Active Filter - Philips PM2521 Service Manual

43
1.
4.4.3.
Active
filter
The input to the active
filter
is
via
the
FET
switch
V603,
activated
by
a signal
from the
anode
of diode
V657
in
the relay control
D1908
(in all
functions except
Hz
and
s).
Filtering
is
obtained basically
by
the frequency-dependent feedback
network
R601
,
C601 R602, R603, C602,
C603 around the operational
amplifier
A601
.
The
loop
coupling to the operational amplifier inputs
is
achieved
via
high
impedance
FET
switches
V605.
Trimmer R624 provides the
offset
adjustment.
The
output from
the active
filter
is
coupled
via
the
FET
switches
V606,
V606', (depending
on whether
AZ
on
^
is
selected)
to the
+Vin (FET V608) or the
-Vin (FET V608)
of the voltage-to-current
convertor
A602.
The output
of the operational amplifier
A601-6
is
also
routed
via
zener
diode
V654
to a
potential divider
circuit
R609,
R610
derived
from
the -t-IOV
supply.
This
drops
the supply to
+8.5V
to equalise
the conduction points of
the V-l
convertor
FET
s
caused by
the
differential inputs.
ACTIVE
FILTER
Fig.
31.
Active
filter