Document text
January 1965
Brief 65- 10001
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 Converts AM Signals to FM for Magnetic Recording
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The problem: To find an improved method of con-
verting AM signals to FM for magnetic recording. The
new circuit should be less complex and more linear
and reliable than existing methods.
The solution: A relaxation-type voltage-controlled
oscillator (VCO) that produces a triangular output
waveform at a frequency that is proportional (either
directly or inversely) to the voltage of the varying AM
input signal. A conventional multivibrator converts
this triangular output waveform to a square-wave
input to the recording device.
How it's done: Oscillation is produced in the cir-
cuit by the charging and discharging of the capacitor
Cj. Its rate of charge, or frequency of oscillation, is
controlled by current flow through Qj and Q 2 . The
This 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
Output Waveform
current flowing in the collector circuit of is pro-
portional to the varying voltage (the AM input signal)
across the base-emitter junction. Adjustment of R {
controls the amount of deviation from its center fre-
quency of the FM signal for a fixed input signal. The
current flowing in the collector circuit of Q 2 provides
a constant current source in addition to the varying
source from Qj. Varying R 2 adjusts the current
through Q 2 , thus controlling the center frequency of
the VCO. CRj is a tunnel diode used to perform cur-
rent sensing and switching. Q 4 , Q 5 , arid R 4 discharge
C { on signal from CRj. Q 3 , Q 6 , and R 3 form an isola-
tion amplifier to prevent the sensing and switching cir-
cuit from loading Cj. As Cj charges, the voltage
across it increases and appears across the series con-
nection of the emitter-base junctions of Q 3 and Q 6 ,
(continued overleaf)
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.
parallel resistors R 5 and R 6 , R 7 , and CRj. Q 5 is held
at cutoff by voltage divider CR p Q 6 , R 7 , R 5 , and R 6 .
Direct coupling of Q 5 to Q 4 prevents Q 4 from dis-
charging Cj. As the voltage across CR { exceeds its
peak point, CR { switches to its high-resistance State
causing current flow in the base-emitter junction of Q 5
and a corresponding increase in current flow in the
collector circuits of Q 5 and Q 4 . The drop in imped-
ance at the collector of Q 4 discharges C { . When the
voltage across Cj drops sufficiently, CRj switches to
its low-resistance state and Q 4 is cut off. The triangu-
lar output waveform appears at the emitter of Q 6 .
Notes:
1 . This circuitry should be of interest to designers and
manufacturers of radar, telemetry, and test equip-
ment.
2. Inquiries concerning this innovation may be di-
rected to:
Technology Utilization Officer
Goddard Space Flight Center
Greenbelt, Maryland, 20771
Reference: B65- 10001
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-227)
Brief 65-10001
Category No. 01