Art Schaldenbrand
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Simon,
Two things:
1) See "Frequency Synthesis by Phase Lock", by W. Egan, in particular, Chapter 6 discusses Higher Order Loop filters.
2) Also, though I can't find a reference, I believe that the condition for stability is little different than you describe. It is more like the loop is stable if the slope of the magnitude at gain=1 is 20dB/dec. So as long as there is one more pole than zero in the loop, the loop should be stable.
A little additional discussion, if you think of the Barkhausen Criterion, oscillation is only sustained at gain=1. So the above rule is just implying the system is well-behaved at the unity gain cross-over, i.e., there is not enough excess phase from the higher order to poles to cause oscillation.
Best Regards,
Art Schaldenbrand
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