Document text
June 1970
Brief 70-10119
NASA TECH BRIEF
m
NASA Tech Briefs announce new technology derived from the U.S. space program. They are issued to encourage commercial
application. Tech Briefs are available on a subscription basis from the Clearinghouse for Federal Scientific and Technical
Information, Springfield, Virginia 22151. Requests for individual copies or questions relating to the Tech Brief program may
be directed to the Technology U tilization Division, NASA, Code UT, Washington, D.C. 20546.
Graphical Method to Predict the Dynamic Response of FM Receivers
Equipment Test Setup
A technique has been devised to determine graph-
ically the rms threshold point, saturation point, and
operating points for an FM receiver utilizing various
modulation indices and degrees of submodulation.
The receiver operating points for various signals
employing different modulation indices and types of
submodulation are normally specified by the signal-
to-noise ratio (S/N) for efficient receiver operation.
The S/N of an FM receiver is a function of rf input,
noise figure, and bandwidth. Measurements for a
generalized noise characteristic graph can be accom-
plished by connecting an rf signal generator to the FM
receiver under test. The FM receiver video output is
then measured by a wave analyzer (tunable voltmeter)
using a suitable bandpass. Selection of a suitable
bandpass requires a close examination of the noise at
the particular frequency utilized, to prevent erroneous
measures of spectral noise density and eliminate ex-
ternal unrelated sources. The signal generator is set
to the carrier frequency and not modulated, since it
is desired to measure the noise produced in an FM
receiver as a function of rf input level. Measurements
of the output carrier-to-noise ratio in db, for various
input values in dbm, are obtained and plotted.
With no rf input applied, the noise amplitude at the
video output is maximum. When the rf input power
to the receiver is slowly increased, video noise magni-
tude first remains constant and then starts to decrease.
When the receiver noise is greater than the rf input,
the receiver cannot distinguish rf input from receiver
noise. Further increases in rf input to the receiver
cause the magnitude of the video noise to decrease
rapidly, until a point is reached when the rate of noise
decrease slows down. This point is called the thresh-
old point to designate the level where the rf input
magnitude is comparable to the receiver noise magni-
tude, and receiver noise-supression begins.
As the rf input is increased beyond the threshold
value, change in video noise in db becomes a linear
function of rf input in dbm. This linear relationship
continues until saturation is reached. Beyond this
point, the output noise shows no further decrease for
any additional increase in rf input. This saturation
point can be graphically determined as the first in-
crement of rf input for which the output video noise
remains constant. When data concerning a specific
modulating system is desired, the receiver rms video
output of the modulating system is measured within
the same bandwidth in which noise is measured, and
then plotted.
Note:
Requests for further information may be directed to:
Technology Utilization Officer
Kennedy Space Center
Kennedy Space Center, Florida 32899
Reference: TSP70-10119
(continued overleaf)
This document was prepared under the sponsorship of the National
Aeronautics and Space Administration. Neither the United States
Government nor any person acting on behalf of the United States
Government assumes any liability resulting from the use of the
information contained in this document, or warrants that such use
will be free from privately owned rights.
Patent status:
No patent action is contemplated by NASA.
Source: KarlMerzof
The Boeing Company
under contract to
Kennedy Space Center
(KSC-101 1 1)
Brief 70-10119
Category 01