[Elecraft] Quadrature down-sampling

[Don Wilhelm]

Gary,

That part has nothing to do with the filtering - the next step is to 
convert both the I and Q signals to digital and do the manipulation 
mathematically - that includes filtering, detection, and most other 
signal processing functions.  The result is then converted to an analog 
audio signal for the final amplification.

Yes, the amount of flexibility is limited only by the amount of DSP code 
and speed of the DSP processing - all manner of "magic" can be performed 
once the signal is placed into the digital "number soup".

That is the normal process for a Software Defined Radio - some use a 
computer soundcard to do the digital/analog conversion and the processor 
does the computation, but there is no reason why that process cannot be 
done using a dedicated processor.  The K3 already works the same way, 
but the frequency for the ADC is higher - that really makes little 
difference.  Using the mixer to convert directly to baseband is quite 
common in SDR receivers.  While we are getting close to the time when 
direct digital sampling from the antenna is feasible, it is not yet 
available at a price we would want to pay.

73,
Don W3FPR

On 7/26/2011 11:43 AM, Gary D Krause wrote:
> Thanks for the explanation, Alan. So, my next question is, how does it compare
> with a crystal filter? It is better? Does it give you more flexibility with
> filtering?
>
> Thanks,
> Gary, N7HTS
>
>
> On Tue, 26 Jul 2011 08:08:38 -0700
>    Alan Bloom<n1al at sonic.net>  wrote:
>
>       On Tue, 2011-07-26 at 08:44 -0600, Gary D Krause wrote:
>
>           I've been wondering what Quadarature down-sampling is? This is the
> first that I have heard of it.
>
>
>       It's like a two-channel direct-conversion receiver. The two channels
>       are on the same frequency but use local oscillators that are 90 degrees
>       out of phase. With suitable processing, the in-phase (I) and quadrature
>       (Q) mixer outputs give both the amplitude and phase of the signal.
>       Among other things, that makes it possible to suppress the unwanted
>       sideband of an SSB signal without a crystal filter.
>
>       Alan N1AL
>
>
>
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