Sinusoidal oscillator has a frequency determined by resistance-capacitance /RC/ values of two charge control devices and a constant-amplitude voltage independent of frequency and RC values. RC elements provide either voltage-control, resistance-control, or capacitance-control of the frequency

Kerwin, W. J.

Westbrook, R. M.

English

NASA Technical Reports Server

September 1970	 Brief 70-10338 
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be directed to the Technology Utilization Division, NASA, Code UT, Washington, D.C. 20546. 
Constant-Amplitude RC Oscillator 
VB 
The problem: 
To design a simple voltage-controlled resistance-
capacitance (RC) oscillator to be used in micropower 
frequency-division telemetry. Nonsinusoidal sub-
carrier oscillators require low-pass output filters prior 
to multiplexing. To vary the frequency of an RC Wien-
type oscillator without waveform distortion, it is 
necessary to vary both the control resistors (or capaci-
tors) and to make both elements equal with the closest 
possible tracking of each other. An automatic gain 
control arrangement must also be added to insure 
minimum distortion throughout the tunable frequency 
range. RC phase-shift oscillators usually have either 
three resistors or three capacitors which must be 
ganged to provide a variable frequency operation 
and still maintain a linear relation between the fre-
quency and resistance (or capacitance).
(continued orerleal) 
low
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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 ownedrights.
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The solution: 
A sinusoidal oscillator is designed with a frequency 
determined by RC values of two charge control de-
vices and a constant-amplitude voltage independent 
of frequency and RC values. RC elements can be 
selected to provide either voltage-control, resistance-
control, or capacitance-control of the frequency. 
How it's done: 
A basic oscillator circuit is designed with two cas-
caded phase-varying all-pass stages. As shown in the 
figure, bias resistors R 3 , R 4, R 7 and R 8 are chosen in 
conjunction with load resistors R 5 , R 6 , R 9 and Rio to 
operate transistors Q i and Q2 with input-to-collector 
and input-to-emitter voltage gains of unity. Resistor 
R i and capacitor C i form an all-pass network. Resis-
toy R 2 and capacitor C 2 form the second all-pass 
network. The two states are shown ac-coupled by 
capacitors C 3 and C4 (dc interconnection is also 
possible). The transfer function of the first stage, 
given by:
=	 R C w 
V, 
demonstrates that variance in the input frequency 
changes the phase angle but not amplitude of V2. The 
first stage provides a positive phase shift from 0 to 180 
degrees, depending on frequency and values of Ri 
and Ci. The second stage has the same capability but 
has a phase shift of opposite polarity. 	 -	 -
Therefore, if the base-to-collector and the base-to-
emitter transistor voltage gains are unity for each 
stage, and if coupling capacitors C3 and C4 have 
negligible reactance compared to capacitors Ci and 
C2, the circuit will oscillate at a frequency such that 
the sum of the two phase shifts is equal to zero. The 
oscillation frequency is 
fo= 2R1R2C1C2 
Choosing R 1 = R 2 = R, and C 1 = C = C, the oscil-
lazation frequency will be f 0 = 27r RC. 
Note: 
Additional documentation is available which pre-
sents an example of a practical voltage-controlled 
micropower sub-carrier oscillator and more details of 
the basic oscillator. This documentation may be 
obtained from: 
Technology Utilization Officer 
Ames Research Center 
Moffett Field, California 94035 
Reference: B70-10338 
Patent status: 
Inquiries about obtaining rights for the commercial 
use of this invention may be made to NASA., Code 
GP, Washington, D.C. 20546. 
Source: William J. Kerwin and 
Richard M. Westbrook of 
-	 Ames Research Center 
-	 (ARC-10262). 
Brief 70-10338	 Category 01


Constant-amplitude RC oscillator

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