Watkins-Johnson Company WJ-8718-19/FE Instruction Manual page 113

CIRCUIT DESCRIPTION WJ-8718-19/FE HF RECEIVER
3.4.8 RF INPUT SWITCH (FE-A2) (794276)
The RF Input Switch is a modular -assembly which mounts one P W
subassembly, RF Input Switch, FE-A2A1.
3.4.8.1 RF Input Switch (FE-A2A1) (270935)
Refer to Figure 6-11, RF Input Switch Schematic Diagram, as an aid in
understanding the following circuit description.
The RF Input Switch consists of two solid RF switches, Ul and U2. Pin
11, the control pin, of each switch is connected together and routed to the control
input line from the Digital Control. When the control input is low, Ul-7 is connected
internally to Ul-4, making an RF signal path from Jl to J2. At the same time U2-7
is connected internally to U2-4, making an RF signal path from J4 to J5.
When the control input is high, Ul-1 is connected internally to Ul-4,
making an RF signal path from Jl to J3. At the same time, U2-1 is connected
internally to U2-4, making an RF signal path from J6 to J5.
3.4.9 1 MHZ FILTER (FE-A3) (794327)
Refer to Figure 6-12, 1 MHz Filter Schematic Diagram, as an aid in
understanding the following circuit description.
The 1 MHz Filter is a 5-pole elliptic low pass filter with a cutoff
frequency of 1 MHz. It exhibits a 50 ohm characteristic impedance and a nominal loss
of < -3 dB. Above 1 Mhz, the loss increases rapidly, being typically > -40 dB for
frequencies above 2 MHz.
3.4.10 INPUT CONVERTER (A3) (791592-7)
The Input Converter is a modular assembly which mounts two P.W.
subassemblies, First Mixer, A3A1, and 2nd Mixer, A3A2. Figure 3-15 is a detailed
functional block diagram of the Input Converter which should be referred to in the
following circuit description. Figure 6-13, Input Converter Schematic Diagram, may be
referred to for greater component level detail, if desired.
3.4.10.1 First Mixer (A3A1) (370611)
RF input signals enter the 1st Mixer, AlUl, in the frequency from 5 kHz
to 30 Mhz. The Mixer, AlUl, is also driven by the 1st LO signal, 42.91-72.90 MHz at
a level of +20 dBm. AlUl converts the RF input signals to the 1st IF frequency of
42.905 MHz. The conversion loss of the 1st Mixer is approximately 6 dB. Therefore,
the 1st Mixer is followed by amplifier A1U2 to overcome the loss. A1U2 is a
broadband hybrid integrated amplifier with a low noise figure, a good terminating
'mpedance for the mixer, and a large signal handling ability. The output load of
ilU2 is 50 ohms, which properly terminates the 1st IF crystal filter A1FL1. This
filter requires a 50 ohms source and load and has a center frequency of 42.905 MHz
and a 3 dB bandwidth of 16 kHz.
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