[Skimmertalk] image rejection

[Andreas Hofmann]

I am also having a hard time to fix mirror image.  In my case it occurs when stations are stronger than about 40db s/n. I had to take off my Skimmer off the air (RBN) as I do not want to send wrong spots (and even worse, not a new spot every minute).

Can someone really elaborate about how the i/q balance thing is supposed to be done?  There are not many documented instructions.  

Here is what I understand:
- It should be done with an antenna connected and a strong signal generated.  I used my normal station antenna for SDR and TX into dummy load.
- The TX signal should be moving across the whole band doing dits.  Is this correct?
- while doing this the I/Q balance must be on, with both check boxes checked.
When I do this, I see both curves adjusting.

Here is what I do not understand:
- is this independent of a band? I think it might, but not sure.
- what has to happen to the two checkboxes in the i/Q balance screen when the adjustment is done?
- should I do this on a band that has stronger signals to begin with (i.e. 40 in the evening)?
-When I run the mentioned routine, what dB of s/n should I expect the main signal and the image signal to be?  I was able to see the 40db difference when I did this. However
People mentioned that the signal was supposed to be turned down, so I am not sure if I should generate a signal stronger than 50db or not.  This is really the most unclear item for me.
When do I know I am done?

Thanks and 73
Andy, KU7T


-----Original Message-----
From: skimmertalk-bounces at contesting.com [mailto:skimmertalk-bounces at contesting.com] On Behalf Of Mark n2qt
Sent: Friday, June 01, 2012 3:31 PM
To: skimmertalk at contesting.com
Subject: [Skimmertalk] image rejection

Today my 6M Softrock skimmer starting spotting stations at their image frequency.  I noticed the I/Q balance had lost it's original smooth curve and now had a lot of scattered results.  I cleared the results and as learning re-occurred the images went away.

Images were occurring whenever the receive signal strength exceeded about 38 db snr.

I normally leave the learning mode enabled but now I think that might be a mistake once a good learning curve has been established.  (as mentioned the best way to get a curve quickly is sweeping a fairly loud signal generator across the band.)


Mark n2qt
(now back to listing all the 6 digit grids from various beacons in the bad call list)


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