# Interpretation of frequency of VCO

1. ## Interpretation of frequency of VCO

A VCO has a transfer function of 100/s, could this mean, it produces frequency which is 100KHz/V?

2. ## Re: Interpretation of frequency of VCO

which VCO?
what does the data sheet say?
what do the applicable ap notes say?
there (likely) is at least one ap note with a sample usage. what does that say?

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3. ## Re: Interpretation of frequency of VCO

The Laplace model of a vco is given by the expression

Phase/Vin = Gain/s

where "s" is the Laplace operator

then in you case Gain=100. Trasforming into time domain we have dPhase/dt = Gain*Vin that is w = Gain*Vin

2*pi*frequency = 100*Vin from which frequency/Vin = 100/(2*pi) = 16 Hz/V

4. ## Re: Interpretation of frequency of VCO

Hi albbg

Suppose the VCO produces output say 1KHz/V (square wave)and I connect it to a 10 bit counter. Then it would take 1 second for counter to roll off. So the time constant is 1 second in this case. Would the transfer function of the combo not be 1/s in this case?

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5. ## Re: Interpretation of frequency of VCO

Hi.

What you say can be mathematically shown as:

f = 1 / t
or
t = 1 / f

Klaus

6. ## Re: Interpretation of frequency of VCO

Hi KlausST

Are you confirming that the expression 1/s is correct?

7. ## Re: Interpretation of frequency of VCO

Would the transfer function of the combo not be 1/s in this case?

Unless you arbitrarily redefine the commonly understood VCO transfer function definition, you should consider what albbg told. The unit is rad/s rather than Hz.

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8. ## Re: Interpretation of frequency of VCO

At this point of view, I was wondering the difference between VFC vs VCO?

9. ## Re: Interpretation of frequency of VCO

Literally, a VCO is a VFC (voltage to frequency converter) and vice versa. Specifically the term VFC refers to converters with large frequency ratio (e.g. 0 to xx kHz) and high linearity. The frequency ratio of a VCO can range between several 100 ppm (tuned crystal oscillator) to decades (current steered rings oscillator).

10. ## Re: Interpretation of frequency of VCO

Originally Posted by garimella
A VCO has a transfer function of 100/s, could this mean, it produces frequency which is 100KHz/V?
When the given expression is really the TRANFER FUNCTION (and not thefrequency with s=seconds), it must be interpreted as the PHASE characteristics.
This is because in a linear PLL model the input and output quantities are given as PHASE values.

In this case, the transfer function is PHI(s)=Ko/s
with Ko=VCO constant given in rad/Vs.

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