Creating a Nyquist plot with Cadence

Cadence has a stability analysis that returns loop gain (return ratio). Typically, this is done using a log sweep of frequency. The reason is that doing a uniform linear sweep results in too large frequency steps for low frequencies and/or too large of a step for higher frequencies.

Unfortunately, when you do a logarithmic sweep, you can’t cover from negative to positive frequencies–which is what you want when you generate a Nyquist plot. With the code below, you can reflect the positive frequencies onto negative to get the full Nyquist plot:


; load “~/cadence/skill/abConcatWaveforms.il”
lg = -getData(“loopGain” ?result “stb”)
; abConcatWaveforms externally defined
lg2 = abConcatWaveforms(flip(conjugate(lg)) lg)
w = newWindow()
ocnYvsYplot(?wavex real(lg2) ?wavey imag(lg2))
stb_x = xmin(abs(lg2-complex(-1.0,0.0))**2)
stb_margin = value(lg2, stb_x)
addTitle(sprintf(nil “stb_margin = %f dB” -dB20(stb_margin)))
xLimit( list(-1.6 0.2) )
yLimit( list(-0.5 0.5) )

You’ll need Andrew Beckett’s abConcatWaveforms function defined and loaded.

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