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Phase Noise of ECL Level Divider (Read 25603 times)
timmc6
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Phase Noise of ECL Level Divider
Sep 20th, 2006, 2:35pm
 
Hello,

I am trying to simulate the phase noise of an ECL level divide by 2 circuit.  My understanding is that since this circuit is more of an analog circuit and not a CMOS level divider, noise at all time needs to be considered, not just at the midpoint of the transition.  So the PNOISE setting should be 'sources' instead of 'time domain'.  Is this correct?  Also, I am not sure if I should use an 'absolute' sweep or 'relative' sweep in PNOISE.  Any thoughts on this are greatly appreciated.  Thanks.  :)

Regards,
Tim
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Ken Kundert
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Re: Phase Noise of ECL Level Divider
Reply #1 - Sep 20th, 2006, 8:58pm
 
Whether the circuit itself is analog or digital is of no consequence. If the ECL divider is followed by another thresholding circuit (such as some form of logic, or the LO input to a mixer), then you should use "time domain" to strobe the noise at the threshold crossings as noise far from the threshold crossings is inconsequential. If instead the divider is driving some other type of circuit (I cannot think of what this might be, perhaps a filter?), then you should use "sources" to compute the time-averaged value for the noise.

-Ken
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timmc6
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Re: Phase Noise of ECL Level Divider
Reply #2 - Sep 21st, 2006, 7:40am
 
Thank you for your response Ken.  The ECL level divider I have is driving other ECL dividers, but I need to simulate the phase noise of this one separately from the others.  So 'time domain' PNOISE sounds like the way to go.  Thanks again.

Tim
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chase.ng
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Re: Phase Noise of ECL Level Divider
Reply #3 - Sep 25th, 2006, 5:47am
 
Hi,

What if the divider is driving a switching mixer? Is a switching mixer consider as threholding circuit also?

Thanks,
chase.
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Paul Geraedts
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Re: Phase Noise of ECL Level Divider
Reply #4 - Sep 25th, 2006, 7:08am
 
Ken,

The same would apply to the phase noise modelling of a VCO, right? Though the time-averaged phase noise is generally specified. Why? Historic reasons, possibly?

Thanks,

Paul
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Ken Kundert
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Re: Phase Noise of ECL Level Divider
Reply #5 - Sep 25th, 2006, 11:09am
 
There are two types of phase variation in oscillators. The dominant source of phase variation is due to oscillator phase noise, which results from noise coupling into the primary oscillation mode. In this case, the phase variation is slow and the same for all signals. In this case, it does not matter if you measure the phase variation at one point in the cycle or time averaged over the whole cycle, because the phase due to this source does not vary significantly over a cycle.

The second source of phase noise is due to sources that are isolated from the primary oscillation mode, such as noise from buffers or dividers. Noise due to these sources are usually modulated by the oscillation signal, in which case they can vary substantially over each cycle, and so should be characterized at the threshold if followed by a thresholding circuit.

And yes, switching mixers are thresholding circuits.

-Ken
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chase.ng
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Re: Phase Noise of ECL Level Divider
Reply #6 - Sep 25th, 2006, 6:55pm
 
Hi,

Let say I have a switching up-mixer, and the LO is coming from a divide-by-2. So does it mean the receiver blocking is actually caused by the strobed noise at the mixer threshold?

That means even with VCO, if it is driving a thresholding circuit (like a switching mixer directly), we should still use the time domain noise instead of the time averaged noise, am I right?

Thanks,
chase
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Ken Kundert
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Re: Phase Noise of ECL Level Divider
Reply #7 - Sep 26th, 2006, 12:06am
 
I'm saying there are two contributions. At low offset frequencies oscillator phase noise dominates, which does not require the strobed noise. At higher offset frequencies, the divider noise may dominate, in which case you should use the strobed noise.

-Ken
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Re: Phase Noise of ECL Level Divider
Reply #8 - Sep 27th, 2006, 8:36pm
 
Ken:

All good points for inherent noise issues.

I would add some items relative to cross coupling noise that leads to jitter - namely capacitive coupling through bias, power and ground paths.

Suggest running a full LPE on the device and using appropriate impedances in the power and ground of the environment.  Also, the power/ground rail noise needs to be modeled.

Jerry
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Paul Geraedts
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Re: Phase Noise of ECL Level Divider
Reply #9 - Sep 28th, 2006, 6:47pm
 
Still wondering: how to define noise power without mentioning a time interval? Without a time interval, I don't see how to relate strobed noise and time-averaged noise.

Paul
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Frank Wiedmann
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Re: Phase Noise of ECL Level Divider
Reply #10 - Sep 29th, 2006, 2:12am
 
Ken Kundert wrote on Sep 25th, 2006, 11:09am:
There are two types of phase variation in oscillators. The dominant source of phase variation is due to oscillator phase noise, which results from noise coupling into the primary oscillation mode. In this case, the phase variation is slow and the same for all signals. In this case, it does not matter if you measure the phase variation at one point in the cycle or time averaged over the whole cycle, because the phase due to this source does not vary significantly over a cycle.

