Electronics > RF, Microwave, Ham Radio

Understanding the phase behaivor of two diplexers

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e.sotillet:
Hi Folks!!
 I've two different diplexers (same architecture HP & LP, order 7 but different component values), they have almost the same amplitude response but the phase response is quite different. In fact, one of those (with the large phase change) cause an oscillation in a posterior amplifier.


The diplexer is part of a bidirectional amplifier. My hypothesis is correct? The abrupt phase change may cause the oscillation?

Best regards

fourfathom:
That top one isn't an abrupt phase change.  Instead, you are seeing the phase transition through 180 degrees, so the display snaps from -180 to +180, which is actually the same value.  It's a display thing, not a phase behavior thing. If you were to offset the phase by about 90 degrees the two displays would look fairly identical. [edit: no they wouldn't]
It's still possible that associated impedance change of the diplexer is causing your amplifier oscillation, but again, this is not an abrupt phase change.

e.sotillet:
So, the phase response of both diplexers are ok? 

Yes, I will try using a broadband load (attenuator) to reduce the impedance variations in the band of interest.


Best regards

fourfathom:
Sorry, I was wrong about the two plots being similar.  The top plot shows a phase shift change totaling about 675 degrees (somewhat monotonic constant phase change vs frequency over the range shown), while the lower one shows a non-monotonic phase change ranging about 100 degrees or so.  What is probably more important is the impedance presented to your amplifier, and from the charts shown, these two designs are likely too have significantly different impedances.

But what I did get right is that the top chart does not have an abrupt phase change.

coppercone2:
I noticed that phase thing on VNA before, first time I saw it I thought WTF.

Usually you deal with phase on a polar plot, which is why its so weird.

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