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RF Sampling using a 3 dB hybrid to generate I & Q

Started by mrberman87 3 weeks ago4 replieslatest reply 3 weeks ago73 views

I know that a 3 dB hybrid can generate a 0 degree and 90 degree output ports.  Can I sample these two ports synchronized in time to generate my I & Q signals? I have seen 3 dB hybrid's used to generate sin and cos signals for a heterodyne receiver, but I don't know if this will generate the signals as needed for the quadrature I & Q.

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Reply by SlartibartfastOctober 4, 2021

Yes, this is a very common way to build a receiver.   The specifications on the hybrid will be important to determine gain and phase balance between the I and Q channels.  The hybrid may be a significant source of distortion in this area, but if it meets your requirements it's a good way to do it.


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Reply by mrberman87October 4, 2021

Thank you for the reply!!


What kind of distortions does the hybrid usually introduce?  This will be part of a wide band digital pre-distortion system.  I guess as long as I can calibrate out the distortions of the analog front end, I can compensate for them digitally.

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Reply by SlartibartfastOctober 4, 2021

Especially with a wide bandwidth of interest it will likely not have a perfectly flat frequency reponse.   The gain and phase balance will likely not be perfect, either.

If you put a sine wave into the real side, the I and Q output will ideally trace a circle.   Gain balance errors will make the circle an oval with the long side on whichever axis has the higher gain.   Phase balance errors will tilt the oval off of the I or Q axes.  Phase balance is essentially the two channels not being exactly 90 degrees apart.

Both of those can be calibrated and adjusted, as well as frequency flatness or phase response.  If you're using it for a pre-distortion system the error compensations can be moved into the pre-distortion filter, but it will take some degrees of freedom or adjustment range from the pre-distorter to do so.

IF sampling is used a lot these days to avoid gain and phase balance distortions, but sometimes that isn't possible or practical.

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Reply by mrberman87October 4, 2021

Thanks for the information.  It sounds like normal I & Q sampling issues.  I assumed that whatever I did, I would have to worry about amplitude and phase imbalance, and that it would have to be calibrated out.  IF sampling will not work for my use case, I am trying to correct all output products of a wide-band amplifier, including spurious, without knowledge of the frequency or bandwidth of the signal of interest.


Thank you again for the help, your information has been invaluable!


Michael