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How to simulate the interference noise in PLL? (Read 5257 times)
VincentLee
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How to simulate the interference noise in PLL?
Apr 15th, 2004, 12:27am
 
Hello,
As we all known, Phase Noise of PLL can be simulated in SpectreRF by PSS & Pnoise analysis. However, the result of PN just includes the nature noise of transistors and resistors. My problems are:
1, The supply  power and the bulk also induce the noise into PLL circuit, then how to consider their effects to the entire PN result?
2, Now PLL circuit is generally in the mixed signal IC or SOC, then the interference noise from digital parts can also contribute to the PN though it's effect can be reduced by reasonable layout. How can we consider this effect?
3, Can these effects be simulated by SpectreRF? If it can, then how to do it?
Thanks & Best Regards,
VincentLee
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Frank Wiedmann
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Re: How to simulate the interference noise in PLL?
Reply #1 - Apr 15th, 2004, 6:06am
 
In SpectreRF, you can specify a piecewise linear noise spectrum of arbitrary shape for all independent sources. So if you add sources with noise spectra which model your interference effects, you can easily include them in your simulation. Generating realistic models is usually the difficult part with this approach.

There are also dedicated tools for substrate noise analysis like for example Substrate Noise Analyst from Cadence.
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Ken Kundert
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Re: How to simulate the interference noise in PLL?
Reply #2 - Apr 15th, 2004, 7:40am
 
In these cases, SpectreRF's PXF analysis is helpful. To use it, first perform a PSS analysis with noise sources in the circuit, but with zero amplitude. Then perform the PXF analysis. It should be configured to observe the output over the desired range of output frequencies. It will then compute the small-signal transfer function from every source in the circuit from every sideband k such that |k| <= maxsideband.  Then, to predict the noise at the output due to external noise sources, simply multiply the noise spectrum of each souce by the appropriate transfer function.

For example, if you are interested in low frequency supply noise being converted up to the fundamental output frequency, perform a PXF analysis that sweeps about the fundamental frequency and use the transfer function for the 0 -> 1 or +1 sideband from the power supply.

-Ken
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VincentLee
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Re: How to simulate the interference noise in PLL?
Reply #3 - Apr 21st, 2004, 7:54pm
 
[quote author=Frank Wiedmann  link=1082014062/0#1 date=1082034397]In SpectreRF, you can specify a piecewise linear noise spectrum of arbitrary shape for all independent sources. So if you add sources with noise spectra which model your interference effects, you can easily include them in your simulation. Generating realistic models is usually the difficult part with this approach.

There are also dedicated tools for substrate noise analysis like for example Substrate Noise Analyst from Cadence. [/quote]
Thanks a lot, Frank!
Recently I have researched some papers which describe how to obtain the noise model of interference effects. As you say, the process is indeed difficult, espeically when the digital part in SoC is large. From the papers I know there are 3 main noise interference sources including switching gates, power supply noise and package parasitics. Can these noise sources all contribute  to substrate noise?

BTW, you mention that Substrate Noise Analyst from Cadence can do that, then is this software as a part of IC5033 or a alone software?

Thanks & Best Regards,
Vincent Lee
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VincentLee
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Re: How to simulate the interference noise in PLL?
Reply #4 - Apr 21st, 2004, 8:25pm
 
Hello, Ken
I really appreciate your help! I think I have known your main idea although a few details still confuse me. Hope you can help me again!

Quote:
To use it, first perform a PSS analysis with noise sources in the circuit, but with zero amplitude.

Do "Noise sources in the circuit" mean a piecewise linear noise spectrum I add to the power supply and substrate (ground)? And why is zero amplitude? I think the periodic small signal should have non-zero amplitude, just as ac source in the AC analysis.

Quote:
For example, if you are interested in low frequency supply noise being converted up to the fundamental output frequency, perform a PXF analysis that sweeps about the fundamental frequency and use the transfer function for the 0 -> 1 or +1 sideband from the power supply.

