Feedback loop extracts demodulated reference signals from IF input and feeds signal back to demodulator. Since reference signal is extracted directly from carrier, no separate reference need be transmitted. Circuit obtains coherent carrier from balanced or unbalanced four-phase signal of varying characteristics

Allen, J. R.

Allen, R. W.

Jackson, A.

Solomon, K.

English

NASA Technical Reports Server

To Delay
And comparator 
Loop 
Filter
To Delay And 
Comparator 
Loop 
Filter 
FILTER
Ovipet To
ls&D 
Demod.
Video Ampi 
	
IF	 Power 
	
lvpvt	 Divider	 Outpst To 
158.0 
Demod.
video A,npl 
rrdj 
DEMODULATOR
900	 1800 
Hybrid	 comb 
Mill ii 
STRIPPER 
 
Op=	 vcxo 
March 1973	 B73-10094 
NASA TECH BRIEF 
Lyndon B. Johnson Space Center 
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 National Technical Information 
Service, Springfield, Virginia 22151. Requests for individual copies or questions relating to the Tech Brief program may 
be directed to the Technology Utilization Office, NASA, Code KT, Washington, D.C. 20546. 
Carrier Extraction Circuit 
S
The problem: 
In a phase shift keyed (PSK) system, a carrier 
reference is needed for demodulation. Frequently, a 
separate reference signal is transmitted, but this requires 
additional transmission power. 
The solution: 
From the IF input, a feedback loop extracts a 
demodulated reference signal that is fed back to the 
demodulator. Since the reference signal is extracted 
directly from the carrier, no separate reference need be 
transmitted. This circuit can obtain a coherent carrier 
from a balanced or unbalanced four-phase signal of 
widely varying characteristics.
How it's done: 
The flow of the principal signals in the feedback 
loop is shown in the accompanying block diagram. The 
loop consists of four main parts: (1) a demodulator 
circuit containing two separate coherent demodulators, 
one producing the in-phase component and the other the 
quadrature, (2) a filter which removes the high-frequency 
noise component, (3) a modulation stripper, where each 
filtered data stream separately remodulates the incoming 
quadriphase signal and is recombined to produce an 
unmodulated but noisy carrier, and (4) a phase locked 
loop which cleans up the unmodulated carrier for input 
back into the modulator. 
The Demodulator. The incoming modulated IF signal 
first enters the feedback loop through the power divider 
where it is split into four equal parts. Two go to the 
O
PHASE LOCKED LOOP 
Channel 
Reverse 
Control
Carrier Extraction Loop Functional
Block Diagram
(continued overleaf) 
This documentwas prepared under the sponsorship of the National	 Government assumes any liability resulting from the use of the 
Aeronautics and Space Administration. Neither the United States 	 information contained in this document, or warrants that such use 
Government nor any person acting on behalf of the United States 	 will be free from privately owned rights.
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demodulators, and two to the modulator stripper. One 
of the demodulators uses the in-phase component of the 
reference, and the other uses the quadrature component. 
The demodulated signals are provided to integrate, 
sample, and dump (IS&D) circuits for detection of the 
binary data streams and are amplified in parallel and 
fed into the filter circuit. 
Filter. The filter circuit employs plug-in loop filters 
from which the signals are applied to comparators to 
obtain a binary data output for each channel. 
Modulation Stripper. The data now go to mixers in 
the modulation stripper where they are combined with 
the original IF signal from the power divider. The 
resulting signal is partially demodulated. Upon recom-
bining the two channels in the proper phase and ampli-
tude relationship, the modulation components cancel, 
and a carrier component in phase with one of the two 
original components is generated. 
Phase-Locked Loop. The extracted carrier from the 
modulation stripper is compared to the voltage-con-
trolled crystal oscillator (VCXO) output in the mixer. 
The error signal is filtered and applied to the VCXO 
voltage control. The VCXO output is split, one part
going back to the mixer, and the other becoming the 
coherent reference signal input to the demodulator. 
It should be noted that there are two possible ways 
to combine the signal making up the modulator stripper 
output. The correct combination is selected and syn-
chronized with the selection of channels in the de-
modulator by the channel reverse control. 
In this system, both channels can be operated at 
different power levels, and the channels may be at 
different bit rates. The circuit has been successfully 
tested and can handle data rates from I KBPS to 30 
MBPS. 
Patent status: 
Title to this invention has been waived under the 
provisions of the National Aeronautics and Space Act 
[42 U.S.C. 2457(1)] , to the RCA Corp./Government and 
Commercial Systems, Camden, New Jersey 08102. 
Source: K. Solomon, J. R. Allen,

A. Jackson, and R. W. Allen of
RCA Corp.
under contract to

Johnson Space Center
(MSC-14262)
. 
. 
B73-10094	 Category 02, 09


Carrier extraction circuit

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