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5.a) Clipper With Dynamic Threshold: ( on Cards # 8 and # 9)
(NOTE: This clipper
is ordinarily disabled when the Hilbert- Transform Clipper
is
operative. The clipper described below is used in OPTIMOD-FM Model 8100A and
in the older OPTIMOD-TV Model 8180A. By moving jumpers, it is possible to re-
activate this circuit in an emergency situation requiring removal of the Hilbert-
Transf orm Clipper cards for service. Therefore, for the sake of completeness this
description is included.)
The
clipper
is
a straightforward
shunt
clipper
which
is
ordinarily
biased
with
+1.5 volts, thus providing a somewhat " soft" characteristic ( but not nearly as soft
as
a pair
of
back-to-back
unbiased
diodes).
The
characteristic
was
chosen
to
obtain
the
best
compromise
between
harmonic
and
IM
distortion
induced
by
clipping, when the IM- cancelling circuitry is considered.
The
output
of the bandpass filter in the high frequency limiter [ see ( 4) above]
feeds
a rectifier
with
threshold.
When
high
frequency
energy
exceeds
this
threshold, the clipper bias voltage is reduced to reduce the clipping threshold by
approximately
1.0dB.
The
purpose
of
this
threshold
reduction
is
to
provide
headroom between the
clipper threshold and the subsequent
overshoot
corrector
threshold.
This
headroom
accomodates
the
distortion
corrector
signal ( see
5.c
below) which is needed to correct the IM distortion produced when large amounts
of
HF
energy
are
clipped.
If
this headroom
were
not
provided,
the
overshoot
corrector would clip off the distortion corrector signal, thus negating its effect.
On
the other hand, when the input signal to the clipper contains predominantly
low
frequency energy, the distortion corrector loop is essentially ineffective. In
this case, the absolute
amount
of clipping is minimized by raising the
clipping
threshold to approximately the threshold of the overshoot corrector.
5.b) Hilbert-Transform Clipper: ( on Cards # 0 and # 1)
The
input
signal
to
be
peak- limited
is
split
into
two
paths.
The
main
path
consists of a chain of phase- shift networks while the second path consists of a
chain of phase- shift networks cascaded with a sharp 4kHz lowpass filter. The two
chains
are
designed
so
that
the
phase
difference
between
their
outputs
is
90
degrees from 30Hz to 4kHz.
The
outputs of
the
two
chains
are
applied to
a vector
sum
generator
which
computes the square root
of the sum of
the squares
of
the two outputs.
The
output of the vector sum generator is applied to the control ports of two VCA ' 5:
directly
to a " high- frequency" VCA, and through a delaying lowpass filter to a
"low- frequency"
VCA.
The
output
of
the
main
phase- shifter
chain
is
applied
directly to the audio input of the HF VCA, and through a lowpass filter to the
audio
input of the LF VCA. Thus the control voltage and audio to the LF VCA
are both delayed equally.
If
a sine
wave
below
4kHz
is
applied to
the
input
of
the
Hilbert- Transform
Clipper, the output of the vector sum generator is ideally DC without ripple, the
control voltage to the VCA's is constant, and no distortion is produced by the
action of the VCA's. However, when more complex waveforms are applied, ripple
does
occur
in
the
control
voltage,
producing
IM
distortion ( but
no
harmonic
distortion). (Due to approximations in the design of the phase shifters, the actual
THD produced on sinewave is typically 1-2% up to 10dB clipping.)
It
can
be
shown
that
the
output
of
the
HF
VCA
is
accurately
peak- limited
without overshoots regardless of its input spectrum, provided only that the phase
difference between the inputs to the vector sum generator is 90 degrees over the
frequency range in which both chains have substantial output energy ( i.e., below
A-8
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