This month, we begin a close look at the synthesiser. It is critical to the operation and performance of the transceiver, and provides the local oscillator on transmit and on receive. It is required to provide an output signal that can drive the 74AC series logic divider in the H-mode mixer. This signal must have as little phase noise as possible and must be crystal-derived for stability and accuracy. It is controlled by the main CPU and must be capable of being set to any frequency in the range 46 to 78MHz. Whilst this may appear to have a 32MHz range and therefore capable of covering the whole of the HF spectrum, remember that there is a divide-by-2 process in the mixer - so for a 30MHz range we would require a VCO swing of 60MHz. This is avoided by using a programmable divider in the VCO output circuit. This can be seen in Fig 26, which shows a block diagram of the synthesiser.
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