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How to simulate the switching kick-back (Read 4269 times)
sulla
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How to simulate the switching kick-back
Nov 10th, 2003, 3:41am
 
Hi,

This is my first time to design a PLL circuit.
Since the charge pump current of my PLL is small, ~2uA, I try to find out how big the switching kick-back of the switches in the charge-pump.
Can you give me suggestions to simulate it?
Thanks,

ps. I use spectre to simulate my PLL.

Sulla
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EVI
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Re: How to simulate the switching kick-back
Reply #1 - Dec 1st, 2003, 6:44am
 
Hi Sulla,

The BSIM3 model has two modes - quasistatic and non-quasistatic. Quasistatic mode is the default mode and it assumes that the channel charge gets distributed equally between source and drain when the switches turn off, which is quite inaccurate model of charge injection effect. Non-quasistatic  mode simulates the transistor channel as a distributed R-C system which presents much more accurate model of the channel Smiley but takes about 50 times longer to simulate Sad. So in order to simulate the kick back effects accurately you have to switch your models to non-quasistatic mode. To do that go to the model file and change nqsmod parameter from 0 to 1.

Eugene.
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rf-design
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Reiner Franke

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Re: How to simulate the switching kick-back
Reply #2 - Dec 1st, 2003, 2:11pm
 
I did not notice a 50x speed reduction if the nonstatic channel effect is turned on. Already if you test only the charge-pump the simulation time is very short.

I think you mention as a switch-back effect the charge injection of the charge pump. That is an additional constant charge which make an equivalent time offset for the input pulses. The best way is to simulate the charge-pump together with the phase-detector. If the reference and the divided frequency are equal and arrive at the same time you get an charge every period. That should be counteract by a time offset. So the charge result from the static and transient imbalance of the up/down current sources.  Furtheron if you make an FFT of the charge you get the spur spectrum which modulate via the loop filter the VCO. With some calculation you also get the reference spur.
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