RFL 9780 RFL Electronics Inc.
April 8, 2003 14-4 (973) 334-3100
14.3.2 INPUT LIMITER
The 4-kHz signal from the narrowband filter on the RFL 9780 CLI module enters the RFL 9780 LM/SL
module at edge connector terminal C13. This signal is passed to the input limiter, which is composed of
comparator U1A and is then passed to the discriminator.
NOTE
THE ACTIVE DISCRIMINATOR IS TIGHTLY TUNED AT RFL. NONE OF THE
PARTS WITHIN THIS CIRCUIT ARE FIELD REPLACEABLE. ANY COMPONENT
CHANGES OR ADJUSTMENTS MADE TO THIS CIRCUIT WILL DEGRADE
SYSTEM PERFORMANCE.
14.3.3 ACTIVE DISCRIMINATOR
U5 and its associated components form an active discriminator, which shifts the incoming signal by 90
degrees at 4 kHz.
14.3.4 DISCRIMINATOR LIMITER
The signal produced by the discriminator passes to the discriminator limiter, composed of comparator
U1B and associated components.
The output of the discriminator limiter is a square wave at the same frequency as the output of the input
limiter. When the center frequency is being received, these signals are 90 degrees out of phase. They
are applied to Exclusive-OR gate U7A; the signal produced by U7A is a square wave that is twice the
carrier frequency, with a 50-percent duty cycle.
As the input frequency changes, the phase across the resonator U5A and U5D will lag or lead with
respect to the input. Depending on the input frequency, the phase difference between the two inputs of
U7A will change from -90 degrees to +90 degrees. These phase differences change the duty cycle of
U7A's square wave output. The output's dc component is proportional to the pulse width, which is
proportional to the input frequency. In this way, the frequency-shifted input creates an output with a dc
level that is a function of frequency.
The NORMAL/INVERT jumper J1 should left in the NORM (Normal) position at all times.
14.3.5 LOW-PASS FILTER
Operational amplifiers U3A and U3B and their associated components form an active low-pass filter.
This filter averages the mixed limiter signals, thus providing a DC level.
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