforms illustrative of the
operation of the invention.
DESCRIPTION OF THE PREFERRED
EMBODIMENT
Referring to the drawings, automatic frequency con-
trol 10, as shown in FIG. 1, includes crystal oscillator
reference 12, error corrector 14 and control 16, which
are interconnected to sample the quiescent carrier fre-
quency of FM transmitter 18, through mixer stage 20,
and apply an appropriate correction, through switch
22, to voltage controlled oscillator 24 of transmitter 18.
A source of video, having scanning rate and sync
standards similar to those required for commercial
broadcasting, is coupled to the input of video amplifier
26. One output 28 of video amplifier 26 is coupled
through capacitor 30 to the input of 225 MHz voltage
controlled oscillator 24 of transmitter 18, and to the
output of switch 22. A second output 32 of video am-
plifier 26 is coupled to the input of sync clipper 34 of
control 16.
The output of voltage controlled oscillator, or
V.C.O., 24 of transmitter 18 is connected as an input
to a first 225 MHz buffer amplifier 36, in turn having
an output coupled to the input of second 225 MHz am-
plifier 38, and to input 40 of mixer 20. The output of
amplifier 38 is, in turn, connected as an input to times-
3 , 800,224
3
ten frequency multiplier 42 which outputs a carrier fre-
quency of 2,250 MHz. This 2,250 MHz carrier is then
fed through band-pass filter 44, which reduces the spu-
rious, or undesired, sideband frequencies present at
the output of frequency multiplier 42 to provide a clean
transmitter output. Waveform A of FIG. 2 is a plot of
time versus voltage at the input of video amplifier 26
and the resultant deviation in carrier frequency of
transmitter 18’during any one horizontal scanning pe-
riod. The back porch 46 of the composite video signal
is clamped to a zero volt reference which also repre-
sents a quiescent carrier output frequency of 2250
MHz. The negative peak 48 of sync pulses SO is 0.4
volts which results in a negative shift of 5.15 MHz of
the 2,250 MHz carrier.
Blanking pedestal 52 occurs between an input of
-K). 1 and zero volts which is the black or blanking level
of the input video, also designated as the retrace inter-
val. A -H volt input is required for a maximum white
level 54 and results in a -H2.85 MHz shift of the car-
rier.
The input of voltage controlled oscillator 24 is
clamped by frequency corrector 14, in a manner to be
further described, to a voltage which corrects for devia-
tions of oscillator 24 from the desired frequency of 225
MHz (the output frequency 2,250 MHz divided by 10)
when the video input is 0 volts, and during an interval
coincident with the back porch 46 of the video signal.
.Sync clipper 34 of control 16 strips the horizontal
and vertical synchronizing pulses from the incoming
video signal which are then coupled as clocking pulses
to the input of switching pulse generator 56, having
outputs 58, 60 and 62 coupled to control inputs 64, 66
and 68 of switches 70, 72 and 74, respectively, or cor-
rector 14, and having output 76 coupled to control
input 78 of sample and clamp switch 22.
Reference standard 12 includes 45 MHz crystal oscil-
lator 80, the output of which is coupled to times-four
multiplier 82, and to the normally open input 84 of
switch 70 of frequency corrector 14. The output of
multiplifer 82 is, in turn, amplified by 180 MHz ampli-
fier 86 and coupled to a second input 88 of mixer 20.
The resultant output frequency of mixer 20 is thus
the difference between the 180 MHz reference fre-
quency and the output of 225 MHz amplifier 36. This
output is also 45 MHz, assuming that the output of
V.C.O. 24 is 225 MHz. Any deviation of voltage con-
trolled oscillator 24 from 225 MHz produces a propor-
tional change in the 45 MHz output of mixer 20, which
is coupled to the normally closed input 90 of R.F. type
switch 70, of corrector 14.
This signal and the 45 MHz signal from reference
crystal oscillator 80 are alternately sampled for approx-
imately one-half of each horizontal scanning period, by
switch 70, having control input 64 connected to output
58 of pulse generator 56. With 0 volts applied to con-
trol input 64, (Waveform D), switch 70 samples the
output of.mixer 20, through normally closed input 90,
for approximately a half horizontal sweep period, that
is, a 32 microsecond interval 92, centered about back
porch 46 of the video signal. During the intermediate
32 microsecond interval 94 (Waveform D) centered
about the midpoint of each horizontal sync period, des-
ignated T<, a positive pulse 96 from output 58 of pulse
generator 56 enables switch 70 to sample the output of
4
reference oscillator 80 through normally open input
84.
