At 08:13 PM 6/4/2005, you wrote:
>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.
I'm interested in the high tech solution, which (currently) is a high speed
analog (electronic) switch driven at 4X the desired receiving frequency. It
has advantages in ultimate sensitivity and simplicity, even though ultimate
sensitivity gains aren't useful at HF due to naturally occurring
terrestrial noise sources.
The high speed analog switches (that I know of) limits the maximum
receiver frequency to about 75 Mhz.
I would think the technology would really shine at 2 meters (and higher),
where many mixers can be eliminate by direct conversion.....but, where do
we get a switch that can operate at 600 Mhz? I think counters can go that
high, so clocking the electronic switch is currently possible.
Is there a lower tech approach that does away with the 4X local oscillator
and switching speed requirements?
I'd like to see some discussion on alternative means of direct conversion
that offer high performance and opposite sideband rejection. I can't even
imagine anything working well at 10 GHz. Show me the way please.
GL to all.
Art
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