[Table of Contents]People old and young enjoy waxing
nostalgic about and learning some of the history of early electronics. Popular Electronics was published from October
1954 through April 1985. All copyrights (if any) are hereby
acknowledged. |
There
is an old adage that goes thusly: "If you want to build an oscillator,
design an amplifier. If you want to build an amplifier, design an oscillator."
Its basis is the difficulty that can be experienced in obtaining the
right combination of feedback phase and amplitude. Of course experience,
use of simulators, and careful circuit construction minimize the opportunity
for validating that saying. The basic requirement for an oscillator
is feedback from the output to the input that is in-phase and great
enough in amplitude to maintain, via the amplifier's gain factor, a
constant output level. Tuned L-C (inductor-capacitor) tank circuits
are often used as simple frequency-determining elements because of their
combined resonance characteristics. Phase shift oscillators are a type
of oscillator that can be built without inductors. Instead, they rely
on the phase shift of a series of capacitors and resistors to obtain
the 180-degree phase shift needed from output to the input to sustain
oscillations. Frequency control is not typically as stable as with a
tank circuit or a crystal, especially as temperatures change. This type
of oscillator definitely feeds the aforementioned adage more so than
those with circuits exhibiting high Q factors. This article from Popular
Electronics covers some of the fundamentals (pun intended).
After Class, Working with Phase-Shift Oscillators
By Harvey Pollee
Special
Information on Radio, TV, Radar and Nucleonics
Most oscillators that
utilize resistance-capacitance tuning generate triangular, trapezoidal,
or square waves. When one thinks of the generation of sine waves, he
usually visualizes an inductance-capacitance tuned type such as the
Hartley or Colpitts circuit. There is a class of RC oscillators, however,
that is capable of yielding excellently formed sine waves and, because
of the absence of coils or transformers, these oscillators are very
attractive to the experimenter.
Of the three common circuits
in the latter group (the Wien bridge, the bridged-T, and the phase-shift
oscillator), the phase-shift type is the simplest to build, contains
the fewest components, and is very easy to get working.
Basic Oscillator.
The fundamental circuit of
the phase-shift oscillator is given in Fig. 1. Like all oscillators,
action is initiated by some random fluctuation in the tube current or
voltage, such as is due to thermal or shot effect.
To explain
the operation, let us assume that the grid of the triode becomes very
slightly positive for an instant. When this happens, the plate current
increases slightly, causing the voltage drop across plate-load R_{L}
to increase somewhat above its standby value. The extent of this increase
depends upon the voltage gain of the tube; the greater the gain, the
larger the change in voltage drop across R_{L}.
Fig. 1. Theoretical phase-shift oscillator circuit. See text.
Practical circuits are shown in Figs. 2 and 3.
Fig. 2. Pentode phase-shift oscillator. Capacitors labeled
"C" have same value; resistors labeled "R" are equal in resistance.
Refer to Fig. 4 for "C" and "R" values for given frequencies.
Fig. 3. Dual-triode phase-shift oscillator. All "C's" are
equal and all "R's" are equal. The nomogram will help you choose
values for given frequencies.
A voltage drop of this nature causes the plate voltage of the tube to
go down, thus making the plate negative-going. Since a positive-going
grid has caused a negative-going plate, we can say that the "signal"
on the plate is out of phase with the signal on the grid by 180 degrees.
The plate variation is now fed back to the grid through three RC
groups: C1-R1, C2-R2, and C3-R3. Each group can produce a voltage phase
shift of its own. Considering only the first group (C1-R1), the voltage
appearing across R1 will lead the signal voltage pulse from the plate
by an amount determined by the ratio of the capacitive reactance (Xc)
of C1 and the resistance (R) of R1. Capacitive reactance depends on
frequency as well as on capacitance, so that there must exist some frequency
for which the phase shift for C1-R1 will be exactly 60°.
Fig. 4. Nomogram for obtaining required
component values.
To determine either "C," "R," or "f" if the other
two values
are known, lay straightedge to intersect vertical axis
at known figures and read unknown figure from the remaining axis.
Now the voltage that appears across R1 is applied across the C2-R2
group. Assuming equal capacitors and resistors throughout the circuit,
then the phase shift across C2-R2 will also be 60° for this special
frequency, making a total phase shift of 120°.
Finally, a third
60° phase shift across the last group (C3-R3) results in an overall
voltage change of 180° from the time the signal leaves the plate to
the time it returns to the grid. Adding the normal triode phase change
of 180° described above to the C-R phase shift of 180° gives us a total
inversion of 360° between the initial voltage fluctuation and the amplified
pulse that returns to the grid.
This, of course, is exactly what
is needed for sustained oscillation - feedback in phase with initial
signal, or positive feedback - so that a sine-wave voltage appears between
the plate of the triode and B-. This voltage may be taken from the plate
through a capacitor (C4) as the oscillator output.
Phase-Shift
Frequencies. The frequency of the output voltage is automatically "selected"
by the oscillator circuit to conform with the required 60° phase shifts
just discussed. This means, of course, that control of frequency is
obtainable by varying either the resistances or the capacitances.
In practice, anyone of the resistors may be a potentiometer to provide
a relatively narrow range of control. Frequency variation over a substantially
wider range may be realized by varying all three resistors simultaneously;
a three-gang potentiometer is ideal for this purpose.
The versatility
of a well-designed phase-shift oscillator is evident when we consider
that it can be constructed for frequencies as low as one cycle per minute
and as high as 100,000 cycles per second. Phase-shift oscillators can't
be beaten for audio testing, code practice, gain control (as in guitar
vibrato amplifiers), or for any other application requiring a stable,
reliable, pure sinusoidal output.
Practical Circuits. It can
be shown mathematically that a minimum voltage gain of 29 is necessary
to provide satisfactory performance at a single frequency. To insure
strong oscillation over a range of frequencies, the gain must be somewhat
higher than this. Hence, a practical phase-shift oscillator requires
either a high-gain pentode or two triodes in cascade for sure-fire operation.
An example of a pentode oscillator is shown in Fig. 2, and a dual-triode
type is shown in Fig. 3. In the latter circuit, the feedback voltage
for sustaining oscillation is taken from the cathode of the second triode.
Since there is zero phase shift between the grid input and cathode output
voltage of a vacuum tube, the second triode does not introduce any complications
when used this way. Instead, it provides a low-impedance source for
the feedback voltage and prevents the output load (headphones, speaker,
etc.) from causing oscillator instability due to loading effects.
The nomogram given in Fig. 4 will provide you with the required
R and C values for any frequency between 5 cps and 100,000 cps. Merely
select a value for C (all three capacitors are equal), then lay a straight-edge
from this value of C through the desired frequency. The intersection
of the edge with the R-axis on the nomograph tells you the value of
all three phase-shifting resistors. The same procedure is used for finding
f if R and C are known, or finding C if R and f are known.
Posted