play_sdr: Problem in Low IF?

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BackgroundNoise
Posts: 6
Joined: Sat Nov 24, 2018 1:54 am

play_sdr: Problem in Low IF?

Post by BackgroundNoise » Thu Nov 29, 2018 11:45 am

Hello!

I have been trying to learn about play_sdr and the different options it provides. I may have found a problem with the Low IF mode.

We have three prominent AM stations in the region:
- 576 kHz;
- 630 kHz; and
- 702 kHz

These frequencies are below 60 MHz, so I believe they are block-converted to above 120 MHz first.

The following spectrogram was generated using data from the following command: playsdr -f 576000 -a1 -b 600 <output_filename>
This is Zero IF mode.
Home_f576kHz_BW600kHz.cs16_Spectrogram - resize.png
Home_f576kHz_BW600kHz.cs16_Spectrogram - resize.png (397.63 KiB) Viewed 8620 times
This plot makes sense i.e. 0Hz corresponds to 576 kHz and the other frequencies are in their expected places.

I then tried: playsdr -f 576000 -a1 -b 600 -i450 <output_filename>
I was hoping this would enable Low IF mode to reduce spurious signals.
Home_f576kHz_BW600kHz_IF450kHz.cs16_Spectrogram-resize.png
Home_f576kHz_BW600kHz_IF450kHz.cs16_Spectrogram-resize.png (419.09 KiB) Viewed 8620 times
This plot is not as is should be i.e. the frequency span of the signals has doubled.

Similar results are obtained, in each case, if a frequency of 702 kHz is selected.

Has the sample rate been automatically been reduced?
How can I check the SDR's internal settings?

Thank you!

p.s.
The program notes these default settings:
[-b Bandwidth in kHz (default: 1536)]
[-i IF bandwidth in kHz (default: 0)]

I think the -i setting relates to a centre frequency. Using "IF bandwidth" is distracting. Is this correct?

Does -b 600 -i 450 mean centre the 600 kHz bandwidth on 450 kHz?
Last edited by BackgroundNoise on Thu Jan 01, 1970 12:00 am, edited 0 times in total.
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BackgroundNoise
Posts: 6
Joined: Sat Nov 24, 2018 1:54 am

Re: play_sdr: Problem in Low IF?

Post by BackgroundNoise » Fri Nov 30, 2018 2:19 am

It seems that decimation is used.

In v2.11 of Software Defined Radio API is the following table:
SDR v2.11 API table.png
SDR v2.11 API table.png (79.8 KiB) Viewed 8493 times
Thus, the output sample rate is changed.

The following is from v3.01 of the API:
SDR v3.01 API table.png
SDR v3.01 API table.png (321.85 KiB) Viewed 8573 times
A change to the sample rate is not mentioned. It even notes that Decimate is "disabled by default".

Suggestion:
Add a "Users Guide" section to the API document or create a separate document.
In that section, or document, provide a selection of standard operating modes and settings.
Perhaps even show a couple of bad settings and the error messages that result.

The spirit would be to pass on the knowledge and experience that the developers have.

This will help everyone tap into the flexibility that SDR's provide.
Last edited by BackgroundNoise on Thu Jan 01, 1970 12:00 am, edited 0 times in total.
Reason: No reason

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