Elenco Electronics EP-130, Manual

Page: 141 / 160

In this project, you will build and study a low-
distortion sinewave oscillator. Build this project after
you have built and studied the previous project
because this one has no transformer; transformers
are likely to cause distortion because of their non-
linear characteristics.
As in the previous project, you should listen to the
tone of this oscillator and adjust the control for the
clearest-sounding single tone (the one with the least
distortion). Again, start with the control near
maximum. The frequency of operation is about
300Hz, at the minimum distortion setting of the
control.
This circuit is called an RC phase shift oscillator and
is considered a basic sinewave oscillator. Oscillations
occur because of the positive feedback of the
signals. The resistors (R) and capacitors (C) make up
the path for the circuits to the transistor base. Every
time the signals pass the RC circuits, a slight time lag
occurs. In other words, the rise and fall of the wave
(the phase) shifts slightly. That’s why we call it phase
shift. After the signal has traveled through the circuit,
the phase shifts 180 degrees. When the collector
voltage rises, this rise is fed back to the collector with
the phase shifted. When the base voltage rises, the
collector voltage falls. This repeating cycle causes
the transistor to oscillate.
When you change the control setting, the frequency
changes. This is because the degree of phase shift
changes. The tonal quality also changes. Set the
control to the point where you can hear the purest
tone. At this point, a clear sinewave is generated.
Notes:
EXPERIMENT #119: LOW DISTORTION SINEWAVE OSCILLATOR
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Wiring Sequence:
o
124-27-48-82-80-EARPHONE
o
47-105-93-77-EARPHONE
o
81-109-108-28
o
94-110-46
o
78-138
o
79-106-107
o
119-137
o
121-122
Schematic