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The
output
of R626 is
also
applied to the "+" input
of IC606A through R628.
R629 pulls this "+" input slightly more negative than the output of R626 to hold
the output of IC606A negative during conditions of no gain reduction. However,
as
soon
as
the "-"
input
of
IC606A
is
pulled
slightly
less
positive
by
the
occurence of gain reduction, the output of IC606A goes positive and lights the
HF LIMIT lamp through R630 and Q601, used as a zener diode.
5.a) Clipper With Dynamic Threshold: ( on Cards # 8 and # 9)
(NOTE:
This circuit
is
only
used
in
an emergency situation when
the Hilbert-
Transf orm Clipper Cards # 0 and # 1 are removed from service. While we do not
expect
that
the
Hilbert-Transform
Clipper
circuits
will
exhibit
any
reliability
problems,
retaining
the
alternate
clipping
circuitry
for
emergency
use
was
extremely convenient and cost-effective, and provides the customer with an extra
measure of redundancy and security.)
The threshold of the first clipper CR801, CR802 is varied dynamically to make
best
use
of
the
distortion- cancelling
circuitry.
When
the
clipper
input
signal
contains
substantial
high
frequency
energy,
then
the
threshold
of
clipping
is
lowered
approximately
1.0dB
to
permit
the
difference- frequency
IM
distortion-
cancelling
signal
to
sum
with
the
output
of
the
main
15kHz
lowpass
filter
without
excessive
clipping
in
the
subsequent
overshoot
compensator.
However,
when
the clipper
input
signal contains predominantly low
frequencies,
then the
clipper
threshold
is
raised
to
minimize
the
amount
of
low
frequency
clipping
which occurs.
This
is
achieved
by
using
the rectified
output
of
IC602B ( the
high
frequency
bandpass filter employed differentially in the preemphasis
filter) to control
the
clipper threshold. The output
of IC602B feeds a rectifier- with- threshold IC806B,
IC807 ( and associated circuitry) whose operation is identical to the rectifier- with-
threshold already described in 2.d. This rectifier feeds an RC filter R844, R847,
R848,
CR807,
C826.
R844
determines
the
attack
time
of
the
circuit
in
conjunction
with
C826.
The
recovery
time
is
determined
by
the
series
combination of R847 and R848.
If the high frequency energy present at the input to the rectifier is insufficient
to
overcome
its threshold, then the "+" input
of IC808A
is held at
ground
by
R847,
R848.
If
output
current
flows
into
the
rectifier,
then
C826
is
pulled
negative
through
CR807.
If
the
voltage
across
C826
attempts
to
go
more
negative
than approximately - 13 volts, the rectifier will saturate and limit the
voltage
swing to this value. The voltage divider R847, R848 attenuates this 13
volt variation such that
it causes a voltage variation of - 0.2v at the output
of
IC808A,
thus
changing
the
threshold
of
clipping
by
approximately
1.0dB.
A
1.5
volt
quiescent
bias
for
the
clipper
diodes
is
provided
by
passing
the
output of a voltage divider through CR808 to R846. IC808A then acts as a unity-
gain
inverting amplifier for the voltage at CR808's anode. CR808 temperature-
compensates the threshold of clipping by reducing the clipper bias voltage as the
voltage
drop across the diodes increases (with temperature variation). The final
diode
bias
voltage
at
the output
of IC808A
is
thus the sum
of
the
quiescent
voltage
contributed by the circuitry connected to the "-" input of IC808A, and
the voltage variation contributed by the circuitry connected to the "+" input
of
IC808A.
IC808B
is
connected
as
a unity- gain
inverting
amplifier,
and
provides
a
complementary negative bias for CR802.
B-8
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