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I
AJ-8718-19/FE HF RECEIVER
CIRCUIT DESCRIPTION
3.3.3.2
Input Converter (A3) (791592)
As shown in Figure 3-4, the Input Converter consists of two major
subassemblies:
1st Mixer A3A1 and 2nd Mixer A3A2.
3.3.3.2.1
1st Mixer (A3A1) (370611-6)
The 1st Mixer receives RF Input signals (0.005-30 MHz) from the RF
Filter (Al) and the LO Signal (42.91-72.90 MHz) from the 1st LO Synthesizer (A5A1)
The 1st LO and RF Input signals are mixed by the 1st mixer to produce a first IF in
the range of 42.90001 to 42.910 MHz.
The 1st IF signal is amplified and filtered by a
150 kHz bandwidth band-pass filter with a center frequency of 42.905 MHz.
3.3.3.2.2
2nd Mixer (A3A2) (370646-6)
The 2nd Mixer receives the 1st IF signal (42.905 MHz) from the 1st Mixer
and the 2nd LO signal (32.21000-32.20001 MHz) from the 2nd LO Synthesizer (A5A2).
The 2nd LO and 1st IF are mixed by the 2nd Mixer to produce a 2nd IF of 10.70 MHz
The 2nd IF signal is amplified and filtered by a 50 kHz bandwidth band-pass filter with
a 10.7 MHz center frequency and drives the 50 ohm input of the 10.7 MHz Filter
Switch, A4A1.
3.3.4
IF/DEMODULATOR SECTION
Figure 3-5 is a block diagram of the IF/Demodulator Section.
The
IF/Demodulator Section is mounted on the IF Motherboard P.W. Assembly A4. As shown
in Figure 3-5, the IF/Demodulator Section consists of the following major modules: 10.7
MHz Filter Switch, A4A1; 10.7 MHz/455 kHz Converter, A4A2; 455 kHz Filter Switch,
A4A3; AGC, A4A6; 455 kHz Amplifier/AM Detector, A4A7; FM/CW/SSB Detector, A4A9;
Audio Amplifier, A4A10.
3.3.4.1
10.7 MHz Filter Switch (A4A1) (791594)
The 2nd IF output from A3 drives the 50 ohm input of the 10.7 MHz
Filter Switch, A4A1-13.
The signal is routed through one of three voltage-selectable
circuit paths within A4A1.
The wideband path, selected by +5 Vdc at pin 15, passes
the full 50 kHz bandwidth from A3 through A4A1.
The other two circuit paths,
selected by +5 Vdc at pin 17 or 19, restricts the bandpass to 6.0 or 3.2 kHz
respectively (see Table 3-1 for summary of 2nd and 3rd IF bandwidth selection).
The
amplified, band-limited output at A4A1-57 drives the 50 ohm input of the 3rd Mixer
A4A2-57.
3-13
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