<html><head><style type="text/css"><!-- DIV {margin:0px;} --></style></head><body><div style="font-family:times new roman,new york,times,serif;font-size:12pt"><div>This is one of those details of Nyquist that is not usually explained very well.<br><br>The Nyquist limit applies to "real sampled signals" which means there are evenly spaced samples in time and assumed to be sampled with the I of an I/Q but not the Q part.<br><br>The Nyquist criterion occurs because you can get amplitude, but not phase information at the Nyquist frequency. Using "complex sampling" gives you a mechanism to go right up to the Nyquist limit because the I and Q samples give you both amplitude and phase. In practical terms you can never really sample any useful data anywhere near the limit with real sampling because of the alias filter requirements. It is only slightly better with complex sampling as evidenced by the 186 kHz display rather than all the way up
to 192 kHz. The other practical effect is that you get better phase and amplitude information with I/Q sampling than you do with real sampling at twice the rate. An I/Q sampling system at 192 ksps gives you the same number of samples as doing real sampling at 384 ksps, but the phase information is more robust with I/Q sampling because the two samples are not evenly spaced.<br><br>Said another way, you get both phase and amplitude information with every set of samples with I/Q basically right up to the limit. With real sampling, it takes a large number of samples to get the phase information to show up when operating close to the limit.<br><br>Ray Mack<br>W5IFS<br>------------------------------------------------------------------------------<br></div><div style="font-family:times new roman, new york, times, serif;font-size:12pt">***** High Performance Software Defined Radio Discussion List *****<br><div style="font-family:times new roman, new
york, times, serif;font-size:12pt"><br> I'm trying to understand why and how the bandscope width (I'm not <br>talking about the full spectrum bandscope on cuSDR) on cuSDR is twice as <br>wide and the bandscope width on PowerSDR at the same sampling rate (192 <br>Khz). PowerSDR provides a bandscope width that is roughly 1/2 the <br>sampling rate. This is what I have come to expect based on the fact <br>that using a 192 Khz sampling rate the highest frequency that can be <br>passed would be 96 Khz. But then I noticed that cuSDR provides a <br>roughly 192 Khz bandscope when using a 192 Khz sampling rate. At first I <br>thought that was impossible, but after playing with it for a while I <br>came to the conclusion that it was working properly.<br><br>So I'm trying to understand how that is possible. Is it because <br>PowerSDR has its heritage with I/Q sampling done in the analog domain <br>with analog "real" aliasing filters so
that for 192 Khz sampling the <br>data contains frequencies from 0-96 Khz, whereas cuSDR is depending on <br>the I/Q filtering being done in the digital domain with complex output <br>and therefore the I/Q data can contain frequencies from -96 Khz to 96 <br>Khz? Or is it that the FPGA is not filtering at ~96 Khz but at ~192 Khz <br>instead, so that the data contains aliased frequencies but somehow the <br>aliases can be determined via the separate I and Q streams? I have <br>severe doubts about the latter since I've always understood that once <br>aliased always aliased. Is there some other explanation?<br><br>How difficult would it be to "upgrade" PowerSDR to double the bandscope <br>width?<br><br>Thanks,<br><br>John<br>_______________________________________________<br></div></div>
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