VCO design for operating at low frequencies

[krivan]

I have to design a VCO which has to operate in a low-frequency region. Say from 1.5 MHz to around 10 Hz.
Can somebody suggest me a way to do it, or send a link to some possible topologies?

Perhaps it was not a good idea I meant an LC-VCO and because of its high frequency 2, or 3 frequency divider after it. Well, it's not a state-of-the-art solution and the freq dividers eventually don't work in the high-frequency region this is why does somebody know a more suitable solution?

Krivan

 

[ieropsaltic]

Re: VCO design

Why don't you try a ring oscillator.

 

[krivan]

Re: VCO design

Being in lack of experience regarding the voltage controlled oscillators, I assumed that an LC-VCO is suitable.

Why is a ring-oscillator better than an LC for low frequencies? I mean, if I remember correctly the ring oscillator is also a high-speed one. Or maybe an

inverter -- delay cell -- inverter -- delay cell -- inverter -- ...

like architecture could give the right frequency?

Krivan

 

[ieropsaltic]

Re: VCO design

For low frequencies, the required inductor and capacitor used for LC-VCO would be extremely large. You can try to calculate the required LC for 10Hz using f = sqrt(2*pi*L*C) and you'll see what I mean.
Also, LC-VCO can't achieve such wide band tuning range.

For ring oscillators, you can control the delay by controlling the number of stages and of delay per stage. Delay per stage can also be varied by varying current per stage or by varying the load.

However, ring oscillators suffer from much worse noise performance than their LC counterparts.

 

[yibinhsieh]

Re: VCO design

For such low frequency operation, I usually use the RC relaxation type oscillator.

It can provide good phase noise and low power consumption.

By the way, you may add a hysteresis circuit to prevent the glitch issue in such low frequency operation.

 

[A_U_J]

VCO design

There is an excellent paper which suggests a simple method to improve phase noise of ring oscillator if you are going to use it in a PLL:
**broken link removed**