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Simulators >> Circuit Simulators >> VCO PSRR simulation set-up Spectre
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Message started by Visjnoe on Mar 7^th, 2007, 9:34am

Title: VCO PSRR simulation set-up Spectre
Post by Visjnoe on Mar 7^th, 2007, 9:34am

Hello,

I'm new to Spectre/SpectreRF and I'd like to discuss the best approach to simulate the PSRR (or actually the influence of supply noise/ripple) on the jitter/phase noise of my VCO.

Now, I know that there's this PXF analysis, but that's not quite what I want (I think) since what I understand from the examples in the manual, this will calculate the xf function from the supply towards the output.

I would like to superimpose a square wave/switching wave on the supply (emulating digital circuitry) and assess than the jitter/phase noise.

I think PSS is not the way to go here, since I'm dealing with a combined autonomous/driven circuit.
QPSS assumes two driving signals (I guess), so I'm a bit stuck here...any suggestions are more than welcome.

Regards

Peter

Title: Re: VCO PSRR simulation set-up Spectre
Post by Visjnoe on Mar 7^th, 2007, 10:18am

Dear all,

I figured out 1 approach which uses transient analysis:

superimpose a sine/square wave on the supply and check the output frequency. For e.g. a sinusoidal wave on the supply, the deviation from the average output period will (most likely) also be sinusoidal.

Since this type of jitter (coming from supply noise) is deterministic, we could stick with transient analysis.
However, we are most likely to incur a simulation time penalty.

Therefore, any pointers on a PSS approach are still more than welcome...

Regards

Peter

Title: Re: VCO PSRR simulation set-up Spectre
Post by Frank Wiedmann on Mar 7^th, 2007, 11:45pm

In order to simulate the jitter due to a sinusoidal wave on the supply, you can use sampled PXF analysis (select "Sampled" under "Specialized Analyses"). In order to convert the voltage transfer function to jitter, you have to divide the result by the derivative (or slope) of the signal at the threshold crossing. I would recommend that you check your result against the result of a transient analysis for a sinusoidal wave on the supply (e.g. using the eyeDiagram calculator function) to make sure you have set up everything correctly. Please be aware that the apparent jitter in the eye diagram will depend on the correct value of the period in the eyeDiagram function. The value that minimizes the apparent jitter will probably be the correct one.

Title: Re: VCO PSRR simulation set-up Spectre
Post by simon2 on Sep 20^th, 2007, 3:41pm

Hi Visjnoe,
as I understand it, the issue is simply one of modulation of the vco's steady state carrier, with the exception that the "control" port is the supply, rather than the VCO's voltage control input.

Therefore any of the "standard" techniques for simulation and analysing the results of those simulations as applied to VCOs should hold true.

Assuming you wish to use a circuit netlist description of the VCO (rather than a mathematical behavioural model) then probably the most direct method is to modulate the supply line with a small squarewave (remember to include an realistic supply impedance) then monitor the output waveform as an "eye diagram". You might like to look at a note I have placed here:

http://www.silicondevices.com/Resources/AppsNotes/ModellingVCOs.html

to see how this might be done. There are also details of how you may look at the frequency deviation of the output without having to resort to a full-blown FFT of the output (and the time-penalty associated with the FFT).

The deviation meter part of the testbench is interesting as not only is it useful for assessing supply pushing, ity also allows you to simulate load-pull.

Load pull is done with multiple simulations where a half wave transmission line is progressively terminated with a near zero through Zo to a near open circuit load resistance, whilst monitoring the frequency deviation (using the above circuit). This will probably tell you much more about the supply related behaviour of the VCO (provided you have modelled the supply accurately to greater that 10x the VCO output frequency) than simply modulating the supply itself.

Email me directly if you want to know more details: Simon.Harpham@ieee.org

Cheers,
SimonH.