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Simulators >> Circuit Simulators >> Long transient simulations with periodic inputs
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Message started by Mobeedikk on Apr 2^nd, 2007, 10:47am

Title: Long transient simulations with periodic inputs
Post by Mobeedikk on Apr 2^nd, 2007, 10:47am

Hello all,
I am simulating a DLL now. I have an ideal sine wave voltage source feeding into a delay line, which feeds into the phase detector, so that the sine wave reaches one input of the phase detector one full period after it leaves the voltage source. The other input to the phase detector comes directly from the same sine wave voltage source. The first cycle I simulate the circuit, the voltages at the nodes in the delay line are still at their dc values. Only after I simulate the circuit for more than a few cycles do the internal nodes in the delay line settle to their steady state waveforms.

My problem comes when I try to simulate a long transient. I want to break up the long transient into many short transients, but each time I run one of the short transients, the internal nodes of the delay line seem to start out from their dc values, rather than points on their steady state waveforms. I think this is because I don't end simulations exactly when the input sine wave goes through a zero crossing. Thus, when I start the next simulation, there is a discontinuity in the input waveform, and spectre recalculates all the internal voltages of the delay line,and sets them to their dc values.

I'm currently dealing with this issue by running the transient for exactly an integer number of cycles of the input sine wave. If I do this, then there's no discontinuity in the input waveform, and the internal nodes of the delay line seem to start out correctly at their steady state values. But there's got to be a better way to do this right? I've tried using the" save period" option under the tran form to save the state of the DLL exactly at the zero crossing points of the input sine wave, but spectre doesn't seem to want to "recover" the internal voltages of the delay line to their steady state values. Can anyone give me any pointers?

Thanks,
Richard

Title: Re: Long transient simulations with periodic input
Post by Andrew Beckett on Apr 11^th, 2007, 7:58am

Hmm, the saveperiod/recover approach ought to work - if it doesn't, you should contact Cadence customer support so it can be investigated.

Andrew.