The output of switch 70 is coupled through 45 MHz
discriminator stage 98 to the input of a second R.F.
5 type., normally open switch 72. Switch 72 is closed for
two intervals during each horizontal scan by positive
pulses 100 and 102 from pulse generator 56, (Wave-
form E).
Pulse 100, of 3.5 microseconds duration, is initiated
by the trailing edge 104 of each sync pulse 106 at time
T, (Waveform C).
The second pulse, pulse 102, is 7.5 microseconds in
width, and is centered abput the midpoint of 32 micro-
seconds sampling interval 94 (Waveform D).
Switch 72 provides sampled outputs 108 and 110
(Waveform G), being outputs of discriminator 98 re-
sponsive to the frequency of reference oscillator 80 and
mixer 20, respectively. These sampled outputs are cou-
pled as inputs to amplifier 112. Output 114 is con-
nected across holding capacitor 116 during pulse 96,
and output 114 is connected in series with capacitor
116 to input 118 of DC amplifier 120 during sampling
interval 92. These alternate circuits, which enable the
subtraction of the mixer derived output from the refer-
enced signal derived output, are achieved by the opera-
tion of clamp switch 74 which grounds to circuit
ground 75 the output side of capacitor 116 responsive
to 7.5 microsecond pulse 122 (Waveform F) coinci-
3 Q dence with similar pulse 102 applied to sample switch
70 (Waveform E). At other times, the output side of
capacitor 116 is connected to input 118 of amplifier
120.
The output of D.C. amplifier 120 is sampled by sam-
35 pie switch 22, being closed by 1 .5 microsecond pulse
124 (Waveform H) centered about the midpoint (Tj)
of 3.5 microsecond discriminator sample pulse 100
(Waveform E). Sample switch 22 feeds the output of
amplifier 120 representing. the transmitter error to ca-
40 pacitor 30 from which it is applied to the frequency
control input of V.C.O. 24.
OPERATION
Initially, the frequency of transmitter 18 is calibrated
45 to 2,250 MHz, as follows.
First, the video signal is removed from the inout of
video amplifier 26. Then sample switch 22 and clamp
switch 74 of error corrector 14 are enabled by means
provided internally of switching pulse generator 56.
50 Under these conditions, the input of D.C. amplifier
120 is referenced to zero volts through clamp switch
74. The output of amplifier 120 is connected, through
sample switch 22, to the input of voltage controlled os-
cillator 24. Bias adjust 126 of amplifier 120 is then ad-
Justed in the appropriate direction to calibrate oscilla-
tor 24 to a frequency of 225 MHz, or the output of
transmitter 18 at 2,250 MHz.
Since the output of mixer 20 is now 45 MHz and is
coupled through the normally closed input 90 of switch
70 to limiter-discriminator 98, the discriminator may
be adjusted for an approximate 0 volt output.
If preferred, switch 70 may be operated in order to
connect 45 MHz oscillator 80 to discriminator 98, and
then the discriminator is adjusted for zero output. •
Once the calibration is complete, each of switches
74, 70 and 22 is returned to a normal operating posi-
tion, and the video input signal is reconnected.
3,800,224
5
6
It is assumed, for the purpose of illustration, that the
calibration of voltage controlled oscillator 24 as de-
scribed above, results in a positive bias voltage (V*) at
the otuput of D.C. amplifier 120 as shown in (Wave-
form B). Accordingly, holding capacitor 30 is charged 5
to this value and holds the charge within very close lim-
its during each horizontal scan interval, since the decay
time of capacitor 30 is selected to be about 30 scanning
periods.
Since the back porch 46 of the video signal at the 10
input of V.C.O. 24 is referenced to this bias level, the
carrier during any back porch interval is unmodulated,
or at a frequency of 2,250 MHz.
