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Measurements >> Phase Noise and Jitter Measurements >> how to simulate the phase noise of Low Pass Filter used in PLL?
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Message started by lwzunique on Feb 6^th, 2017, 7:01pm

Title: how to simulate the phase noise of Low Pass Filter used in PLL?
Post by lwzunique on Feb 6^th, 2017, 7:01pm

Hi, everyone.
I am now try to using matlab code to get the overall PLL phase noise, and I need to get each block's noise. Regarding LPF, how to build the testbench to simulate its output voltage noise. my question here is, what input source should I use? vdc or idc?
vdc and idc are all ideal source, they have no noise contribution to the output, why the output voltage noise are different when using vdc or idc?

Title: Re: how to simulate the phase noise of Low Pass Filter used in PLL?
Post by cheap_salary on Feb 6^th, 2017, 11:13pm

Can you understand topology of your Loop Filter surely ?

lwzunique wrote on Feb 6^th, 2017, 7:01pm:

my question here is, what input source should I use? vdc or idc?

It depends on Loop Filter which you use.

lwzunique wrote on Feb 6^th, 2017, 7:01pm:

vdc and idc are all ideal source, they have no noise contribution to the output,
why the output voltage noise are different when using vdc or idc?

Surely study and consider Loop Filter which you use.

If your loop filter is a type where an input is a current and an output is a voltage, vdc shorts input and so you have to apply idc.
Rather you don't have to apply any source to input.
This is true for Loop Filter after Charge Pump.

Surely study and consider Loop Filter which you use.

And noise from Loop Filter is not phase noise.

Output Noises of charge pump and loop filter are not phase noise even if you use them in phase domain model of PLL.

Title: Re: how to simulate the phase noise of Low Pass Filter used in PLL?
Post by Ken Kundert on Feb 7^th, 2017, 11:55am

Whenever you build a testbench, you must mimic the eventual operating environment of the circuit. In this case, by switching between a voltage source and a current source you are changing the source impedance as seen by the filter. This could change the filter characteristics dramatically. For example, a very common loop filter is just to place a C and a series RC in shunt with the input, with the output shorted to the input (see listing 5 of Predicting the Phase Noise and Jitter of PLL-Based Frequency Synthesizers). In this case, using a voltage source at the input would have the effect of shorting out the filter components, and so the filter would have no discernible effect.

Concerning cheap_salary's comment about how a loop filter does not generate phase noise. A better description would be to say that the loop filter produces stationary noise that is converted to phase noise by the VCO. All of this is covered in some detail in the paper I cited above.

-Ken