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11
SERVICE NOTES
Technical description (see Fig. 2 and Fig. 8)
•
A. INTRODUCTION
The
generator is a Wien bridge
oscillator using
variable resistance
for
tuning.
The
output voltage may
be
sinusoidal
or square,
according
to
the
position
of switch
""vI
'1
"
(SK3).
The frequency
range
is 10
c/s
to
100
kc
/
s,
subdivided
into
four
decades which can be
selected
by the
range
selector
(SK2).
The amplitude of
the
outpUt
voltage
is continuously
variable
up to
2 Vrms for sinusoidal voltages
or
-4 V
p _ p
for square-wave voltages.
B.
THE OSCILLATOR
In the block diagram (Fig. 2) the oscillator section
is indicated
by
the letter A. The oscillator is
a
Wien bridge, the frequency
of
oscillation of which
is given
by the formula:
1
f
o
=
2nRC
From this formula it
is
dear that the frequency is
varied
by chang
ing
the
values
of R or C. The four frequency decades are obtained
by
switching
on and off capacitors in pairs,
in
the
reactive
bridge
branches, while the continuous adjustment
in
each decade is made
possible by the two gang potentiometer Rl. R18 is
a
trim ming
potentiometer, with
which
the frequency calibration can be adjust
ed.
Trimmercapacitors are used for an accurate
adjustment in
the
highest frequency range,
when
the phaseshift of the oscillator
am
plifier tends to increase.
Trimmer C2 is used for
adjusting
at 100
kc/s,
while C22
is
used
at
10
kc
/s
in the
X
1 Krange.
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