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FM circuit
There were two approaches used for the FM circuit of the conventional all-mode transceiver. Either
there was a dedicated modulation circuit using a 10.695MHz crystal, or the modulation was
performed by the VCO on the 2nd OSC. The latter was not an option for the TS-480, and since the
whole transceiver had to be compact, we did not adopt the former approach. How then is FM
handled by the TS-480?
What we have employed for the TS-480 is something that is rarely seen these days in ham radios:
the reactance modulation approach, which does not have the modulation applied directly to the
oscillating circuit. This type of circuit was widely used in the days when a crystal was used to
change channels in FM car transceivers, but it dropped from sight when PLL became the norm. It
is not a new circuit, but it has excellent characteristics. In the TS-480, this reactance modulation
circuit is connected to the output of the DDS, which serves as the source for the PLL reference
frequency, so effectively it is modulating the 1st OSC.
This approach offers several advantages:
Since frequency modulation is not conducted in the TX signal circuitry, even if the TX RF signal
is passed through a roofing filter, it will not suffer from any delay distortion caused by the filter;
Since there is no need for an oscillator to perform modulation, "one-shot" frequency
management is permitted when transmitting on FM with the same precision as SSB;
This approach saves on space and cost.
Fig. 6: FM modulation block diagram
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