2-3 Crystal Filter - IF Amplifier - AGC Module
Crystal Filters
The received signal is fed to this module and routed to
one of two eight-pole crystal filters by steering diodes.
The bandwidths are selected by the front panel
SELECTIVITY control. For an IF bandwidth of
2.1KHz, this control places +15 VDC on the 2.1KHz
filter select input and GND on the 400Hz filter select
input lines. To choose the 400Hz filter, the
SELECTIVITY control reverses this connection.
These filter selection signals are also supplied to the
product detector for inserting a second 400Hz filter
and to the IF amplifier for filter gain equalization.
IF Amplifier
The signal leaving the crystal filter is connected to a
tuned amplifier (3.395MHz) which uses an RCA
40673 dual-gate MOSFET. The purpose of this
amplifier is to establish the noise figure for the IF
section and to provide gain equalization for the crystal
filters. This low noise device provides ample gain that
is easily controlled by a voltage at it’s second gate.
When the 400Hz filter is selected, +15 volts is applied
to the GAIN EQ input, resulting in a 3.3 volt level on
gate #2 for maximum gain. When the 2.1KHz filter is
selected, the GAIN EQ input is grounded and the
MOSFET’sgate #2 voltage is then controlled by a
0-3.3 volt signal from the 2.1KHz FILTER GAIN EQ
trim pot.
The signal is then amplified by a pair of cascaded
Motorola MC1350 amplifiers. These devices furnish
most of the IF amplifier’s gain and over 120 dB of
gain control. During no-signal conditions, the IF
amplifier’s gain control line is set to 5.38 volts. Gain
is reduced by increasing the gain control voltage.
AGC
The output of the IF amplifier is coupled to the AGC
detector diode. This diode charges a timing capacitor
through a resistor which establishes the AGC attack
time of approximately 1 millisecond. This value is
sufficiently fast to maintain a constant AF output
when a strong signal appears while preventing most
pops and noises from “hanging” the AGC system.
The AGC decay time is determined by the parallel
combination of a timing capacitor and a 22 ΜΩ
resistor at the JFET input of the AGC amplifier. The
timing capacitor is one of three selected by the front
panel AGC control. This arrangement provides AGC
decay times of about 20ms, 500ms, and 5 seconds,
thus allowing the operator to match the receiver’s
AGC characteristics to the received signal’s rate of
fade. In addition, the front panel AGC control switch
has an “OFF” position, which shorts the JFET’s input
to ground, thereby defeating the AGC system.
The AGC amplifier’s output is connected through a
diode to the IF amplifier’s gain control line. Two
other signals are similarly connected such that the
most positive signal determines the receiver’s gain.
The front panel manual RF GAIN control raises the
voltage on the control line to limit the maximum
receiver gain with the AGC enabled. It will also
provide total control of the receiver’s gain when the
AGC is defeated. The second signal mutes the
receiver.
Mute Control
The external mute control system is compatible with
that used by the R. L. Drake Co. The receiver is
muted when a companion transmitter removes the
short to ground (normally supplied when not
transmitting) from the EXT MUTE connector. This
causes +12 volts to be placed on the IF amplifier’s
gain control line, effectively silencing the receiver.
The EXT MUTE DEFEAT switch on the rear panel
provides a ground to the EXT MUTE connector so the
receiver can be operated without a transmitter.
The monitor function also defeats the external mute
feature by grounding the EXT MUTE input when the
front panel function switch is turned to MON. This
permits the operator to temporarily override the
external mute for monitoring the transmitted signal or
setting the transmit frequency.
Circuit Description
Page 4
