With good receiver hardware design, there's little reason to use
multiple frequency conversions in a SDR receiver. Every time you
introduce a frequency conversion into a receiver, you inevitably
compromise its dynamic range.
As Alberto said, leakage around the mixers can result in a tone at the
centre of the bandwidth, but this can be controlled by careful design.
Noise is another matter, but it shouldn't cause unncessary concern.
Amplitude domain noise will be limited and cancelled in balanced
switching mixers. The treatment of the phase noise component is
exactly the same as for a conventional receiver with an IF offset from
0Hz. It becomes a matter of reciprocal mixing. The fact that the IF is
around 0Hz is irrelevant from the point of view of reciprocal mixing
performance.
Of course there are problems with sound cards. Most laptops have awful
soundcard performance: they usually don't implement a stereo/two
channel input, and the actual sampling frequency can be several
percent removed from the theoretical value.
It's important to get a proper (allegedly) 'professional audio' sound
card if you are to get a PC based SDR receiver to work well. I use a
M-Audio 'Transit' USB box without problems under Windoze. (Trying to
get it work under Linux has been another matter).
I'm not interested in HF operation. I've had Alberto's software
operating in a 1.3GHz SDR receiver employing an AD 8347 demo board
with good results. I'm currently looking at a design for a 10GHz zero
frequency IF receiver. There's no reason why a zero frequency IF
receiver (or transmitter) can't be implemented at any frequency. I'm
aware of professional work on millimeter wave transmitters which used
a zero IF linearisation process, cartesian loop linearisation, which
effectively contains a two-phase zero frequency IF receiver.
Historically hams were making zero IF ssb (and AM/FM/PM) transmitters
and receivers back in the 1950s using analog tube technology. In fact,
it goes back further than that! The classic regenerative TRF detector
which was in use from the early 1920s onwards can be analysed as a
self-oscillating direct conversion receiver!
Implementing the same basic equations in the digital domain makes an
awful lot of sense.
73
Chris
GW4DGU
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