To examine the operation of automatic frequency
control 10, assume initially that discriminator 98 is 15
properly adjusted to provide an output of 0 volts upon
receiving a precise 45 MHz signal from reference oscil-
lator 80 (Waveform G). Assume further that the out-
put frequency of V.C.O. 24 has drifted lower in fre-
quency and therefore that the output of mixer 20 has 20
drifted below 45 MHz. During interval 94 (Waveform
D) switch 70 is operated to pass the reference signal
from oscillator 80 to discriminator 98. There will, ac-
cordingly, result a 0 voltage output 108 (Waveform G)
of discriminator 98 which will be passed during interval 25
94 (Waveform D) by switch 72 (Waveform G) to am-
plifier 112 which in turn applies this potential across
capacitor 116 which is at this time connected across
the output of amplifier 112 by clamp switch 72. There-
after the output terminal of capacitor 116 is discon-
nected from ground 7S and during interval 92 (Wave-
form D), discriminator 98 is responsive to the assumed
"off” frequency output of mixer 20 and thus there is
applied to D.C. amplifier 120 the difference between
the capacitor voltage being 0 at this time as described
above, and output voltage of discriminator 98, being a
negative pulse 110, representative of the "off” fre-
quency output of mixer 20. Thus the reference derived,
capacitor voltage of 0 volts is simply subtracted from
the mixer derived discriminator voltage, negative in
this case, and the difference voltage is applied to D.C.
amplifier 120. Amplifier 120 amplifies the error 110
and reverses its polarity, applying an error correcting
pulse 125, Waveform 1, through switch 22 to holding
capacitor 30. This sequence of sampling of error 110
occurs once for each sync cycle and the voltage thus
derived is cyclically fed from amplifier 120 by switch
22 to capacitor 30 and thus to the control input of
V.C.O. 24, which causes its frequency to be raised to
a correct value of 225 MHz.
If V.C.O. 24 tends to drift higher in frequency, the
output of mixer 20 tends to become higher than 45
MHz, resulting in a positive error signal to amplifier
120. Then each time the output is sampled by switch
22, during back porch 46 of video input (Waveform
A), a discrete negative pulse, would be applied to hold-
ing capacitor 30 to thus cause the frequency of V.C.O.
24 to lower to a correct frequency.
If there is no error in frequency and thus the outputs
of discriminator 98 responsive to both the signal from
oscillator 80 and mixer 20 are zero, no corrective volt-
age will be applied by automatic frequency control 10
to V.C.O. 24.
If due to environmental or other effects, the discrimi-
nator output should drift, for example in a negative di-
rection, shown by dashed lines 126 (Waveform G), this
will, of course, cause a negative voltage to be applied
to capacitor 1 10 during the sampling of reference oscil-
lator 80. Since, however, the same negative drift volt-
age will also be applied to the discriminator output dur-
ing the sampling of mixer 20, and the two voltages are
subtracted by virtue of the switching of the output ter-
minal of capacitor 1 10, no net error js introduced by
discriminator drift. This is true whether the drift be
negative as shown by dashed line 126 or positive as
shown by dashed line 128. Thusfor example, if it were
assumed that discriminator 98 produced a minus I volt
at a “bn” frequency of 45 MHz and the transmitter
were “on” frequency there would be a minus 1 volt
across capacitor 116 during the effective sampling of
referenced oscillator 80 and thereafter with a correct
45 MHz signal from mixer 20, discriminator 98 would
also output a minus 1 volt. Since the capacitor voltage
is placed in series with the discriminator voltage during
error sampling by D.C. amplifier 120 and the latter
voltage thus subtracted from the former, a correct 0
volts, indicating a no error condition, is fed to amplifier
120. The same process of elimination of discriminator
drift as a factor is applicable when there is an actual
error of voltage output representative of an “ofC’ fre-
quency signal from mixer 20. In such case the voltage
representative of discriminator drift appears identically
in both the reference derived signal and mixer derived
signal and are cancelled leaving only a voltage repre-
sentative of the actual error.
The output frequency stability factor derived from a
mathematical model of the FM TV transmitter is pres-
ented in the following equation.
d« = + (dv/A„) + (dC/5) (d„ - d„) + ( l//fo) \Kcd,. +
+ Ksds\
Where
d„ — OUTPUT FREQUENCY STABILITY FAC-
TOR
— OPEN LOOP GAIN (A„ 1 )
d„ — REFERENCE OSCILLATOR STABILITY
FACTOR
dy — VCO STABILITY FACTOR
dt — LIMITING ERROR FACTOR
do — DISCRIMINATOR STABILITY FACTOR
Kcdc — CLAMPING ERROR FACTOR
Ksds — FREQUENCY CONTROL BIAS DRIFT
FACTOR
Ksds — SAMPLING ERROR FACTOR
Thus it is seen that the steady-state frequency stabil-
ity (do) is a function of the following factors:
1 . The stability factor d* of the reference oscillator;
2. The term dv//Co representing the stability factor of
the V.C.O. divided by the open loop gain;
3. The disturbance term (dLjS) (d„ — dn) resulting
from imperfect limiting which causes the discriminator
zero drift to affect the stability, although this factor is
reduced by the down conversion ratio;
4. The disturbance term, (\! Ko){Kcdc-\- Kede-)- Ks
ds], representing clamping inaccuracy, frequency con-
trol bias voltage drift and sampling error.
