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measuring the spectrum of an ideal PWM output wave

terminator12

friends,

    I generated the PWM wave by giving a 1kHz sine wave of amplitude 200 mV(peak) at the positive input of an ideal VCVS and a 1V, 640kHz (peak-peak) triangular carrier to the negative input of the VCVS. Then I pass the PWM signal into a second order butterworth filter (using an SVCVS). When I probe the spectrum at the output of the filter, I see that the input signal is heavily distorted. Theoretically an ideal, continuous-time PWM (NPWM) is distortion-free. Am I missing some setting in cadence.

 The settings are:

VCVS gain : 10000

VCVS Min output voltage : 0

VCVS Max output voltage : 1

Strobe period: 1/(Triangle-freq*32)

Maxstep :  1/(Triangle-freq*32*16)

Reltol : 1e-4

Transient mode: Conservative

 

thank you in advance

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