[hpsdr] PowerSDR vs. cuSDR bandscope width

[John Sager]

Glenn,

You need to get your head around the concept of negative frequency -
Google for it. The signal has both in-phase (I) and quadrature (Q)
components and they, together, consist of both positive and negative
frequencies. Because both are sampled at 192kHz, they each contain
components up to 96kHz, and so the band that they represent ranges
from -96kHz to +96kHz. The centre is whatever the first (digital)
local oscillator is tuned to. Hermes can capture frequencies from 0
to ~60MHz. If the digital LO in Hermes is set to 28.1MHz, then the
I&Q output at 192kHz sampling rate represents 28.004 to 28.196MHz
(minus a bit due to necessary filtering).

regards,

John G8ONH

On 22/11/12 20:18, Glenn Thomas wrote:
> ***** High Performance Software Defined Radio Discussion List *****
> 
> 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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