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pnoise, LF contribution in PLL (Read 2109 times)

greety

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pnoise, LF contribution in PLL
Sep 01^st, 2005, 10:29pm

Hi,

I am simulating a closed-loop PLL with a sample-and-hold
PFD/Loop Filter. To simulate the noise contribution of the loop filter I am using noiseless behavioral models for all blocks except for the loop filter, then running pnoise to check the phase noise at the vco output.

The pnoise results match my expectations at frequencies above the loop bandwidth, but for freqencies below the loop bandwith the noise continues to increase, which is not what I expect should occur. Any ideas as to why this is happening? I have tried modifying tolerances, tstab, nothing seems to affect it.

thanks,
greety

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Eugene

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Re: pnoise, LF contribution in PLL
Reply #1 - Sep 2^nd, 2005, 1:27pm

I would get a few more clues by isolating the main noise contributor. You can do this with the noise summary or by selectively turning off the noise sources in the loop filter resistors. Once you identify the main culprit, you can compute the transfer function from that element to the output. You may find that the noise is what you expect afterall.

The other thing to look for is even order nonlinearities. If high frequency noise hits an even order nonlinearity, it can mix down to lower frequencies.

Also, since you are using SpectreRF, it's always a good idea to make sure you are selecting the proper sidebands.

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