SDR Play low IF mode and images
Posted: Mon Nov 02, 2015 8:22 pm
I don't normally use the Low IF mode but with the removal of the I/Q imbalance correction in SDR# later versions I thought the LOW IF mode would be a good workaround - albeit only around 1.6MHz bandwidth.
Unfortunately - and it was not totally unexpected I found weak but annoying images at 2 x the sampling rate of 2.048 MHz on strong FM broadcast stations - these appear 4.1 MHz below the station. i.e 89.1 appears at 85.0, 91.3 at 87.2. Ok I know these ones are outside the broadcast band but the next on - 93.5 will appear on 89.4, 102.8 (my nemesis) will appear on 98.7 and 100.9 will appear on 96.8. The images are about 60dB below the strength of the actual station - but that gives 15dB or noise-free mono.
This is not quite as bad as the images that you get in zero IF mode without the I/Q balance being adjusted and is a limitation of the low IF mode.
I had hoped to suggest to techsupport that they look at whether a low IF mode with x2 decimation at 8.912 MHz would be possible as it would give a bandwidth of around 3.6 MHz (if I understand things correctly from the API documentation) - but unless the resulting image 8.1 MHz is attenuated by at least another 10dB it is not worth the effort - though a comment from techsupport would be appreciated.
It is consideration for any SDR using Low IF architecture
Unfortunately - and it was not totally unexpected I found weak but annoying images at 2 x the sampling rate of 2.048 MHz on strong FM broadcast stations - these appear 4.1 MHz below the station. i.e 89.1 appears at 85.0, 91.3 at 87.2. Ok I know these ones are outside the broadcast band but the next on - 93.5 will appear on 89.4, 102.8 (my nemesis) will appear on 98.7 and 100.9 will appear on 96.8. The images are about 60dB below the strength of the actual station - but that gives 15dB or noise-free mono.
This is not quite as bad as the images that you get in zero IF mode without the I/Q balance being adjusted and is a limitation of the low IF mode.
I had hoped to suggest to techsupport that they look at whether a low IF mode with x2 decimation at 8.912 MHz would be possible as it would give a bandwidth of around 3.6 MHz (if I understand things correctly from the API documentation) - but unless the resulting image 8.1 MHz is attenuated by at least another 10dB it is not worth the effort - though a comment from techsupport would be appreciated.
It is consideration for any SDR using Low IF architecture