3-2 Brief Description 8525B/8528 Technical Service Manual
Issue 6 Publication No: 15-02036
3.3 Transmit
The audio signal is amplified and levelled in the microphone amplifier and
compressor, then fed to the balanced modulator where it is mixed with the
1650kHz (USB) or 1647kHz (LSB) local oscillator to produce a DSB signal.
The resulting signal is filtered by the 2.5kHz crystal filter to produce the
SSB signal (USB or LSB) required for transmission.
In the transmit mode, the IF amplifier is switched to produce a buffer to feed
the mixer where the SSB signal is mixed with the output of VCO2 to
produce the 45MHz IF signal.
After the ‘roofing’ filter, the signal is mixed with VCO1 output to produce
the required channel frequency (2MHz to 24MHz). This is taken, via the
transmit/receive switch, to the PA and Filter PCB where it is amplified and
filtered (band-switched filters) before being passed to the antenna.
Forward and reflected power circuits are used to control the power
amplifier. If a high VSWR is detected, the PA’s output power is reduced to
prevent damage.
3.4 Synthesizers
Two single-loop synthesizers are used, the main synthesizer (VCO1)
generating a 45.25MHz to 75MHz signal i.e. 0.25 to 30MHz, plus 45MHz,
in 2kHz steps, and a ‘vernier’ synthesizer (VCO2) generating 43.352 to
43.350MHz in 10Hz steps.
The synthesizers are controlled by the microprocessor which controls the
transceiver. Serial data is loaded into both synthesizers; the data varying
according to the required channel frequency preprogrammed into memory.
The system uses a single ovened crystal reference oscillator of 6.6MHz. This
reference is also used to provide the 1650kHz (6.6MHz divided by 4) signal
for the audio modulator/demodulator.
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