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Phase Noise Setup doesn't work in port/vsource

VIP of NH

I'm simply simulating pss and pnoise for a port/vsource, with added phase noise "freq, noise(dBc)" pairs.

In my testbench, there's only the vsource and it's labelled output, and gnd. 

In ade, I setup pss output as voltage between vsource positive and vsource negative(gnd), no input.

But the simulation result in Direct Plot -> pnoise -> Phase Noise gives me a straight line with same value for all frequencies.

Then it means either the added Phase Noise pairs don't work, or there's sth wrong with my simulation.

Could you please tell me why?

Thank you very much.

Here's my testbench and vsource setup (when I use port, the setup is almost the same, only with a 50 Ohm impedance for the port)

Here's the ade setup for pss and pnoise:

Here's the simulation result plot of phase noise:

 

Parents

  • This is very hard to diagnose without seeing the circuit and setup in more detail, but I will make these points:

    1. Your tstab is very short. It will simulate to the onset of periodicity (maybe you have some startup sources which take ~5ns before they become periodic?) plus the tstab plus one cycle of the fundamental. You're effectively only including just over 3 cycles of the input frequency. Your tstab really needs to be long enough for the PLL to have locked - I think it's unlikely to have locked in 3 cycles of the input frequency. If it's not locked, the circuit is not periodic but the shooting method can get confused because the circuit has a strongly nonlinear response to the initial conditions and so it's hard for the shooting algorithm to be able to predict what it needs to change for the circuit to be periodic.
    2. Not sure what type of oscillator you have, but it must be pretty sinusoidal if 410 harmonics is enough to cover the spectral content of the oscillator (since the VCO output is at the 405th harmonic of the input frequency). You may need more harmonics to force more time steps in the VCO to make it more likely to continue oscillating.
    3. It's very hard for PSS to converge with such high divide ratios - you certainly need longer tstab. What appears to have happened is that the shooting algorithm has caused the oscillator to die out - probably because it was not stable enough in the first place.

    This would be better dealt with via customer support because then hopefully you can share the circuit.

    Regards,

    Andrew.

Reply

  • This is very hard to diagnose without seeing the circuit and setup in more detail, but I will make these points:

    1. Your tstab is very short. It will simulate to the onset of periodicity (maybe you have some startup sources which take ~5ns before they become periodic?) plus the tstab plus one cycle of the fundamental. You're effectively only including just over 3 cycles of the input frequency. Your tstab really needs to be long enough for the PLL to have locked - I think it's unlikely to have locked in 3 cycles of the input frequency. If it's not locked, the circuit is not periodic but the shooting method can get confused because the circuit has a strongly nonlinear response to the initial conditions and so it's hard for the shooting algorithm to be able to predict what it needs to change for the circuit to be periodic.
    2. Not sure what type of oscillator you have, but it must be pretty sinusoidal if 410 harmonics is enough to cover the spectral content of the oscillator (since the VCO output is at the 405th harmonic of the input frequency). You may need more harmonics to force more time steps in the VCO to make it more likely to continue oscillating.
    3. It's very hard for PSS to converge with such high divide ratios - you certainly need longer tstab. What appears to have happened is that the shooting algorithm has caused the oscillator to die out - probably because it was not stable enough in the first place.

    This would be better dealt with via customer support because then hopefully you can share the circuit.

    Regards,

    Andrew.

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