[hpsdr] Selectivity design/measurement

[Lyle Johnson]

> ...My question should have been much more specific, as:  "Has anyone built a 
> receiver out of HPSDR components and if so have they done any selectivity measurements?"

Janus+Ozy have been used with PowerSDR and an SDR-1000.

The QSD A/D converter system will likely impact the minimum discernible 
signal (MDS) unless there is a low-noise RF preamp in use.  It will most 
certainly have a great impact on the dynamic range and distortion products.

The selectivity is done in DSP and is pretty independent of the soundcard.

Mercury prototypes have been built and are documented on the wiki.  The 
don;t use a "soundcard" but instead do direct digital conversion of RF.

> I think your point about the DSP defining the selectivity means that the hardware components do not 
> contribute or detract meaningfully from selectivity. Is this correct... 

That is correct.  In more general terms, a bandpass filter is needed at 
RF to limit the signals the SDR is exposed to.  The usual selectivity - 
e.g., a 500 Hz CW filter, or 2.8kHz SSB filter -- is done in software 
and shoudl not be materially affected by the hardware.

Lyle


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