[Elecraft] K3: course wanted: Panadaptor 101

[Larry Phipps]

Corrected text...

I don't have a SDR, but have been pursuing this idea on my LP-100 
reflector with some sharp guys who do. What I have been able to gleen is 
that with the best sound card currently available (which requires a 
Firewire interface), you can get 192 kHz sampling. *After filtering to 
avoid aliasing, I assume this would provide maybe 90 kHz of clean 
display. *With the best board tested by QST, the bandwidth would be 
about half that.

The dynamic range would be in the 100-110 dB range... adequate for a 
panadapter in a normal RF environment. The ADC will overload in the 
presence of strong signals, however, unless you pad the signal from the 
IF. I assume this would result in a flat line on the display at the top. 
There is the chance that signals outside the display width could cause 
interference as well, since the only filtering would be the bandpass 
filters in the K3, and a 3-pole filter on the input of the softrock, but 
they would have to be very strong.

Adding some attenuation between the K3 and softrock (beyond what would 
be needed for proper interfacing to the K3 IF output), would help 
protect the ADC in the soundcard, but you would lose some sensitivity. 
The amount would depend on the strength of the ADC. Depending on 
soundcard, 10 dB would give you a range of  about -10dBm to -110 / 
-120dBm between MDS and 1dB compression point... a bit less if the upper 
limit is more stringent. This should be fine for most 
locations/situations, although I can think of a couple very specific 
exceptions.

To sum up, I would expect that you would spend about $250-400 for this 
setup, assuming you already have the PC, and depending on the level of 
sound card you use, with the high end required for a BW > ~45 kHz. You 
would not have the ability to point and click to jump to a signal like 
the Z90/91 offers... although it's possible that some enterprising soul 
could add this feature to one of the open source SDR apps I suppose. And 
you don't have much portability.

73,
Larry N8LP



Jack Smith wrote:
> A 192 kb/s sample rate gives you a usable bandwidth (Nyquist = sample 
> rate/2) of 96 KHz, assuming a brick-wall anti-alias filter. More 
> likely, the usable bandwidth will be about 80 KHz with a practical 
> anti-alias filter.
>
> Jack
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> .
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