For up-conversion, I should use the transfer function for +1 sideband. What's the meaning of "0 -> 1"?

Finally, I am not sure about the difference between the PAC and PXF analysis. From the SpectreRF theory, I know PXF analysis computes the transfer functions from every source at any frequency to a single output at a single frequency. And the PAC analysis just computes the response from a single stimulus to every node in the circuit.
Then does it mean that they are the same if the circuit has only a single stimulus and only the output node is observed?

Thanks & best regards,
Vincent Lee
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Frank Wiedmann
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Re: How to simulate the interference noise in PLL?
Reply #5 - Apr 22nd, 2004, 12:18am
 
Quote:
From the papers I know there are 3 main noise interference sources including switching gates, power supply noise and package parasitics. Can these noise sources all contribute  to substrate noise?

All these effects should be taken into account, although I would not call all of them "substrate noise".

Quote:
BTW, you mention that Substrate Noise Analyst from Cadence can do that, then is this software as a part of IC5033 or a alone software?

Substrate Noise Analyst is a part of the Virtuoso design platform from Cadence. For details see http://www.cadence.com/products/dfm/substrate_noise_analysis/index.aspx.
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Ken Kundert
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Re: How to simulate the interference noise in PLL?
Reply #6 - Apr 23rd, 2004, 9:10am
 
Vincent,
What I am proposing is something very similar to using AC analysis to compute power supply rejection. If you were simulating something simple, like an opamp, you could determine the effect of small variations in the supply voltage by computing the transfer function from the supply to the output, and then multiplying the spectrum on the supply by the transfer function to determine the portion of the output spectrum that results from supply noise.

In this case, you cannot use AC because it does not account for frequency conversion. Of particular concern is noise at low frequencies being up converted to the output frequency. However, you could use PAC as it does properly handle frequency conversion.

However, it is even more efficient to use PXF, which is like PAC except that it computes the transfer function from every source in the circuit to the output in a single analysis. To run PXF you need to specify a pair of nodes. This defines the output port. You also need to specify a start and stop frequency. This defines the output frequency range of interest. Finally, you need to assure that there are sources present in the circuit to model noise sources. If the needed sources already exist in the circuit, as it would for the power supply, then you do not need to touch them. If they do not exist, add sources in such a way so as not to affect the behavior of the circuit. In other words, add zero volt voltage sources in series or zero amp current sources in shunt. We don't need these sources to do anything really. The mere fact that they exist and are wired into the circuit is sufficient. Then run the PXF analysis. It will compute the transfer functions from each source to the output.

One last detail is that you are probably primarily interested in up-converted noise. In particular, if the output is at fo=fc+fm, where fc is the carrier frequency and fm is the modulation or offset frequency, then you are probably interested in noise on the supplies at fm up-converting to fo. This is a conversion from the -1 sideband to the 0 sideband (in PXF the output frequency defines the 0 sideband, in otherwords; the index of the sideband is defined relative to the output frequency). Thus, maxsideband must be set to be at least 1, and the transfer function of interest would be the one associated with the -1 sideband. (On my last post I mistakenly gave you the sidebands that would be appropriate for PAC rather than PXF).

So, for example, if you had a PLL configured as a frequency synthesizer that produced a 1GHz output, and you were concerned about power supply noise from 1kHz to 1MHz corrupting your output signal, you could do the following:
0. Assume the output is at node "out" and that the power supply source is named "Vdd".
1. run PSS with the reference clock applied. It will compute the steady-state response with the 1GHz output. At this point, there is no noise signal injected in the supplies.
2. perform a PXF analysis with "out 0" defined as the output nodes and sweep the frequency from 1GHz+1kHz to 1GHz+1MHz (you could do this using either an absolute or relative frequency range).
3. Plot the signal Vdd[-1] from the PXF results. This is the transfer function from fo-fc to fo, where in this case fc=1GHz.

-Ken
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