Document text
June 1970
Brief 70-10179
NASA TECH BRIEF
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 Clearinghouse for Federal Scientific and Technical
Information, Springfield, Virginia 22151. Requests for individual copies or questions relating to the Tech Brief program may
be directed to the Technology Utilization Division, NASA, CodeUT, Washington, D.C. 20546.
A 225 MHz FM Oscillator with Response to 10 MHz
A frequency-modulated transistor oscillator has approximately 1.2 MHz/V was achieved with excel-
been designed for application in wideband television lent linearity, and the oscillator exhibited very little
transmitters. Tests on an experimental 225-MHz FM spurious amplitude modulation.
oscillator circuit showed that the design met all sped- The design effort included a theoretical analysis
fication requirements. Frequency stability was within of the oscillator. A mathematical model of the circuit
1.0 percent over the temperature range of -25° to adequately predicted the oscillation frequency, and
85°C. Output power of approximately 120 mW into an iterative computer program yielded results that
a 50-ohm resistive load was easily obtained. The base- agreed closely with the slope of the deviation charac-
band modulation response was flat within ±0.5 dB teristic measured experimentally. This analytical
from dc to 10 MHz. A modulation constant of technique could be employed to aid in the design of
(continued overleaf)
This document was prepared under the sponsorship of the National
Aeronautics and Space Administration. Neither the United States
Government nor any person acting on behalf of the United States
Government assumes any liability resulting from the use of the
information contained in this document, or warrants that such use
will be free from privately owned rights.
other oscillators and to analyze existing circuits.
The technique might possibly be applied to existing
oscillators in order to eliminate frequency drift due
to temperature changes.
The oscillator circuit, represented in the schematic,
is of the LC Colpitts type. This tank circuit configu-
ration will provide near-sinusoidal output waveforms
even when excited by nonsinusoidal currents. The
circuit also provides a high rate of phase change at
the resonant frequency, which has the effect of pro-
ducing good frequency stability.
A common-collector configuration for the transis-
tor (Ql) is used primarily because the collector is
internally connected to the case. Thus, the combined
effects of case capacitance to ground and heat sink
capacitance to ground are an aid, rather than a hind-
rance, for effective rf grounding of the collector.
The varactor (Q2) is located in series with the
coil because unusual modulation response was often
observed when the varactor was placed in parallel
with the tank circuit. At high frequencies there is
sufficient internal inductance and lead inductance
to cause the varactor to enter the series resonant
mode. If this condition is allowed to exist, a drastic
reduction in Q will occur producing unpredictable
modulation response.
The problem of injecting the modulation signal and
the bias voltage to the varactor was solved by the
use of a series trap tuned to 225 MHz and a blocking
capacitor Cl. This trap, composed of L2,R5, and C4,
presents a low impedance to modulation frequencies
and a high impedance to 225 MHz. The design
premise for such a trap in a modulated oscillator was
that the Q of the trap should have a low value. This
restriction was made to avoid undue attenuation of
the resulting sidebands back through the trap and
the video amplifier to ground.
The dc biasing network is of a conventional nature.
The transistor was biased at approximately 40 mA
collector current with at approximately 15 V.
This arrangement will force the transistor to operate
at a point which will tend to produce small-signal
class-A oscillation.
Note:
Requests for further information may be directed to:
Technology Utilization Officer
Marshall Space Flight Center
Huntsville, Alabama 35812
Reference: TSP70-10179
Patent status:
No patent action is contemplated by NASA.
Source: Auburn University
under contract to
Marshall Space Flight Center
(MFS-14977)
Brief 70-10179
Category 01