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HEWLETT-PACKARD
JOURNAL
T E C H N I C A L I N F O R M A T I O N F R O M T H E - h p - L A B O R A T O R I E S
Vol.11 No. 8-10
JBLISHED BY THE HEWLETT-PACKARD COMPANY, 1501 PAGE MILL ROAD, PALO ALTO, CALIFORNIA APR.-JUNE, 1960
The Effect of jut-Circuit Non-Linearity
on the Amplitude Stability of RC Oscillators
Ed. Note: In the RC oscillator the amplitude which the oscillator has exhibited an exag control element is presented in the following
of oscillation is controlled by a thermally- gerated response to such disturbances (see article. We believe the analysis will be of
sensitive resistance in the form of a small Fig. 2 below). In a few cases actual envelope general interest because it takes into ac
tungsten lamp. This control element gives instability has been observed. Although this count the effect of the small non-linearity
the oscillator its characteristic constancy of effect was generally attributed to tube that exists in the amplifier portion of the
output level over a wide range of operating anomalies, since aging or replacing the oscillator circuit, where previous analyses
conditions. In practice, for example, the os tubes removed the effect, over a period of have assumed an ideally linear amplifier.
cillator's response is quite slight to such dis
time the impression began to form that a The analysis shows that, if the amplifier were
turbances as hum, microphonics, a change
more definite factor was involved. in fact ideally linear, the envelope response
with tuning in the impedance ratios in the
frequency-determining network, or a change The envelope response was then investi to amplitude disturbances would be some
in loading at the oscillator output. gated analytically by Bernard M. Oliver, JOO c/fa more than it is. It shows further that
At the same time, and in contrast to the -hp- vice-president for R & D, and his analy the amplifier must have a slight compression
great majority of cases, experience with the sis of the RC oscillator envelope response to for the oscillator to be practical. This agrees
RC oscillator has disclosed instances in amplitude disturbances introduced into the with observed results.
In the years since the introduction of the RC oscil yields results which are in close agreement with ob
lator, many analyses have been made of the power- served performance.
sensitive feedback-controlling mechanism that It is shown that amplifier non-linearity so slight as
regulates the level of oscillation. To the author's to cause insignificant waveform distortion has a pro
knowledge, these analyses have all assumed perfect found effect on envelope stability. In fact, were it not
linearity in the amplifier for this slight non-linearity, it would be virtually
SEE ALSO:
Using VLF Standard
circuit. Predicted perform impossible to build a simple lamp-stabilized RC os
Broadcasts p. 8 ance, however, does not al cillator with good envelope stability over a wide
De Gaulle visits -hp- p. 3
ways agree with experience. frequency range. This is borne out by the fact that oc
In particular, the envelope stability is usually much casionally oscillators show up in production which,
better (i.e., transient disturbances in oscillation am because of a fortuitous combination of tube charac-.
plitude die out much more rapidly) than the linear teristics, exhibit extremely low distortion. Invariably
theory predicts, especially for the higher oscillator these units give trouble with envelope "bounce", i.e.,
frequency ranges. The analysis presented here in they have a highly oscillatory response to envelope
cludes the effect of non-linearity in the amplifier, and disturbances. In some cases, actual instability occurs
Fig. 2. Oscillogram showing unusual squeg-
ging-type transient response produced in
Fig. 1. Basic circuit of RC oscillator. Constancy of RC oscillator by making linearity of ampli
output level is achieved by controlling amplitude of fier portion of circuit extremely high. Slight
oscillation with thermally-sensitive resistance Ri, a amplifier compression gives normal response
small tungsten lamp in negative feedback circuit. shown in Fig. 7(a).
P R I N T E D I N U . S . A . C O P Y R I G H T 1 9 6 0 H E W L E T T - P A C K A R D C O .
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