v1.223: CIC filter compensation implemented, possible DRM benefits
I should have done this long ago, but I finally figured out a simple way to implement CIC filter compensation by empirical measurement. The before and after pictures below show the result. No more 10 to 15 dB roll-off at the ends of a wide passband. This will improve wide-band AM frequency response and more importantly wide-band IQ mode uses. DRM in particular might have an easier time without the roll-off (see DRM PSD plots below).
IQ mode, 12 kHz passband, 10 dB/div:
(display is from the new audio FFT)
With CIC filter compensation. Note flat passband:
DRM PSD before and after compensation:
Attachments:
https://forum.kiwisdr.com/uploads/Uploader/08/6df1555724dd0ffc7e95695fdabe22.png
https://forum.kiwisdr.com/uploads/Uploader/a1/7bca33c3144de8b203b9f8a19d862b.png
https://forum.kiwisdr.com/uploads/Uploader/6b/95bbfc2e91e03bde34047052a51da9.png
https://forum.kiwisdr.com/uploads/Uploader/36/6de24562129d9d418a68613df820db.png
IQ mode, 12 kHz passband, 10 dB/div:
(display is from the new audio FFT)
With CIC filter compensation. Note flat passband:
DRM PSD before and after compensation:
Attachments:
https://forum.kiwisdr.com/uploads/Uploader/08/6df1555724dd0ffc7e95695fdabe22.png
https://forum.kiwisdr.com/uploads/Uploader/a1/7bca33c3144de8b203b9f8a19d862b.png
https://forum.kiwisdr.com/uploads/Uploader/6b/95bbfc2e91e03bde34047052a51da9.png
https://forum.kiwisdr.com/uploads/Uploader/36/6de24562129d9d418a68613df820db.png
Comments
Cheers
PK
Cheers
PK
V1.222 FIR
v1.223 CIC
Hi John,
Could this also potentially affect the TDoA calibration ?
Regards,
Martin - G8JNJ
Regards,
Martin - G8JNJ
Before looking at wide-band digital signals I recorded the OTHR signal from Cyprus and one CODAR signal in order to convince myself that the signal reconstruction out of three IQ datastreams works as expected, see
https://hcab14.blogspot.com/2019/01/kiwisdr-iq-data-streams-with-2025-khz_29.html
There you can find also an example of how to use kiwirecorder.py which might be helpful.