The second source of phase noise is due to sources that are isolated from the primary oscillation mode, such as noise from buffers or dividers. Noise due to these sources are usually modulated by the oscillation signal, in which case they can vary substantially over each cycle, and so should be characterized at the threshold if followed by a thresholding circuit.

And yes, switching mixers are thresholding circuits.

-Ken


Thanks for this very clear presentation, Ken. Unfortunately, when the people at Cadence implemented the pnoise jitter analysis for autonomous circuits, they seemingly forgot about the second source of phase noise you mention. So, instead of making the pnoise jitter analysis a superset of the pnoise timedomain analysis just like for driven circuits, they made it a superset of the pnoise modulated analysis (for those with SourceLink access, see http://sourcelink.cadence.com/docs/db/kdb/2005/Oct/11196272.html). As far as I understand it, this has the consequence that only the first source of phase noise you mention is taken into account in the simulation. In the setup form for pnoise jitter analysis for autonomous circuits, you can specify neither a circuit node nor a threshold value. In my opinion, this way of implementing the analysis is not very useful because it might neglect important sources of jitter in an autonomous circuit.
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Paul Geraedts
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Re: Phase Noise of ECL Level Divider
Reply #11 - Sep 29th, 2006, 4:33am
 
Quote:
As far as I understand it, this has the consequence that only the first source of phase noise you mention is taken into account in the simulation.

I think that's not true. The noise contribution of the circuitry outside the ring of the oscillator (buffer, divider, ...) is still taken into account, although now its noise is time-averaged over a PSS cycle. This seems to result in a *higher* value noise power, see http://www.designers-guide.org/Forum/YaBB.pl?num=1109068827. So the analysis is probably overestimating the noise?

I'm personally still struggling a bit with the word *higher*. I know I tend to confuse continuous time noise with discrete time noise, just like Ken mentions in the post. Though I would like to understand how Spectre calculates the time-averaged noise. Does it do something like a noise analysis at every time point of the PSS and adds these terms together (where every term is a measure of the noise energy in the specific PSS step size)? Or is that a wrong way of looking at it?

Furthermore, is the noise as present during the threshold crossing all phase noise: is there no amplitude noise present? As strobed Pnoise is only looking at one sideband (instead of both and their correlation)!

Thanks very much in advance,

Paul
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Frank Wiedmann
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Re: Phase Noise of ECL Level Divider
Reply #12 - Sep 29th, 2006, 6:42am
 
I am not quite sure if the noise contributions from outside the oscillator are taken into account in an averaged way like you say or if it is really only the noise coupling into the primary oscillation mode that is taken into account. Maybe Ken or Andrew can help us out here. However, I feel pretty sure that a better way of implementing the pnoise jitter analysis for autonomous circuits would have been to base it on the pnoise timedomain (or strobed) analysis just like it was done for the driven case.

Regarding the issue of phase noise versus amplitude noise: this was the subject of my very first contribution to this forum, see http://www.designers-guide.org/Forum/YaBB.pl?num=1036850366/.
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Paul Geraedts
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Re: Phase Noise of ECL Level Divider
Reply #13 - Sep 29th, 2006, 10:46am
 
Frank,

Thanks for the link you posted. I had not seen it before, and it is very interesting!

Quote:
However, I feel pretty sure that a better way of implementing the pnoise jitter analysis for autonomous circuits would have been to base it on the pnoise timedomain (or strobed) analysis just like it was done for the driven case.

Don't get me wrong: I fully agree with you on this point.

Paul
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Ken Kundert
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Re: Phase Noise of ECL Level Divider
Reply #14 - Sep 29th, 2006, 9:59pm
 
In an autonomous PNoise analysis all the noise is accounted for correctly. Thus the noise from both the oscillator and the divider would be included. You can see this if you analyze an oscillator followed by a buffer. If you use SpectreRF to observe the noise at the resonator, you will see that there is no natural noise floor, but if you observe the noise after the buffer there is a noise floor. The noise floor is due to the white noise added by the buffer. The same would be true with a divider. And this also tells you when it would be helpful to use the strobed noise. If you are in the 1/Δf 2 region, then the noise from the phase mode in the oscillator is dominating. In this case, using strobed noise to determine the phase noise should give the same result as using the time-averaged noise. However, if you are concerned about the noise in the region were it is white or near white, then you would be best served using the strobed noise.

The strobed noise analysis conceptually just adds a sampler to the output of your circuit and then reports on the noise in the resulting sequence. Because it is a discrete-time sequence rather than a continuous time waveform, the spectrum is periodic in f.

You might want to take a look at An Introduction to Cyclostationary Noise, which can be found on http://www.designers-guide.org/Theory/.

-Ken
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