Inovonics 235, Инструкция по эксплуатации и техническому обслуживанию

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overtones is not discernible to the listener, but is quite effective in
inhibiting filter overshoots.
For a more detailed discussion of this strange and wonderful filter
overshoot compensation scheme, the reader is directed to U.S. Patent
No. 4,737,725.
LOW-PASS FILTER (Schematic Sheet 2, Page 30)
The 9-pole, elliptic-function (Cauer) low-pass filter is an active version
of the classic L-C design worked-out in Germany during the late 1940s,
probably with a slide rule! The particular active configuration used in
the 235 is sometimes called the “FDNR” because each of the legs to
ground simulates a Frequency-Dependent Negative Resistance.
Referring back to the classic L-C design, resistors in series with the
signal replace series inductors, and each of the active circuits to ground
simulates an inductor/capacitor series-resonant element.
A great “cookbook” which includes this and other filter circuits possibly
useful to the DIY broadcaster is the Electronic Filter Design Handbook
by Arthur B. Williams, published by McGraw-Hill. The reader is kindly
directed to this source for a more informed discussion of how the FDNR
circuit works than we could possibly deliver here, due not only to the
limited scope of this Instruction Manual, but to our own marginal
understanding of how these confounded things work!
IC22A buffers the output of the low-pass filter and IC21A provides
phase inversion and gain to compensate for filter insertion loss. The
signal level corresponding to 100% symmetrical modulation at the
output of IC21A is +5dBu.
OUTPUT STAGE (Schematic Sheet 2, Page 30)
Q13 and Q14 form an output “safety” clipper biased to the same ceiling
as the integrated limiter/clipper. Tight limiting and clipping, and
effective filter overshoot control ensure that only very seldom will Q13
and Q14 act on the program signal. Nevertheless, they are included,
“just in case.”
IC26A buffers the output signal and includes a gentle low-pass function
for frequencies beyond the primary low-pass cutoff. IC27B and IC27C
route the Processor input signal around the active circuitry in the
PROOF mode.
The OUTPUT LEVEL control, R211, feeds gain stage IC25A. This stage
includes current-booster transistors Q11 and Q12 to help drive
interconnect cabling and transmitter input loads. CR19 and CR20 set
bias for the two output transistors, and CR21 and CR22 give short-
circuit protection. This is one-half of a “bridge” output configuration,