Circuit for generation and demodulation of quadrature double side band signals in frequency division multiplexing system is described. Circuit is designed to produce low distortion automatic phase control. Illustration of circuit and components is included

Hauge, G.

Pederson, C. W.

English

NASA Technical Reports Server

Input 
Signal
Output Signal 
Shifted 90° 
October 1972	 -	 B72-10682 
AA	 NASA TECH BRIEF 
Marshall Space Flight Center	 -4r 
NASA Tech Briefs announce now 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. 
Low Distortion Automatic Phase Control Circuit 
In the generation and demodulation of quardrature 
double side band signals in a frequency division multi-
plexing system, a 900 phase shifter is required. The 900 
phase shift must be independent of frequency over small 
frequency changes and must also be capable of handling 
large amplitude sine wave signals without distortion. In 
this automatic phase control circuit (see figure) the 
phase shifter uses an operational amplifier with external 
resistors and capacitors which control the gain and phase 
shift of the output voltage relative to the input voltage. 
The operational amplifier, in conjunction with R 1 and 
.
R2 forms an inverting amplifier with a gain approxi-
mately equal to the ratio of R2 /R 1 and a 180 degree
phase shift. An additional phase shift of 90 degrees 
results from the use of the second (positive input) of 
the two differential inputs of the operation amplifier. 
Signal applied to the positive input through a resistor-
capacitor network increases the overall phase shift by 
an additional 90 degrees. For R 2 R 1 the phase lag, 
0lag may be found from the expression: 
Olag_( 1 80-2 tan' .CtR3) 
Where C is the series-parallel combination of C 1 , C2, 
CR 1 and CR2 and w = 27ff where f is the modulated 
carrier frequency. Varactors CR 1 and CR2 are biased 
R2 
Ff4 
Automatic Phase Control Circuit
(continued overleaf) 
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Aeronautics and Space Administration. Neither the United States	 information contained in this document, or warrants that such use 
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in opposite directions and driven from inverted voltages 
provided by amplifier A2 such that the reverse bias on each 
varactor is equal to the other for any given control voltage. 
The capacitance across the varactor diode junction 
depends on the reverse bias voltage. The two varactors 
are operated on the same portion of their capacitance 
versus voltage curve. For matched curves, the capaci-
tance, and therefore phase lag, is independent of the 
input voltage swing. As a result, there is no distortion 
of the input signal either in phase or in amplitude such 
as would occur when a nonlinear element such as only 
one varicator or FET is used. 
It can be seen from the phase lag equation that 0 is 
a function of frequency as well as capacitance. There-
fore, to provide a constant phase over a small frequency 
range, a closed loop phase control system is required. 
The control loop consists of a phase detector and.asso-
ciated amplifiers. The input and output voltages of the 
phase shifter are compared in the phase detector with 
the resulting error voltage amplified by A 1 and A2 to 
control the bias applied to the varactors. This auto-
matically compensates for any phase variation caused by 
variations in input frequency or component changes. 
The output phase can be held to 0.1% of any given 
phase lag between 1800 and 3600
 with frequency changes 
of up to 10%. R4
 in the phase detector allows for a
small amount of phase adjustment for phase alignment 
of the system. This circuit has been operated with an 
input and output voltage of 6V p-p with harmonic 
distortion down 70 dB or better. 
Notes: 
No additional information exists on this innovation. 
Specific questions, however , may be directed to: 
Technology Utilization Officer 
Marshall Space Flight Center 
Code A&PS-TU 
Marshall Space Flight Center, Alabama 35812 
Reference: B72-10682 
Patent status: 
Inquiries concerning rights for the commercial use of 
this invention should be addressed to: 
Patent Counsel 
Marshall Space Flight Center 
Code A&PS-PAT 
Marshall Space Flight Center, Alabama 35812 
Source: G. Hauge and C. W. Pederson of 
Martin Marietta Corporation
under contract to

Marshall Space Flight Center

(MFS-21671) 
B72-10682	 Category 02


Low distortion automatic phase control circuit

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