EMRFD Message Archive 12159
Message Date From Subject 12159 2016-01-04 19:51:25 lmeeny Phase / Gain Adjustments in a Phasing DCR Hello,
I'm redesigning a double balanced Tayloe mixer based phasing DCR incorporating changes driven by the prior 'not as good as it should be' effort with significant guidance from W6JL. I want to adjust out initial differences in gain and 90 degree phase shift between the audio pre-amp outputs after the mixer through the analog IQ phase delay output paths.
Gain calibration's relatively easy. A resistor summer across the delay circuit I & Q outputs with a small value pot in the center allows a one point balance of path gains.
I'm unable to find an example of a phase difference trim for the two audio pre-amps.
Any suggestions welcome.
12160 2016-01-05 02:43:59 Alberto I2PHD Re: Phase / Gain Adjustments in a Phasing DCR 12162 2016-01-05 06:24:18 jorschei Re: Phase / Gain Adjustments in a Phasing DCR
Hi Ed W2GHD,
In the NC2030 documentation from Dan Tayloe is given an explenation how to adjust the amplitude and phase correction. Study Dan his NC2030 documents
In the PE1KTH files you see the amplitude correction over the LT 6231 Op - Amp suppresing the opposite site band - 35 dB.
12163 2016-01-05 12:56:25 lmeeny Re: Phase / Gain Adjustments in a Phasing DCR Jons,
Thank you for the reply. I have read the NC2030 documentation in detail. If you're going to steal steal from the best hi hi.
Here's what puzzles me. I have been told that if the outputs of the audio pre-amps are not exactly 90 degrees apart that the all passnetworks can not correct for that. I've always thought that the phase shift circuits COULD compensate for phase delay errors but have been told otherwise. What am I misunderstanding?
12164 2016-01-05 13:32:06 J van Scheindelen Re: Phase / Gain Adjustments in a Phasing DCR
Dan has done a great job designing the NC2030 and describe the amplitude compensation and the Phase shift compensation.
Amplitude compensation is one part bud to get the maximum opposite sideband suppression also the phase shift is necessary to improve it up to - 40 dB or better,schould be exact 90 degrees.
Dan design is in my opinion the best direct conversion sampler detector there is because it has also an filter function and low in channel los.
To get the best results you need both amplitude and phase compensation with the op- amps.
Not al frequency components in the audio channel are shifted exact the same phase in the shifter bud for narrow 300-1500 Hz bandwidths it is not a problem. Try it out ....
My front PCB is an optimal RF layout and experiment to try the Si5351A clock generator as VFO and it work well.
Thanks to Dan’s design.
12166 2016-01-05 16:29:47 Tayloe, Dan (Noki... Re: Phase / Gain Adjustments in a Phasing DCR
You can fix the phase error by using an “all pass” that is not a perfect “all pass”. There is an I and a Q side of the phasing network. The phase shift vs. frequency starts dropping lower in frequency on one side than the other side. One side starts dropping early in phase shift vs. frequency, and the other starts late. The big picture is that when the difference in these two phase shifts vs. frequency plots is 90 degrees, then we can get good opposite sideband direction.
Suppose one side of the phase network uses fixed R/Cs, so that the phase/frequency slope is fixed, and suppose the other side has some trimmers. These trimmers allow the second phase vs. frequency to be moved around a bit. This allows the one side to shift so that a 91 or 92 degree difference is maintained or allows the difference to be 89 degrees at the low frequency side and 91 degrees at the high frequency side (for example). Thus if the detector is a few degrees off between I and Q, the second “variable” side can be tweaked to compensate for a constant 2 degrees of error (for example). If the detector does not maintain exactly 90 degrees over a range, the trimmers of the “variable” side allow the slope of the phase fall-off to be modified from nominal to compensate for this to a certain degree.
The NC2030 did this for a two stage phase compensation network. It would probably be pretty tough to do that for a broader 3 stage SSB network without some processor driven algorithm or via measurements and calculated changes.
- Dan, N7VE