Deep space frequency modulation system permits transmission of data where the signal deviation is greater than 1/2 the predetection bandwidth. It provides satisfactory performance at great distances or with low signal levels

Doland, G. D.

English

NASA Technical Reports Server

August 1968	 Brief 68-10289 
NASA TECH BRIEF	
/& 
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., 
Deep Space FM System, A Concept 
Info in	 Info in 
Subcarrier	 Frequency	 FM	 RF 
VCO	 Modulator	 Transmitter	 Path 
Low Pass I	 I	 FM(1) 
Filter (1)
	
(4	 I Demodulator 
iro 
Info Out
FM (2)
Demodulator 
11 
Band pass 
Filter (3)
Bandpass 1	 1	 FM Rec. 
Filter(1)	 RF&IF 
Bandpass 
Filter (2)
	
12	 13 
Subcarrier	 Low Pass 
	
Demodulator	 Filter (2) 
Info Out 
In the diagram, item I is a frequency modulator 
which is modulated by a baseband information signal. 
It is also modulated by a subcarrier from the sub-
carrier's voltage controlled oscillator (VCO), shown 
as item 2. This VCO is also modulated by an infor-
mation signal. The frequency modulated signal from 
item I is transmitted by the fm transmitter, item 3. 
The rf path, item 4, consists of the transmitting 
antenna cable, transmitting antenna, free space, 
receiving antenna, and receiving antenna cable. This 
portion of the system is quite conventional. However, 
the information bandwidth and subcarrier frequency 
must be selected so as to provide frequency separation 
at the receiving terminal.
Item 5 is a portion of the fm receiver consisting of 
the rf and i.f. sections. The rf and i.f. bandwidth must 
be sufficiently wide to pass the normal spectrum for 
the signal transmitted. The fm demodulators are not 
included in item 5. Item 6 is a bandpass filter, #1. 
The predetection bandwidth for most systems is 
taken to be equal to twice the sum of the deviation 
and the highest modulating frequency. For some 
systems, the predetection bandwidth may be reduced 
to equal twice the highest modulating frequency or 
twice the frequency deviation whichever is larger. In 
this system the bandpass filter #1, is made substan-
tially less. It is less than twice the modulating fre-
quency and is also less than twice the deviation. For 
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example, a 1 MHz signal may be used for the sub-
carrier frequency and the total deviation may also be 
as high as 1 MHz. While the bandpass filter #1, item 
6, may be only 1 MHz wide, conventional design 
practice would require the filter to be 4 MHz. With a 
bandwidth only 1/4 normal bandwidth, the fm thresh-
old will be 6 db lower, compared to conventional 
equipment. This will double the range and permit the 
operation with a signal 6 db weaker than normal. 
Since the signal is deviated beyond the passband 
and also has frequency components well outside the 
bandlimits, this filter, item 6, will cause a loss in 
signal components. Specifically, the subcarrier compo-
nents will be lost. The amount of energy outside the 
passband will be dependent upon the deviation of this 
subcarrier. If the deviation is ±600 kHz for example, 
there is only about I db energy outside of the pass-
band as the deviation ratio is 0.6. To maintain the 6 db 
improvement in threshold, the noise bandwidth of the 
bandpass filter #1, item 6, must be reduced to 800 
kHz. FM demodulator #1, item 7, is used to demodu-
late the baseband signal. The spectrum of the trans-
mitted signal produced by the baseband modulation, 
with the subcarrier off, must be confined to the 800 
kHz bandwidth. For the conditions specified in this 
example, the baseband signal will be demodulated by 
fm demodulator #1, item 7. The low pass filter #1, is 
used to limit the post detection bandwidth to that 
necessary for the information baseband being trans-
mitted. 
Since the information carried by the subcarrier is 
lost in the bandpass filter #1, it is necessary to provide 
another channel for this information. Bandpass filter 
#2 should be 4 MHz using conventional design 
practice but may be reduced some. The flat portion of 
the passband is required to be at least 2 MHz (for the
example) so that the first order sidebands of the sub-
carrier are not suppressed. It also must be wider to 
pass the modulation components of the subcarrier. 
When fm demodulator #1 is operated at the fm 
threshold, fm demodulator #2, item 10, will be operat-
ing below threshold. Consequently, noise spikes will 
be generated at fm demodulator #2, item 10. The out-
put of fm demodulator #2 is fed to bandpass filter #3, 
item II. Bandpass filter #3 is centered at the subcarrier 
frequency components of the modulated subcarrier. 
Therefore, it is considerably narrower than bandpass 
filter #2. Bandwidth reduction improves the signal-
to-noise ratio so that the signal from bandpass filter 
#3, item 11, is above the threshold for the subcarrier 
demodulator, item 12. As long as the signal-to-noise 
ratio is adequate at the subcarrier demodulator, the 
information carried by the subcarrier can be satis-
factorily recovered. The low pass filter #2, item 13, 
limits the output bandwidth to that required for the 
information transmitted using the subcarrier. 
Notes: 
I. This system would permit transmission of data 
where the deviation is greater than 1/2 the pre-
detection bandwidth. It would provide satisfactory 
performance at greater distances or with lower 
signal levels than present systems. 
2. This development is in conceptual stage only, and, 
as of date of publication of this Tech Brief, neither 
a model nor prototype has been constructed. 
Patent status: 
No patent action is contemplated by NASA. 
Source: G. D. Doland 
of Lockheed Electronics Company 
under contract to 
Manned Spacecraft Center 
(MSC-1 1825) 
Brief 68-10289	 Category 01


Deep space FM system, a concept

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