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Philips PM 5101 - Service and Maintenance of the PM 5101; Technical Description of PM 5101 Circuits

Philips PM 5101
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11
SERVICE
NOTES
Technical description (see
Fig.
2 and
Fig.
8)
•
A.
INTRODUCTION
The
generator
compr
ises
an
RC
oscillator.
The
sinewave voltage
produced
by
the
RC
oscillator
is
a
pp
lied
either
direct
or
via a limiter
to
the
output
voltage divider.
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.
This means
that,
in
order
to
sa
tisfy
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
ha
s a high degree
of
negative feedback,
so
that 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
resis
tor
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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