play_sdr: Problem in Low IF?
Posted: 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. 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. 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?
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. 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. 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?