How is frequency selection (tuning) accomplished in a direct conversion receiver?

Question

How is frequency selection (tuning) accomplished in a direct conversion receiver ? A crystal filter can't be used is that correct ? Because there is no IF to mix with the incoming frequency band ...

Answer 1

For single-channel direct-conversion to baseband, tuning is done by setting the heterodyne or mixer oscillator frequency, and possibly by tuning band-pass filters, if any, in front of the direct-down-converter mixer. For AM or SSB, the oscillator is tuned to zero-beat with the carrier or suppressed carrier. For CW, the mixing oscillator is tuned slightly higher or lower than the CW signal frequency of interest to produce an audio side-tone. For digital modes a similar offset is used to mix the signal of interest into the audio input bandwidth of a computer sound-card or mono audio input interface.

The upper and lower sidebands around the tuned frequency end up aliased, so cannot be separated. But audio or baseband low-pass filtering can be applied to the resulting aliased baseband signal.

For dual-channel direct-conversion to quadrature/complex/IQ sampled data, DSP processing on the IQ data stream can separate out the upper and lower sidebands, and filter out (via FFT or otherwise) multiple signals somewhat offset from the mixer oscillator frequency, but sufficiently below half the sample rate in offset. This offset acts as a very low IF.

Answer 2

The IF (intermediate frequency) is not mixed with the incoming signal, but the LO (local oscillator).

Tuning in a direct conversion receiver is achieved just as it is with a superhet: by varying the LO frequency. A direct conversion receiver simply has an "IF" of 0 Hz. In fact another name for a direct-conversion receiver is a "zero-IF receiver".

A crystal filter would not be used because with the signal mixed down to baseband, typically the frequencies of interest are too low for a crystal filter to be applicable. Furthermore, the lower frequency means the fractional bandwidth required to get an acceptably steep roll-off is obtainable with a simple RC filter. And in common modern practice the analog filtering is limited to antialising for an ADC, with the final channel selection being performed by digital filters.