SDR Play low IF mode and images

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DaveB
Posts: 129
Joined: Sun Feb 01, 2015 6:04 pm

SDR Play low IF mode and images

Postby DaveB » 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
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Tech_Support
Posts: 196
Joined: Mon Jun 01, 2015 7:00 pm

Re: SDR Play low IF mode and images

Postby Tech_Support » Tue Nov 03, 2015 8:52 am

Dear DaveB,
We hope to have a better long term solution to the problem of unwanted images in Zero IF mode available in the not too distant future. Please keep an eye out for announcements.

Sincerely

SDRplay Tech_Support
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DaveB
Posts: 129
Joined: Sun Feb 01, 2015 6:04 pm

Re: SDR Play low IF mode and images

Postby DaveB » Tue Nov 03, 2015 11:00 pm

We hope to have a better long term solution to the problem of unwanted images in Zero IF mode available in the not too distant future. Please keep an eye out for announcements.


That is the fascination and excitement about Software Defined Radio - seeing and being part of on-going developements.

Your efforts are appreciated.

David
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