The Modulator/Demodulator

The balanced modulator/demodulator stage (BalMod) converts the received IF signal to audio and also converts Tx audio to a 9 Mhz DSB IF frequency signal. The circuit can also generate a CW signal by driving the AUDIO port with a DC current instead of an audio signal. It’s based on an FST3257 mixer, similar to that used for the main front-end mixer. It accepts square-wave BFO drive from an Si5351 or Si570 synthesizer chip. Initial test results look promising, with about 50 dB carrier suppression and over 0 dBm output. A concept schematic of the BalMod is shown below:



A more detailed schematic showing bias circuitry to make this work with the FST3257 bus switch will be published soon.


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