l
f
f
l
I
I
I
I
I
I
I
I
l
I
lator input is also connected in a push-pull configuration control
ling current deflection of each pair of deflecting electrodes.
Thus, the local oscillator and noise components are suppressed
approximately 30 db at the output of Tl.
Optimum balance is
provided by proper adjustment of Rl.
(Refer to Section
V
for ad
justment procedure.)
Of the remaining frequencies in the mixer output, only
the 9 MHz difference frequency is coupled to the IF stage.
The
other frequencies are eliminated by the highly selective crystal
filter circuit.
The crystal filter is a modularized 6-pole crystal-lattice
filter with
a
center frequency of 9 MHz and a bandwidth of 3.0 kHz.
The filter is driven from the secondary of Tl. In operation, the
9 MHz mixer output is passed through the filter and connected to
the input of the first IF amplifier.
Unwanted signals in
the mixer output are rejected by the sharp skirt selectivity of the
crystal-lattice filter.
4.2.3
LOCAL OSCILLATOR
(Refer to Figure 6-2.)
The local oscillator uses a single transistor (Q3) as a wide
band crystal oscillator circuit that provides a nominal 32 pf
load for any one of six switch-selectable parallel resonant crys
tals.
Individual crystal frequencies are chosen so that the
difference between the received signal frequency and the crystal
frequency is always 9000.0 kHz for FSK reception and 9001.4 kHz
for AM reception.
Crystals for the AM mode are 1.4 kHz above the
IF crystal filter frequency so that the carrier is positioned at
the edge of the skirt of the filter.
This permits an upper side
band to pass thru the 3 kHz filter and provide an audio bandwidth
of at least 2500 Hz after detection.
On bands A,
B,
C, and L the
crystal frequency must be above the received signal; on bands
D
and
E
crystal frequency must be below the received signal.
Individual crystal frequencies can be varied slightly to
compensate for small frequency discrepancies by means of an
individual crystal trim capacitor (located above and adjacent to
the crystal sockets).
Operational adjustment of the local os
cillator frequency is provided by front panel OSC TRIM control C8
in any crystal position.
Adjustment of either trim control alters
the value of the tuned circuit capacitance to a small degree,
thereby varying the resonant frequency.
The amount of frequency
variation possible is proportional to the frequency of the crystal.
The crystal oscillator has a direct sample output from its
emitter to rear panel connector J26, pin 1.
The oscillator in
put to the mixer is routed by Sl to buffer amplifier Q2 before
connection to balance transformer T2.
4-6
