Graphical method determines the rms threshold point, saturation point, and operating points for an FM receiver utilizing various modulation indices and degrees of submodulation

Merz, K.

English

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June 1970	 Brief 70-10119 
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Graphical Method to Predict the Dynamic Response of FM Receivers 
RF Generator	 Precision	 FM	 Wave 
HP 202 J	 RF Attenuator	 Receiver	 Analyzer 
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: TSP70-101 19
(continued overleaf) 
low
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https://ntrs.nasa.gov/search.jsp?R=19700000115 2020-03-17T00:25:59+00:00Z
Patent status: 
No patent action is contemplated by NASA. 
Source: Karl Merz of 
The Boeing Company 
under contract to 
Kennedy Space Center 
(KSC- 10111)
S 
Brief 70-10119 Category 01


Graphical method to predict the dynamic response of FM receivers

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