X-2 Technical Service Manual Technical Description 4-9
Publication No: 15-02047
4.3.2 Modulator
The microphone output is capacitor-coupled by C314
[D4]
to the input of the
balanced modulator IC301
[D3]
. The modulator is enabled when DC is applied
to the bias input pin 5 via R312 and transistor switch V9 (sheet 1
[B8]
).
The audio mixes with the local oscillator of 456.5 kHz (453.5 kHz for LSB)
applied to pins 8 and 10, to produce a DSB output at pin 6 that passes via
D301
[C3]
to the 455 kHz sideband ceramic filter Z301.
4.3.3 455 kHz Filter and First Mixer
104-02907 Sheet 1
Filter Z301 passes only the wanted sideband, via tuned transformer T5
[D9]
, to
the input pins 12 and 13 of the first balanced amplifier/mixer IC9
[D7].
The
mixer is enabled by operating transistor switch V9
[B8]
and applying DC via
R35 to the VCC input pin 4 and bias current via R38 to pin 11.
The 455 kHz transmit signal is filtered with the local oscillator VCO2 and
applied to pin 5 to produce a second IF output signal centred on 45 MHz at
pins 3 and 14. The mixer/amplifier has a gain of approximately 20 dB.
4.3.4 45 MHz Roofing Filter
The 45 MHz IF signal is filtered through a 15 kHz roofing filter consisting of
T4, Z2, L10, Z1 and T3
[C5]
. This removes unwanted products of the first
mixer. The filter output is applied to the balanced input of the second mixer
IC2
[D3]
at pins 12 and 13.
4.3.5 Second Mixer and Exciter Output Filter
The second mixer IC2 is enabled by transistor switch V2
[B4]
, applying DC via
R7 to the VCC input pin 4 and bias current via R8 to pin 11.
The 45 MHz transmit signal mixes with local oscillator VCO1 and is applied
to pin 5, producing the selected channel frequency at the mixer output pins 3
and 14. The mixer/amplifier produces approximately 20 dB gain.
The mixer output is fed via a 20 MHz low pass filter to the transmit exciter
output connector P1 pin 1 and 2. From here it couples via coaxial cable to
the PA assembly.
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