It may not be obvious that the balanced modulator at D15,D16 is multiplying anything. ?Consider this: ?When the 12mhz bfo is on the positive side of its cycle, both diodes are fully conducting. ?When the 12mhz bfo is on the negative side of its cycle, both diodes are not conducting. ?When conducting, whatever voltage the audio waveform is at passes on through the conducting diodes and shows up at the crystal filter. ?(The audio changes very slowly relative to the 12mhz bfo, so we might think of the audio as a DC voltage here). ?When the diodes are not conducting, the resistor at R39 holds any signal passed on to the crystal filter to ground. ?The result is that the crystal filter sees the 1khz audio sine wave multiplied by a value of 1 when the diodes are conducting and a value of 0 when the diodes are not conducting, where those 1's and 0's are effectively a square wave at the frequency of the bfo. ?
Jerry, KE7ER
On Fri, Feb 24, 2017 at 10:04 am, Jerry Gaffke wrote:
Multiplying a sine wave with a square wave is very much like multiplying two sine waves
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