[hpsdr] PowerSDR vs. cuSDR bandscope width

[Glenn Thomas]

Restating John's question, how is it possible to recover 186 KHz of 
bandwidth from a set of samples with a Nyquist frequency of only 98 KHz?

My uneducated guess (from the 40,000 foot level) is to observe that 
HPSDR isn't really using a 192 KHz sample rate. The HPSDR hardware uses 
a sample rate of many tens of MHz. Presumably, the cuSDR display is 
driven by data derived from this much higher sample rate.

As John suggested, PSDR assumes that it has an analog front end and that 
the sample rate really is 192 KHz, so the PSDR pan display can't be any 
wider than the Nyquist frequency..

Corrections or confirmations from someone who really does know?

73 de Glenn wb6w

On 11/22/2012 11:40 AM, Doug W5WC wrote:
> ***** High Performance Software Defined Radio Discussion List *****
>
> John,
>
> The PowerSDR displays ~186kHz of the 192kHz spectrum with the 'Zoom' control
> to the far left. The center of the panadapter is 0. A percentage of the high
> and low ends are not displayed because it has little useful information and
> makes for a cleaner display. Totally the choices of the user and the
> software designer.
>
> 73, Doug
> W5WC
>
> -----Original Message-----
> From: hpsdr-bounces at lists.openhpsdr.org
> [mailto:hpsdr-bounces at lists.openhpsdr.org] On Behalf Of John Marvin
> Sent: Thursday, November 22, 2012 1:13 PM
> To: hpsdr at lists.openhpsdr.org
> Subject: [hpsdr] PowerSDR vs. cuSDR bandscope width
>
> ***** High Performance Software Defined Radio Discussion List *****
>
>    I'm trying to understand why and how the bandscope width (I'm not
> talking about the full spectrum bandscope on cuSDR) on cuSDR is twice as
> wide and the bandscope width on PowerSDR at the same sampling rate (192
> Khz).  PowerSDR provides a bandscope width that is roughly 1/2 the
> sampling rate.  This is what I have come to expect based on the fact
> that using a 192 Khz sampling rate the highest frequency that can be
> passed would be 96 Khz.  But then I noticed that cuSDR provides a
> roughly 192 Khz bandscope when using a 192 Khz sampling rate. At first I
> thought that was impossible, but after playing with it for a while I
> came to the conclusion that it was working properly.
>
> So I'm trying to understand how that is possible.  Is it because
> PowerSDR has its heritage with I/Q sampling done in the analog domain
> with analog "real" aliasing filters so that for 192 Khz sampling the
> data contains frequencies from 0-96 Khz, whereas cuSDR is depending on
> the I/Q filtering being done in the digital domain with complex output
> and therefore the I/Q data can contain frequencies from -96 Khz to 96
> Khz? Or is it that the FPGA is not filtering at ~96 Khz but at ~192 Khz
> instead, so that the data contains aliased frequencies but somehow the
> aliases can be determined via the separate I and Q streams? I have
> severe doubts about the latter since I've always understood that once
> aliased always aliased.  Is there some other explanation?
>
> How difficult would it be to "upgrade" PowerSDR to double the bandscope
> width?
>
> Thanks,
>
> John



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