Document text
May 1965
Brief 65-10152
NASA Tech Briefs are issued by the Technology Utilization Division to summarize specific
technical innovations derived from the space program. Copies are available to the public from
the Clearinghouse for Federal Scientific and Technical Information, Springfield, Virginia, 22151.
Circuit Reduces Distortion of FM Modulator
I
The problem: To reduce second harmonic and in-
termodulation distortion of a voltage-variable capac-
itor used to modulate an oscillator.
The solution: A correction circuit designed to
improve the linearity of the voltage-variable capacitor
used to modulate a free-running oscillator.
How it's done: The circuit diagram shows the use
of the voltage-variable capacitor Cj, with the correc-
tion circuit enclosed in dashed lines. The modulating
signal input is phase-inverted and amplified (by Qi-
Q 2 ) and applied to the full-wave-rectifier diodes D (
i
and D 2 to generate the second harmonic without in-
troducing any of the fundamental frequency into the
correction circuit. Operation takes place in the square-
law portion of the rectifier-diode characteristic, so
that Dj and D 2 also perform a squaring function. The
output from the rectifier-squaring network is applied
to the potentiometer, R, which then provides a correc-
tion signal to the voltage-variable capacitor C 2 across
the tuned circuit of the oscillator. The correction signal
must have the proper polarity to reduce total tank-
circuit capacitance on both the positive and the nega-
tive peak swings of the modulating signal input. A
gain control at the rectifier output provides for adjust-
ment of the correction-signal level.
(continued overleaf)
Ttfis document was prepared under the sponsorship of the National
Aeronautics and Space Administration. Neither the United States Govern-
ment, nor NASA, nor any person acting on behalf of NASA: A. Makes
any warranty or representation, express or implied, with respect to the
accuracy, completeness, or usefulness of the information contained in
this document, or that the use of any information, apparatus, method,
or process disclosed in this document may not infringe privately-owned
rights; or B. Assumes any liabilities with respect to the use of, or for
damages resulting from the use of, any information, apparatus, method,
or process disclosed in this document.
Notes:
1. Although this circuit was designed to improve the
linearity of modulation of a free-running oscillator
where modulation is accomplished by use of a vol-
tage-variable capacitor, it may also be applied to
either the master oscillator in a radio transmitter
or a subcarrier oscillator in a telemetry system.
However, this improvement is applicable only to
audio frequency modulation and will not correct
for slowly varying dc inputs in some telemetry
systems.
2. Inquiries concerning this innovation may be di-
rected to:
Technology Utilization Officer
Goddard Space Flight Center
Greenbelt, Maryland, 20771
Reference: B65-10152
Patent status: NASA encourages commercial use
of this innovation. No patent action is contemplated.
Source: Radio Corporation of America,
under contract to Goddard Space Flight Center
(GSFC-257)
Brief 65-10152
Category No. 01