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Design >> Mixed-Signal Design >> vco's and all that jazz
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Message started by svensl on Jul 4^th, 2005, 11:10pm

Title: vco's and all that jazz
Post by svensl on Jul 4^th, 2005, 11:10pm

I have a basic question about VCO models and their actual circuit implementation. I know how to simulate and model a VCO mathematically but have difficulties translating this to a real circuit.

1)
For example, in a N-stage ring oscillator (simple differential pair with variable output time constant) where is the internal phase? Can I plot it in a real circuit as I can in a model? In the mathematical model it is just the integration of the frequency, i.e. idtmod(frequency, 0 ,1).

2) Speaking of the idtmod function, how does the real circuit reset itself when crossing a certain internal phase value and how can I change this?

Thanks.

Title: Re: vco's and all that jazz
Post by svensl on Jul 5^th, 2005, 11:41pm

Hmm...I might not have expressed myself in the right way. In VerilogA I can model a VCO using the idtmod function and then find out when the phase crosses multiples of PI/2 by using the @cross function. This is described in the VCO model from the Verilog-AMS page. My question is the following: How would I, in a circuit implementation of a VCO, have the internal phase reset at a certain value, and when it does, generate some sort of pulse? Most literature talks about ring oscillators and I am not quite sure where in such a circuit the internal phase is and how I can use it? Hopefully someone can help me understand this.

Thanks

Title: Re: vco's and all that jazz
Post by Ken Kundert on Jul 6^th, 2005, 12:41am

In general, circuit do not operate in the same way a behavioral model of a VCO based on an idtmod() function. The closest would be a relaxation oscillator. With the idtmod() function you always integrate up, whereas a relaxation oscillator integrates up half the time and down the other half.

-Ken