Question about VCO and rising level of input magnitude

[svensl]

VCO question

Hello all,

I have a simple question about VCO's. We all know that with a rising level of input magnitude the VCO output frequency will go up. What happens if the input signal is a sine wave centred on zero, ie, amplitude levels of +-0.5V? Is the output frequency still highest at 0.5 and lowest at -0.5, meaning, can the VCO distinguish between positive and negative input levels?
With the mathematical model I could offset the input, but then the output PSD would have a dc signal in it which I don’t want.

Thanks,

 

[Btrend]

Re: VCO question

1.the VCO has equation like this: fvco=f_free+Kvco*Vctrl
if ur VCO accept -Vctrl, then u surely can include the negative half of sine input.
2. if Vctrl=Vp*sin(ωt) then fvco=f_free+Kvco*Vp*sin(ωt) -----(a)
3. if Vctrl=Vdc+Vp*sin(ωt),
then fvco=f_free+Kvco*Vp*sin(ωt)+Kvco*Vdc =f_free2+Kvco*Vp*sin(ωt) ---(b)
4. eq. (a) and (b) are different only in the free running term. So why there is a dc term in ur PSD (Power Spectrum Desnsity) ? can u explain it !

 

[svensl]

Re: VCO question

Thanks for your feedback Btrend. I understand what you said, but consider this. We know that the VCO can be modeled with a simple integrator, or with a transfer function of Kvco/s. So assume we have an input Vin which feeds into a gain Kvco which is added to w_free to give w_free+KvcoVin. This is then fed into in an integrator 1/s.
We apply negative feedback to keep everything within limits so the the output tracks the input. When the input does not have a dc term, then there will no dc term in the PSD. However, if I do offset it or have a w_free term then there will be a dc in the PSD.
Please let me know what your thoughts are on this.

Cheers

 

[arsenal]

VCO question

in my opinion, so long as u wanna the vco to oscillate when vctrl<=0, you will see the dc term in PSD! Even if you use differential control voltage. Because if it oscillates when vctrl=0 then there must be a f_free, which will then be integrated and results to the dc term.

 

[Borber]

Re: VCO question

Just a practical remark. VCO having varicap diode in it or two diodes in antiseries connection may stop oscillating when input voltage is near 0V. This is so because the RF oscillator voltage allso polarizes varicaps in to conducting. Usual minimum input voltage is about 1-2V.

 

[Btrend]

Re: VCO question

We know that the VCO can be modeled with a simple integrator, or with a transfer function of Kvco/s. So assume we have an input Vin which feeds into a gain Kvco which is added to w_free to give w_free+KvcoVin. This is then fed into in an integrator 1/s.

1. the VCO can be modeled as a integrator if and only if what u want from the VCO is its phase (Φ) !
cause Φ=∫2*pi*fvcodt= =∫2*pi*[f_free+Kvco*Vctrl(t) ]dt
=2*pi*f_free*T+2*pi*Kvco*∫Vctrl(t)dt =Φdc+Φac
2. How to calculate PSD of Φ or clock?
3. And how to design a VCO has no free running frequency (i.e. f_free=0 )?