Watkins-Johnson Company WJ-8718-19/FE Instruction Manual page 95

CIRCUIT DESCRIPTION
WJ-8718-19/FE HF RECEIVER
3.3.2.1
VHF Preselector/lst LO (FE-A1A1) (794315)
VHF signals from the Input RF Switching Section drives the input to the
Preselector. The Preselector is a narrow band tunable filter. Tuning is organized intn
two bands, 30-54 and 54-100 MHz by the Band Switch lines from Digital Control ThP
Tuning Voltage from Digital Control forces the Preselector to track the receiver'tuned
frequency. Preselector output drives the 1st Converter input in FE-A1A2.
The 1 MHz Reference and the Tuning Data word drive the Synthesizer
luPUlwJhe band SWltC,h signals and tunin£ voltage generated in the Synthesizer force
the VCO to oscillate from 190 to 260 MHz in two tuning bands. The VCO output is
fed back to the Synthesizer to achieve a "lock" condition in the Synthesizer loop. The
VCO output (1st LO) drives the 1st Converter input in FE-A1A2.
3.3.2.2
Input Converter/2nd LO (FE-A1A2) (794316)
VHF signals (30-100 MHz) from FE-A1A1 drive the 1st Converter Input
T^tf? uugna\S nrc xTuXS? Wirlh the 1St L0 (19°-260 MHz) from FE-A1A1 to produce a
159.5 MHz +/- 0.5 MHz 1st IF output. The 1st IF output drives the 2nd converter.
The 1 MHz Reference signal from FE-A1A1 drives the 2nd LO. The 2nd
LO is a fixed synthesizer producing a 2nd LO output of 189 MHz.
The 2nd converter mixes the 1st IF of 159.5 MHz and the 2nd of 189
MHz to produce the 2nd IF of 29.5 MHz +/- 0.5 MHz. The 2nd IF output is sent to
the Input RF Switching Section where it is routed to the RF/IF Conversion Section.
3.3.3 RF/IF CONVERSION SECTION
Figure 3-4 is a block diagram of the RF/IF Conversion Section,
shown in Figure 3-4, it consists of one major module: Input Converter, A3.
RF GAIN
CONTROL IN
i
INPUT
CONVERTER
A3
1ST IF
As
RF IN
A3A1
A3A2
~l
10.7 MHz 2ND IF OUT
Urd. -J
1ST LO 2ND LO
Figure 3-4. RF/IF Conversion Section Block Diagra
m
3-12