Hi Frank, thanks for your quick comment.
Frank Wiedmann wrote on Dec 12th, 2012, 1:09am:First of all, the cmdmprobe is obsolete and does not work well if a circuit is not completely symmetric. You should use the diffstbprobe instead.
diffstbprobe was introduced in MMSIM10.1. Currently I only have access to MMSIM06.21.431_LNX86, where there is only cmdmprobe.
Frank Wiedmann wrote on Dec 12th, 2012, 1:09am:To simulate the loop gains in this circuit, I would probably first simulate the feedback amplifiers by themselves and verify their stability (as you did).
The amplifier in in-phase path without I/Q cross-coupling has no stability problem, as shown in Fig. 3. Thanks.
Frank Wiedmann wrote on Dec 12th, 2012, 1:09am: To examine the complete circuit, I would then place the diffstbprobe in the global feedback path between the amplifiers (in series with the resistors). This treats the amplifiers together with their local feedback as "feedback amplifier units" (that have already been verified for stability).
By putting the probes in the local feedback path while the global feedback is active, you combine the effects from both loops so that the results are very difficult to interpret.
Please see Fig. B1 and Fig. B2 for the "global feedback" loop response, where the cmdmprobe was added in the coupling path from I to Q. We can see that the global feedback is stable, as suggested by the PM of 30 deg and GM of 35 dB. So my doubt is that when the global feedback is stable, shall we care about the "local feedback" stability as shown in Fig. 2? Are we supposed to have "absolute" stability in every feedback loop? The simulation of the "local feedback" loop response should be correct. I derived the local feedback loop response and obtained the bode plot using matlab, which is close to Cadence stability results.
Is the "local feedback instability" a special phenomenon for polyphase filter and can be ignored?
Thank you.