[hpsdr] PowerSDR vs. cuSDR bandscope width

[Doug W5WC]

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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