Differential probability FM system measures deviation of a carrier modulated by a complex signal. The peak-to-peak amplitude is measured and related to the frequency shift of the carrier signal. The deviation is described in terms of a probability as well as a peak value

Daquin, A. F., Jr.

Haddican, J.

English

NASA Technical Reports Server

June 1967	 Brief 67-10213 
NASA TECH BRIEF	 1 
NASA Tech Briefs are issued to summarize specific innovations derived from the U.S. space program, 
to encourage their commercial application. Copies are available to the public at 15 cents each 
from the Clearinghouse for Federal Scientific and Technical Information, Springfield, Virginia 22151. 
______________	 •	 IiiTiI•i:i 
FM Carrier Deviation Measured by Differential Probability Method 
JLJI_J_1W 
FM Carrier	 Switch
 Comparator 
frM 
Detectorj	
,	
Kf(t) 
--	
Counter 
Clock 
Threshold Bias  
FThreshold
as I 
Reference
CW
Generator 
The problem: 
To devise a system that will accurately measure 
deviation, caused by adjacent channel interference, 
of an FM carrier modulated by a complex signal. 
The solution: 
A differential probability FM deviation measuring 
system in which the peak-to-peak amplitude, or a 
certain portion of the peak-to-peak amplitude, of a 
complex signal is measured and related to the fre-
quency shift of the carrier signal. With this system, 
the deviation is described in terms of a probability, 
as well as a peak value. The system utilizes commer-
cially available equipment consisting of a reference 
cw generator, an FM detector, a comparator circuit, 
a 1-MHz crystal controlled clock, an and gate, and 
a counter. 
How it's done: 
The FM carrier enters the FM detector through 
a coaxial switch. The intelligence signal, Kf(t) (where 
K=gain of the FM detector and f(t) complex modu-
lation data), is extracted in the FM detector and
passed to the comparator, a low drift, high gain, dc 
differential amplifier, where it is compared with a 
threshold bias. The output of the comparator is a 
"one", if Kf(t) is greater than the threshold bias. The 
comparator output and the high frequency clock signal 
(250 KHz counted down from the crystal controlled 
1-MHz clock) are fed to an and gate. A coincidence 
of a "one" Kf(t) output and a clock signal gates 
the counter, which sums the enabled clock pulses 
over a given time interval. The value of the threshold 
bias, which is equal to the peak value of Kf(t) and 
prevents the clock from being enabled, corresponds 
to the peak deviation of the modulated carrier. The 
frequency of this peak deviation is determined by 
closing the coaxial switch to the reference cw genera-
tor and tuning to the frequency whose demodulated 
output just exceeds the threshold bias. The system 
clock is thereby enabled 100 percent of the time; 
thus the reference frequency corresponds to the peak 
deviation frequency. 
The system can also determine the probability of 
exceeding a given deviation limit. Consider an FM 
(continued overleaf( 
is
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communication system limited to a peak deviation 
frequency with an allowable deviation from the 
limit for a given fraction of the time. The probability 
that this deviation will exceed the limit can be deter-
mined by tuning the reference generator to that value 
of the peak deviation frequency for which the 
threshold bias of the comparator just prevents the 
system clock from being enabled. When the reference 
generator signal is removed and the intelligence signal 
is compared to the threshold bias, the system clock 
will be enabled for only that fraction of the time the 
peak deviation is exceeded. The resultant clock count 
divided by the maximum possible clock count (i.e., 
for the clock enabled 100 percent of the time) would 
be the fraction of the time that the deviation limit 
is exceeded. This fraction is numerically equal to the 
probability of exceeding the deviation limit. 
Notes: 
1. Three types of implementation errors are asso-
ciated with this system: clock resolution error, 
comparator hysteresis error, and reference fre-
quency error. The clock resolution error and the 
comparator hysteresis error effectively cancel each 
other, but the reference frequency error is signifi-
cant. Tests made on the deviation measuring sys-
tem to verify its feasibility showed high accuracy 
on an individual component basis. The compara-
tor circuit was found to have an error of less than 
0.1 percent, and the system clock stability was 
better than 0.005 percent. Thus, for a large clock 
frequency to maximum modulating frequency ratio 
f,./f, the measurement accuracy depends primar-
ily on the generator and counter used in the 
system.
2. Inquiries concerning this innovation may be di-
rected to: 
Technology Utilization Officer 
Marshall Space Flight Center 
Huntsville, Alabama 35812 
Reference: B67-10213 
Patent status: 
No patent action is contemplated by NASA. 
Source: J. Haddican and A. F. d'Aquin, Jr. 
of The Boeing Company 
under contract to 
Marshall Space Flight Center 
(M-FS-2 166)
. 
Brief 67-10213	 Category 01


FM carrier deviation measured by differential probability method

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