Re: LF: Conundrum - image cancelling mixer

[MarkusVester@aol.com]

Dear Andy and LF group,

we had that very same discussion at work about a year ago... can the  multi-octave IF quadrature network not be replaced by a simple narrowband RF phase-shifter? No, it can't.

As you say, the first circuit with quadrature RF and LO drives will actually cancel one of the IF output frequencies, eg. the sum frequency, and deliver only the difference. But that is not what you normally want, because it does not differentiate between the two possible RF input frequencies, signal and image. With low IF, these are closely spaced and difficult to separate by a preselector filter.

Example: Direct conversion receiver for 14.2 MHz USB voice. Circuit Nr. 1 is not a true SSB RX, it will convert both 14199 and 14201 kHz to 1 kHz audio. It would however cancel the IF sums near 28.4 MHz, but this is much more easily done with even the most simple IF lowpass. - Circuit Nr.2 is right, it will accept 14201 and produce both 1 and 28401 from it, and it will reject any input at 14199 kHz.

For a transmitter, the same holds true. Circuit 1 would be insensitive at 28.4 MHz "audio", but not be able to split between RF outputs at USB and LSB.

There is a third variant with quadrature LO and IF which will also provide a true SSB mixer. This is advantageous if you need a large tuning range, as the LO is only a single frequency at a time, and digital LO-divider schemes can easily provide matched 90-degree outputs. Also, thanks to mixer saturation, amplitude balance is less critical.

73 de Markus, DF6NM