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11
SERVICE NOTES
Technical description (see Fig. 2 and Fig. 8)
•
A. INTRODUCTION
The
generator
comprises
an
RC
oscillator. The sinewave voltage produced
by the RC
oscillator
is
a pplied
either direct
or via a
limiter to the
output
voltage
divid
er.
In
the latter case,
a
time
symmetrical square wave signal
(meandering
signal,
T
=
T/2
+
T/2)
will
be
available on the output.
B.
OSCILLATOR
The
oscillator
consists
of
a
frequency-determining
Wien fourpole with
equal
branches
and an amplifier, which is stabilised
in the operating
frequency range.
The
resonant frequency of
the
Wien bridge
can
be calculated with the
following
formula:
f
o
=
211:
RC
At this
frequency the inpl1t and the
output voltage of
the Wien
bridge are
in pha
se.
The
amplitude
of
the output voltage of
the Wien network is
three
times
smaller than that of the input
voltage.
Thi
s
mea
ns that, in order to satisfy the
oscillation condition, the phase
shift of
the
amplifier should
be 0°
and
its
gain
factor should
be
3.
Coarse
frequency controI
(in
steps
of
one
decade)
is
achieved
by
switch
ing the
capacitors Cl2
...
C22-C29-C30.
A
tandem potentiometer Rla-Rlb is employed for
fine controi of
the
frequency within the seJected
range
Potentiometer RI8
and capacitors Cl
and
C22
serve for
calibration.
The
amplifier
has
a high degree
of
negative feedback,
so
th
a
t
its
gain is
reduced
to
a
factor
3.
By
means of potentiometer
R26 the gain factor can
be
adjusted.
The resistance setting
of
R26 is the
shunt
resistance in the
feedback
circuit.
NTC resistor RJ6
provides
the
series
resistance. As
a
result
of this, the feedback will
be amplitude
dependent.
If
for
example
the output
voltage
of
the
amplifier increases,
the
current
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