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APPENDIX B:
Circuit Description
The
following section provides
an extremely
detailed description of
the circuitry
used in OPTIMOD-TV on the component level.
It
may be wise to read Appendix A first, and to consult the block diagram on p.
J-25.
Referring
to
the
appropriate
Schematics
and
Parts
Locator
Drawings
in
Appendix
J will
help
you
to
follow
the
text
and
will
aid
component- level
troubleshooting.
In
those
cases
where
the
circuitry
is
duplicated
in
the
left
and
right
channels,
only
the
left
channel
circuit
and
component
designators
will
be
discussed.
1.a) Input Amplifier: ( on Cards # 3 and # 4)
The
input
is
applied
to
the
RF
filter
chamber,
and
there
encounters
an RF
filter
and
10K
bridging
pad
R303,
R304,
R305.
Strapping
R305
into
the
pad
introduces 20dB loss, which is the normal condition of the pad.
The output of the pad is connected to a low- noise true instrumentation amplifier
consisting
of
IC301A,
IC301B,
IC302A,
and
associated
resistors.
R306,
R307
provide bias current for IC301, which is a low- noise bipolar- input dual IC opamp.
R308, R311 are feedback resistors for the two sections of IC301. The differential
gain
is controlled by the series resistance of R310 and GAIN control R309. The
common- mode gain of the IC301 pair is 1.
The
differential
output
of
IC301A
and IC301B
is
converted
to
a single- ended
output
and the
common mode component of the output
is nulled by means
of
differential amplifier IC302A and associated resistors. R316 adjusts the balance of
the
resistor
network
to
assure
maximum
common- mode
60Hz
rejection.
NOTE:
Nearby
lightning
strikes
may
induce
sufficient
energy
into
OPTIMOD-TV's audio input wiring to pass through the RFI protective
networks and destroy IC301 or IC401. If OPTIMOD-TV is installed in
a lightning- prone location, it is advisable to keep spare NE5532's in
stock.
Installation
of Varistors
or
other
protective
devices
between
each
side
of
the
audio
input
line
and
earth
may
help
prevent
such
problems. ( IC301 is socketed, and is thus easily replaced.)
1.b) 30Hz Highpass Filter: ( on Cards # 3 and #4)
The non-bypassable 30Hz highpass filter IC302B, C303, C304, C305, R317, R318,
R319 is a third- order Chebychev filter with 0.5dB passband ripple ( nominal) and a
ripple bandwidth ( i.e., - 0.5dB frequency) of 30Hz. It is realized as a unity gain
Voltage- Controlled
Voltage- Source ( VCVS)
active
filter.
This
filter
is
non-
inverting, has a gain of exactly OdB in the passband, and uses positive feedback
to " sharpen up" the response around the cutoff frequency. Most modern books on
active
filters
extensively discuss this type
of
filter. ( See for example
Wong
and
Ott:
Function
Circuits.
New
York,
McGraw-Hill,
1976,
pp.
230-231.)
B-1
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