The actual effect of these factors is not large and in
practice the carrier frequency of a transmitter con-
trolled in frequency by the invention may be held to
within less than 0.01 percent of the desired frequency.
7
3,800,224
What is claimed is:
1. An automatic frequency control for an F.M. radio
frequency transmitter which transmits a signal having
a periodically recurring period in which a constant fre-
quency is generated, comprising: 5
reference frequency signal generating means for gen-
erating a stable signal at a selected frequency:
signal means responsive to the frequency of said
transmitter for providing a frequency output corre-
sponding to said selected frequency when said lo
transmitter is “on” frequency:
discriminating means responsive to an input signal
for providing a pulse output of a polarity represen-
tative of whether an input signal is above or below
said selected frequency and of a magnitude proper- 1 5
tional to the magnitude that the input signal devi-
ates from said selected signal:
switching means for selectively feeding said output of
said signal means to said discriminating means dur-
ing said periodically recurring periods and for feed- 20
ing the output of said reference frequency signal
generating means to said discriminating means be-
tween said periods:
difference means, including means for temporarily
storing the output of said discriminating means re- 25
sponsive to one of its signal inputs, for providing an
error signal representative of the difference be-
tween outputs of said discriminating means respon-
sive to successive signals from said reference fre-
quency signal generating means and said signal 30
means, respectively; and
coupling means responsive to said difference means
for applying said error signal to said transmitter for
correcting its frequency.
2. An automatic frequency control as set forth in 35
..claim 1 further comprising a long time-constant capaci-
tor adapted to feed a video, frequency modulating, sig-
nal to said transmitter and said coupling means com-
prises means for feeding said error signal to the output
side of said capacitor. 40
3. An automatic frequency control as set forth in
claim 2 wherein said difference means comprising:
a second capacitor having an input terminal coupled
to the output of said discriminating means and an
output terminal connected to said coupling means; 45
and
second switching means for connecting said output
terminal of said second capacitor to ground for the
said temporarily storing of the output of said dis-
criminating means for a discrete period; 50
whereby said second capacitor is charged to a value
of one of its signal inputs when said output terminal
of said second capacitor is grounded, and thereaf-
ter with said output terminal ungrounded and with
the occurrence of the other Of the signal inputs to 55
8 ' ^
said discriminating means, the resulting signal out-
put is combined with the stored voltage across said
second capacitor as an error voltage to said cou-
pling means and to said long time-constant capaci-
tor.
4. An automatic frequency control as set forth in
claim 3 wherein said voltage temporarily stored across
said second capacitor is a voltage provided responsive
to said reference frequency signal generating means.
5. An automatic frequency control as set forth in
claim 4 further comprising:
a video signal source adapted to provide a video sig-
nal with periodic synchronizing pulse outputs dur-
ing retrace intervals and its circuit output is con-
nected to the input terminal of said long time-
constant capacitor;
third switching means for selectively coupling the
output of said discriminating means to the input of
said second capacitor; and
switching control means responsive to said synchro-
nizing pulse outputs of said video signal source for
controlling said switching means, said second
switching means, and said third switching means,
whereby:
the signal output of said dicriminating means respon-
sive to the output of said reference frequency sig-
nal generating means is applied through said third
switching means across said capacitor to ground
through said second switching means during a se-
lected period between said periodically recurring
periods; and
said signal from said signal means is connected
through said switching means to said discriminating
means and the responsive output of said discrimi-
nating means is connected through said third
switching means in series with said second capaci-
tor to said coupling means during said periodically
. recurring periods corresponding to a portion of
each retrace interval of said video signal.
6. An automatic frequency control as set forth in
claim 5 wherein said recurring periods correspond to
the “back. porch” portion of the retrace interval por-
tion of said video signal.
7. An automatic frequency control as set forth in
claim 6 further comprising fourth switching means con-
nected in circuit with said coupling means and said
switching control means includes means coupled to
said fourth switching means for operating said fourth
switching means for selectively coupling the said error
signal to the output side of said long time-constant ca-
pacitor only during a portion of each said “back porch”
portion of said video signal.
*****
65