Panadapter Frequency Span

spinomaly
Posts: 33
Joined: Fri Oct 20, 2017 5:44 pm

Panadapter Frequency Span

Postby spinomaly » Sat Dec 02, 2017 12:31 am

Besides the zoom slider and buttons is there a way to modify the span in panadapter. For example, can I view the entire 7M band? I cannot seem to find a way to do this.
Dr. Loren Betts
W6VH
spinomaly
Posts: 33
Joined: Fri Oct 20, 2017 5:44 pm

Re: Panadapter Frequency Span

Postby spinomaly » Sat Dec 02, 2017 12:45 am

(edited) For some reason I could not tune above 7.195 MHz for some reason with all the various tune methods. Closing the app did not fix it. I ended up looking around at the various dialogs, collapsed the display and expanded it and it then let me tune above.

However, it seems like the total span of the panadapter is much less than it was before.
Dr. Loren Betts
W6VH
spinomaly
Posts: 33
Joined: Fri Oct 20, 2017 5:44 pm

Re: Panadapter Frequency Span

Postby spinomaly » Sat Dec 02, 2017 1:32 am

Well not sure if I should delete the post or not. Decided to keep it in case someone else needs the answer. For the span issue I had adjusted the Audio->Primary Sample Rate to a lower value when I was playing around with the audio. This causes the span to be reduced. Increasing it to max solved that issue.
Dr. Loren Betts
W6VH
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w-u-2-o
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Joined: Fri Mar 10, 2017 1:47 pm

Re: Panadapter Frequency Span

Postby w-u-2-o » Sat Dec 02, 2017 2:00 am

Loren,

If you are not familiar with the RF architecture of direct sampling SDRs, then perhaps the following will be beneficial...

The ADC, running at 122.88MHz, provides an instantaneous bandwidth of half that (Nyquist theory) or 61.44MHz, less some guard band, etc.

At 14 or 16 bits per sample this creates a bit rate that is unsupportable over 100Mbit/s Ethernet, or even over Gigabit Ethernet. Therefore there needs to be a downconversion performed to an IF frequency and bandwidth. This is done digitally, of course, in the FPGA, and the resulting digital IF is at baseband and consists of quadrature (I and Q) samples at the specified Primary Sample Rate. Since this digital IF data stream consists of I and Q samples the resulting bandwidth is equal to the sample rate (more Nyquist theory). This creates a data stream of 32 bit samples (16 bits of I, 16 bits of Q) at the specified sample rate. For example, 6.144Mbit/s when 192KHz is selected as the sample rate. Such a data stream is easily supportable over 100Mbit/s Ethernet.

Historical note: the reason this setting is on an "Audio" section is because, in the very beginning days of the openHPSDR project, data was delivered from the radio over a stereo audio output (left = I, right = Q), input to a PC audio input, and re-digitized by the PC. Those days are long gone, but the sad legacy of the original PowerSDR user interface lives on.

Hence the instantaneous bandwidth of the panadapter/waterfall is equal to the Primary Sample Rate setting.

Within PowerSDR, there is then performed another digital downconversion to a second IF (note that the actual math used is not really analogous of an analog mixer, but the functional effect is the same). This downconversion starts with the receiver passband and results in a real time domain signal at baseband and with a sample rate of 48KHz. As the internal second IF sample rate is always 48KHz, this limits the instantaneous bandwidth for modulation/demodulation to 24KHz per Nyquist, the actual implementation includes some guard band and thus limits it to 20KHz.

73,

Scott
spinomaly
Posts: 33
Joined: Fri Oct 20, 2017 5:44 pm

Re: Panadapter Frequency Span

Postby spinomaly » Sat Dec 02, 2017 2:29 am

Thanks Scott. I am familiar with what you describe below but it took me a little time to determine this was in the audio section.
Dr. Loren Betts
W6VH

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