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5.c) 2.2-15kHz Phase-Linear Bandpass Filter: ( on Cards # 8 and # 9)
The signal from the Hilbert- Transform Clipper's HF VCA is applied to inverting
amplifier
IC801A.
This
amplifier
drives
two
lowpass
filters
with
cutoff
frequencies of aproximately 15kHz and 2.2kHz. The 15kHz filter is a fifth- order
Cauer
lowpass filter R806, R807, 0801, 0802, 0803, 0804, 0805, 0806, C807,
L801,
L802.
This
lowpass filter
is realized as
a " passive
ladder" for
maximum
stability, and minimum noise and distortion. The filter's response is nominally + 0,
-0.6dB
from 0 to 15.4kHz, with a sharp rolloff thereafter. There are notches at
approximately 19kHz and 26.6kHz.
The load resistor
for the
filter, R807,
is connected not
to true
ground but
to
the " virtual ground" of the summing junction of IC801B.
IC801B is an inverting,
frequency selective amplifier whose feedback network R808, R809, C808 provides
a 2dB shelving rolloff. The purpose of this rolloff is to match the gentle rolloff
of
the
2.2kHz lowpass filter to achieve
a maximally sharp slope
on the lower
skirt of the bandpass filter produced by subtraction of the outputs of the 15kHz
and
2.2kHz filters. A complementary shelving filter R811, R812, R813, C809 in
the feedback loop of 10803B restores flat response later.
The output of IC801B feeds a differential sidechain which creates a fourth- order
allpass function when its output is correctly summed with the output of IC801B
(i.e.,
the
main
signal).
This
allpass
function
does
not
change
the
frequency
response of the 15.4kHz lowpass filter, but does add phase shift as necessary to
make
the
overall
time
delay
of
the
15.4kHz
filter
plus
allpass
network
more
constant than the time delay of the 15.4kHz filter alone.
Basically,
this
phase
corrector
sidechain
consists of
a pair
of
active
inverting
bandpass filters built around IC802A, IC802B. The IC802A filter is driven by the
output
of
IC801B
through R815.
Its
output
is
summed
into
summing
amplifier
IC803B through R817.
The
second bandpass filter ( associated with IC802B) is driven by both the main
signal ( through R814) and the output of the first bandpass filter IC802A ( through
R818). The output of IC802B sums into IC803B through R820. The third input to
IC803B is the main signal ( through R810).
In addition to driving the 15kHz lowpass filter, the output of IC801A also drives
2.2kHz lowpass filter 0814, L803, Al, whose magnitude and phase, when cascaded
with additional rolloff R821, R822, 0815, match the magnitude and phase of the
15kHz
lowpass filter from 0 to 2.2kHz. IC803A is a unity- gain buffer to drive
this
final
rolloff network, the output
of which is
directly summed into IC803B
through R822.
If
the
output
from the Hilbert- Transform
Clipper's LF
VCA were not
summed
into
10803,
the
output
of
10803
would
exhibit
a sharply
selective
bandpass
response
due
to
the subtraction
of
the
2.2kHz
lowpass
filter
output
from
the
15kHz
lowpass
filter
output. ( The " subtraction"
is
achieved
because
the
15kHz
lowpass filter is inverting due to IC801B.)
However, the output of the Hilbert- Transform Clipper's LF VCA is summed into
10803 as well ( through R855). Thus the output of 108113B is the final output
of
the Hilbert- Transform Clipper, and is equivalent to a linear- phase 15kHz lowpass
filter. Because this output contains substantial overshoots, it is followed by the
FCS Overshoot Compensator ( see 6 below).
B-11
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