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4
Page 307
Refer to the "X-Ray View" and "Voltage Chart" (fold-out from Page 306).
CIRCUIT DESCRIPTION
These
circuits prwide highly selective filtering of the 3.395
MHz IF, and generate the
BFD signals in both receive and
transmit modes. Solid-state switching is employed to
prwide
bilateral filtering and to select between the single sideband
filter and the optional, narrow-bandwidth CW filter.
Two switches are used on each side of the crystal filters to
direct the signal flow. One set of switches selects between
transmit and receive paths. Transistors
CEO1 and (1603 are
energized only in the receive mode and pass the 3.395 MHz
signal from the second mixer
((1705) through the filter to
the input of the receive IF board. In addition to their
switching function, these stages provide impedance matching
to the filter. During transmit operation, stages
0604 and
0602 selve the same function, passing the modulator output
through the filter to the transmit IF board.
The second set of switches selects either the
SSB filter
or
the
CW
filter. In US8 or LSB modes, diodes D678 and D681 are
forward biased and the SSB filter is used. Diodes D683 and
D685 are
back
biased by current flow through D684 and
D686. This blocks signal flow to the
CW
filter and shunts
any potential
"Mow
by" signals to ground through the
.O1
pF capacitors at C681 and
C686.
In
CW,
diodes D683 and
D685 are folward biased and diodes
D678
and
D681 reverse
biased
so
that
signal flow is now through the
CW
filter.
Diodes D679 and
D682 shunt the SSB filter input and
output to ground.
0608. CE11, and 0613 are the
--
beat frequency
-
oscillators
(BFO's) which provide the appropriate mixing frequency
to
the product detector in receive and to the _ba!anc_ed_
modulator in transmit. These three oscillators are identical
except for the
cvs& frequency.
The
CW
generator (061 1)
is used only in the transmit mode. In receive
CW,
the USE
generator is used to provide the offset from center
frequency required to produce an audio tone from the
product detector. Transistor
0609 permits the
CW
generator
to operate only in the transmit mode, and
0612 permits the
US8 generator to oscillate in transmit US8 or in receive
USE
and
CW.
Four hot carrier diodes are used in the balanced modulator.
Thxrier generated by the appropriate BFO is amplified
by transistor
0607 and
is
mixed with an audio signal from
the transmit audio board to produce the 3.395
MHz
transmit IF which
is
fed to the crystal filter.
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