[Skimmertalk] FYI, Some answers from Alex, VE3NEA, regarding automatic I/Q balance function

[Andreas Hofmann]

I had an email thread with Alex, the maker of CW Skimmer about this. Here are some good details that he wants me to share:

The correct way of using the automatic I/Q balance function is to enable both data collection and balance correction options, and let the software do its job. 
If there are no strong signals on the band, then you do not need balance correction - there are no images strong enough to be rejected! Once the strong signals appear, the balance will self-adjust.

DO NOT use any local signal sources, such as your transmitter or a signal generator, to build a balance curve. Such signals enter the receiver in many different ways, not only through the antenna connector, and the phase of the stray components is very different from that of the main signal. Worst of all, after the program learns the wrong curve from a strong local signal, it will have difficulties updating the curve using the weaker signals off the air.

Both Rocky and CW Skimmer in the SoftRock mode learn the balance correction curve per band. Every time you change the center frequency, the program loads the balance data for that frequency.

If the program has collected incorrect data, due to hardware malfunction or operator error, click on the Clear button and let the software collect new data. 
If a good curve suddenly gets replaced with a set of scattered points, this is probably caused by some strong signal, e.g., your own transmit signal. If you cannot get rid of such signal, collect the curve as explained above, and disable further data collection. Remember that every center frequency you use has its own balance data set.

Image rejection of automatic balancing is equal to the SNR of the signals used to build the balance curves: as soon as the software finds an image, it adjusts the balance to put that image below the noise. In other words, image rejection is adequate to the band conditions. I have seen images rejected by more than 90 dB.

Disable data collection only if you have balance curves destroyed on a regular basis. If this does not happen too often, it is better to keep the balancer running and updating its data: I have discovered that the antenna and receiver parameters change with time, and this requires some balance adjustment from time to time. Auto balancer is designed to track such changes, so keep it running whenever possible.

Thanks
Andreas, KU7T

-----Original Message-----
From: skimmertalk-bounces at contesting.com [mailto:skimmertalk-bounces at contesting.com] On Behalf Of Pete Smith N4ZR
Sent: Sunday, June 03, 2012 10:08 AM
To: skimmertalk at contesting.com
Subject: Re: [Skimmertalk] image rejection

What you know, you know right, Andy.  Other comments below.

73, Pete N4ZR
The World Contest Station Database, updated daily at www.conteststations.com The Reverse Beacon Network at http://reversebeacon.net, blog at reversebeacon.blogspot.com, spots at telnet.reversebeacon.net, port 7000 and arcluster.reversebeacon.net, port 7000


On 6/3/2012 11:47 AM, Andreas Hofmann wrote:
> 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.
yes - it is happening at audio frequencies
> - what has to happen to the two checkboxes in the i/Q balance screen when the adjustment is done?
correct balance must remain checked.
> - should I do this on a band that has stronger signals to begin with (i.e. 40 in the evening)?
Over-the-air signals can always be used for IQ correction, if they are loud enough (>25 dB SNR)
> -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.
Best not to overload the receiver, so 40 dB is plenty.
> When do I know I am done?
When the curve is matched to data points across the entire range.  At that point you should see another 30-40 dB of image rejection.  For all practical purposes, the images should be gone.
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