Manual

-150-
T
his oscillator circuit produces a sine wave signal. A
sine wave (or sinusoid) is a wave of pure single-
f
requency tone. As an example, a 400Hz sine wave
is a wave that oscillates 400 cycles in one second
and contains no other frequency contents. Non-sine
waves (such as square wave or triangular wave
signals) have harmonics - waves with frequencies
that are multiples of the single-frequency
fundamental wave. A non-sine 400Hz wave can
include the 400Hz wave (its fundamental wave)
along with an 800Hz wave (its 2nd harmonic wave)
and a 1200Hz wave (its third harmonic wave). A
square wave signal contains many harmonics.
Experienced technicians can test a circuit by putting
a sine wave into it and listening to its output - and
you can too. If you put in a sine wave, and
something else comes out, the undesired harmonic
frequencies must have been generated somewhere
in the circuit.
The key parts of this circuit that produce a 400Hz
sine are:
• A 0.1mF capacitor connected across terminals
3 and 5 of the transformer. This forms a tank
circuit that resonates at about 600Hz.
• A 470kW resistor to turn on the base of the
transistor only a little.
• An adjustable feedback circuit that includes
the control and the 0.05mF capacitor.
• A 100W resistor connected to the emitter. This
helps to stabilize the circuit and keep the
sound from being distorted.
Connect the earphone to terminals 1 and 2 of the
transformer. Start with the control on maximum (10
on the dial) and slowly decrease the control setting
while listening to the tone quality of the output.
Before the oscillations stop, you will reach a point
where you hear only one tone. This last clear-
sounding tone is the sine wave. Repeat these
control adjustments until you have no trouble
distinguishing between a sine wave and a distorted
wave.
N
otes:
EXPERIMENT #128: SINE WAVE OSCILLATOR
Schematic
Wiring Sequence:
o 1-EARPHONE
o 2-EARPHONE
o 3-28-109
o 4-94-106-124
o 5-41-110
o 26-40-93
o 27-105
o 42-71
o 72-119
o 121-122
EP-130_62315RevC.qxp_EP-130_062812 6/23/15 11:18 AM Page 150