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Design >> Analog Design >> Why constant output range is preferred in VCO?
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Message started by pipeline on Oct 27^th, 2006, 6:02am

Title: Why constant output range is preferred in VCO?
Post by pipeline on Oct 27^th, 2006, 6:02am

What is the benefit to keep a constant output range for VCO when frequency changes between the minimum and maximum values, especially for a wide control range VCO? Razavi emphasizes constant output range in VCO design in his book. But I don't really get it.

Title: Re: Why constant output range is preferred in VCO?
Post by SATurn on Oct 27^th, 2006, 10:16am

The reason is that without a loop controlling the output swing (Vsw), the tuning of oscillation frequeny (fosc) respect to the controlling current (IC) or controlling voltage (VC) will not be uniform! For example sometimes it may happen that fosc decreases with increasing the IC and it means that your PLL (for example) will be no more stable. Actually fosc depends on Vsw and IC and by this technique fosc will only depends on IC, so the control would be much simpler. So, it is very important to have this simple loop incuded in your design.

SATurn

Title: Re: Why constant output range is preferred in VCO?
Post by loose-electron on Oct 29^th, 2006, 3:56pm

Hm.. A couple of things that I am aware of -

Synthesis PLL's operate with low loop bandwidths and some amplitude changes there should not affect loop gain and associated stability and phase margin an awful lot. Some, but 3-6db of change should not destroy the system.

You have two different families of PLL's and they are
LC-VCO PLL's for synthesis and mixer use in the RF world,
and
Ring Oscillator VCO PLL's for timing and clocking types of applications.

With ring oscillator sytems you want a constant amplitude output in the form of a saturated state output prior to transitioning, otherwise you get a very noise (jitter prone) sensitive system. Pretty much all ring oscillators are run this way.

With LC VCO systems you are back in sinusoid land and you are trying to maintain a spectrally pure sinusoid with minimal phase noise (and the associated spectral spreading) - That means to a certain point you are dealing with linearity issues (not too big in amplitude or you get harmonics) and signal to phase noise issues. (not too small in amplitude or you get a poor SNR and the associated spectral spreading)

All of the above scenarios point to a fixed amplitude being a bit more viable than variation in the signal output.

Probably some other reasons, but thats what comes quickly to mind.

Jerry