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RET-041 Instructions
95307-000-01 6/87
Circuit Description — Card #1 F10
Card #1F1O is divided into two " subsystems " , which are described below in order of signal
flow. The block diagram in Fig. 6 illustrates this description.
1) 5kHz Low-Pass Filter
This is a unity-gain, phase-corrected, 5th-order Chebychev filter with O.ldB frequency re
sponse irregularity and a -O.ldB bandwidth of 5.0kHz.
IC8a and associated components form a third-order non-inverting filter with a gently rolled-
off frequency response. The 5th-order filter ' s frequency response shaping is completed by
IC9a and associated components, which form a 2nd-order inverting filter when the circuit
path is completed by strapping jumper B DAY) or A NIGHT) to the " 5kHz " position.
IC8b and associated components form a phase corrector, which is an all-pass filter with
flat magnitude response and frequency-dependent phase response. This filter adds delay as
necessary to make the delay of the entire filter approximately constant with frequency, thereby
minimizing overshoot.
DAY/NIGHT switching is effected by very simple logic. Pin T on the card is low -15V)
when OPTIMOD-AM is in NIGHT mode, and high ground) when it is in DAY mode. The
logic level at pin T drives JFET Q2 directly. QI inverts the logic and drives Q3. Q2 and Q3
are on when their gates are at ground, and off when their gates are at -15V. Thus Q3 turns
on in NIGHT mode and connects jumper A into the circuit path, while Q2 turns on in DAY
mode, and connects jumper B into the circuit path.
2) 10kHz (NRSC) Low-Pass Filter
The signal can be routed to a 10kHz overshoot-compensated low-pass filter, consisting of a
first and second filter in series, with embedded clippers.
The signal enters the filter system and encounters clipper CR1, CR2, which provides main
peak control for the signal. After buffering by ICla, the clipped signal is applied to the
first 10kHz filter IC2, IC3, IC4 and associated components). This filter is very steep, and
removes the harmonics introduced by the clipping in CR1, CR2, as well as any significant
program energy above 10kHz.
The first filter is an active-RC analog of a passive LC ladder filter. It is realized by resistors,
capacitors, and frequency-dependent negative resistors FDNRs). An FDNR is realized with a
dual opamp, three resistors, and two capacitors. When the passive LC filter is transformed into
an active RC filter, inductors become resistors, resistors become capacitors, and capacitors
become FDNRs.
Each FDNR resonates with a series resistor to create a notch in the frequency response of
the filter. This is analogous to a series LC circuit to ground.) The notches are located in the
" stopband " beyond approximately 10.05kHz). The circuit associated with IC2 produces a
notch at 10.65kHz ±4%. The circuit associated with IC3 produces a notch at 12.00kHz ±4%.
The circuit associated with IC4 is tuned by R39 to produce a notch at precisely 10.10kHz to
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