sequence
by
the
AND
gates
3106/8107
(see
oscillograms
Fig.
6a).
The
switching
sequence
of
the
38—kHz
subcarrier
frequency
within
each
cycle
starts
with
T124
towards
T121,
is
reversed
to
T128
via
T124
and
terminates
with
T125.
4.}
Summing
Amglifier,
MPX
Lowgass
Filter
and
Outgut
Amplifier
See
circuit
diagram
230.8518,
sheets
2
and
3.
The
summigg
amglifier
consists
of
the
input
stage
designed
as
a
differential
amplifier
comprising
T151
and
T132,
and
of
the
driver
stage
T133
whose
collec-
tor
resistor
is
the
constant
current
source
T135
to
increase
the
open—loop
gain.
The
operating
point
of
the
complementary
output
stage
T134/T135
is
de-
termined
by
the
emitter
resistors
R293
and
R294
together
with
the
diodes
01
121
and
G1
122.
The
negative
feedback
is
adjustable
with
R288.
The
following
sig—
nals
are
summed
at
the
amplifier
input
with
the
correct
relative
levels:
L
g
R
and
difference
signal
S
=
L
g
R
a
carrier
and
delivered
by
the
coder
(see
section
4.2).
a)
Sum
signal
M
=
superimposed
on
b)
Pilot
tone
P
(19 kHz)
obtained
from
the
pilot
generator
circuit
(see
section
4.4).
c)
Correction
signals
for
reducing
the
crosstalk
via
R265
+
0146
and
R264
+
0147
(lines
A
and
B).
Interfering
switching
spikes
(532 kHz)
from
the
coder
are
eliminated
by
the
sample
&
hold
circuit
T137/0155.
The
emitter
follower
T138,
whose
resistor
is
designed
as
a
constant
current
source
comprising
T139,
feeds
the
MPX
low—
pass
filter
(Via
line
J).
The
MPX
lowpass
filter
using
L111
and
L112
is
a
Cauer-type
filter
with
two
attenuation
peaks
at
768
kHz
and
498
kHz.
The
harmonics
of
the
subcarrier
—
which
theoretically
may
be
present
—
are
sufficiently
suppressed
without
de—
teriorating
the
phase
and
amplitude
response
up
to
53
kHz.
A
contact
of
the
bipolar
relay
R5101
selects
the
MPX
signal
or
the
mono
signal
(from
ERlOE).
The
outgut
amElifier
is
made
up
of
the
summing
circuit
T151
to
T154
and
the
two
output
stages
MPXl,
T155
to
T157
and
MPX2,
T158
to
T160.
The
first
stage
adds
the
following
signals
with
the
correct
relative
levels:
a)
MPX
signal
(coded
signal
including
the
pilot
tone).
b)
SCA
signal,
if
the
option
"SCA
modulator"
is
used.
This
signal
from
the
input
ST101.50
(.60
=)
is
frequency—modulated
and
has
a
carrier
in
the
range
60
to
74
kHz
whose
centre
frequency
is
referenced
R
38122
-
l8
