Channel Skew Adjustment Procedure

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hfsdpk11
Posts: 28
Joined: Mon May 01, 2017 5:00 pm

Channel Skew Adjustment Procedure

Postby hfsdpk11 » Wed May 24, 2017 2:46 pm

Would someone please give an explanation how best to adjust the controls in the MAIN "Channel Skew Adjustment" panel? I take it these are controls to adjust the I/Q channels but haven't been able to find an explanation on how to adjust them or what parameter(s) to monitor when making an adjustment. I'm guessing that these adjust the orthogonality and amplitude balance of the I/Q channels but don't know what best to look at; the only thing I can think of is the level of the residual LO that's seen on the spectrum display however I don't see any significant changes when moving the sliders or changing the channel delays. All I can say is that disabling "Automatic I/Q Correction" seems to reduce the LO leakage even with the slider and delay adjustments at their default zero values. Thanks.
Last edited by hfsdpk11 on Thu Jan 01, 1970 12:00 am, edited 0 times in total.
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g1hbe
Posts: 240
Joined: Sat Jan 17, 2015 3:28 pm
Location: Cheshire

Re: Channel Skew Adjustment Procedure

Postby g1hbe » Fri May 26, 2017 3:50 pm

I've found the best way to adjust I/Q balance is to tune in a very strong signal and position it on one side of the waterfall display. Look in the same position on the other side of the display for a mirror image. If you can't see one, then you probably don't need to adjust anything!
If you do see one, adjust the phase and amplitude sliders (or whatever controls you have) to minimise the image. I've found it's easy to get 60dB of rejection and with some more careful adjustment you can get even better.
All the above is done with the software running in 'zero IF' mode.

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Andy

hfsdpk11
Posts: 28
Joined: Mon May 01, 2017 5:00 pm

Re: Channel Skew Adjustment Procedure

Postby hfsdpk11 » Sun May 28, 2017 1:09 pm

Thanks. I'm not seeing any images although the noise floor is only ~35 dB. below the strongest signals I can find so I guess the settings are ok where they are. Curious that in my case (RSP2 pro) the residual LO level is much better with automatic I/Q correction disabled and setting the controls to zero. It will be interesting to try optimising with a signal generator at the input to maximise the SNR.

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hfsdpk11
Posts: 28
Joined: Mon May 01, 2017 5:00 pm

Re: Channel Skew Adjustment Procedure

Postby hfsdpk11 » Tue May 30, 2017 2:01 pm

g1hbe:

I did some adjustments after finding a VHF signal 50 dB. above the noise and managed to null out the image with some amplitude and phase adjustment, so thanks again for your guidance. The adjustment wasn't much, only a tick on amplitude and about 1.3 deg in phase.

There are four slider selections "B, 1, 2 and 3", all of which require the same ampl and phase settings at the same test frequency, however it's not clear why there should be four selections for effectively the same adjustment. Why not just "B" ? and what do "B, 1, 2 and 3" stand for? Do the four selections serve as presets in the event that the I/Q balance varies with frequency? e.g.: B="baseline", 1,2,3 = "alternate settings 1, 2 and 3"?

Cheers,

hfsdpk11, aka ve3ktn.

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g1hbe
Posts: 240
Joined: Sat Jan 17, 2015 3:28 pm
Location: Cheshire

Re: Channel Skew Adjustment Procedure

Postby g1hbe » Wed May 31, 2017 7:43 am

I must admit I haven't seen the 1,2,3 settings, and I'm away from home now so I can't check. Optimum balance does seem to change with frequency, so perhaps they are pre sets as you suggest. I've found the auto setting works well enough. Your noise level seems high (or your signals low). Is everything ok with your system? My general daytime noise level is around -120 dBm on HF and MF, and strong B.C. stations can be -35dBm. All with SPRUno and a Wellbrook 1530 loop outside at 20 feet.

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Andy


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