Adjacent channel interference...I wonder

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kb5udf

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Hi all, food for thought/consider the following:

While monitoring the LWIN system (scott, la) site with the usual cheap dongles, I noticed Unitrunker
periodically indicated that control channel was usually a solid decode (green), but periodically dipped
into the red, and sometimes, briefly dropped altogether. I began to wonder if this is the LSM distortion
that many talked about, but really wasn't sure the reason for the dropouts.

So, after a time, I fired up my airspy with SDRSHARP. This allowed me to have a graphic representation
of all the 700mhz repeater outputs on the LWIN system within range of my location. What I found was that when
I was experiencing strong dips or dropouts on the control channel using the dongles, it coincided with a repeater coming on the air very near by in frequency, usually within a few khz. Thus, the cause of my dropouts looks like adjacent channel interference. The front end of the dongle is too wide to deal with very close strong signals, and so decoding of the desired signal is harmed.

This situation leads me to wonder, how often (if at all) is what people my fellow scanists believe to be LSM distortion or the like, actually related to adjacent channel interference?

Please note, I am in no way suggesting simulcast problems are not real. Rather, I am proposing an additional, and potentially confounding variable, that I suspect consumer scanners may also suffer from.

regards

JB
 

jonwienke

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The RTL dongles have no front end filtering, so they are highly susceptible to images and adjacent channel interference. Scanner receivers such as the Uniden BCDx36HP have excellent adjacent channel filtering. It can happen, but is far less likely with a decent scanner receiver than a RTL dongle.
 

kb5udf

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Thanks

Probably also limited filtering (but I don't know) my airspy suffers from the same problem in unitrunker, albeit not quite as bad as the dongles.
 

RFI-EMI-GUY

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Front end filtering normally cannot help this, if it is in band, where the IF filtering (channel bandwidth ) is the solution. How many KHZ? Narrowband Channels are on 12.5 and 25 KHZ channel centers. If it is something odd, you might have an image or spurious problem. If it is spurious, or image or IMD, then yes front end filtering may be needed. It is good practice.



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