When to use a single VCO

[brookning]

Hello there,

I have a question when to use a single VCO in the real world? I read lots of materials and I find it's popular to use PLL as LO or frequency source. However, I think since the VCO has a tuning range and it is able to be tuned to a indicated frequency point, why the writers of those materials do not simply use single VCO to provide LO signals?

What about the situation in the millimeter wave band like V band and W band?

Thank you very much.

 

[ycchang]

Because the stand alone VCO without PLL is a free-running component, which means it has the variation on frequency and it's time variant. And in most applications, the stable source is preferred.

 

[brookning]

Thanks, Chang.

So how those systems, where the alone VCO structure is adopted, avoid the unstable problems you mentioned?

Or in other words, what kind of applications could suffer those unstable problems?

 

[biff44]

You can use a single VCO in systems where you transmit and receive in the same box, and can share the VCO between them. A short range FMCW radar is an example--the vco can not drift very much between transmit and recieve times, so you do not have to lock it. Toys, cheap radios, etc can also use unlocked VCOs.

In applications where you need to receive at a distance what is transmitted, and you do not want to tune the receiver LO vco to try to find the signal, will need a locked VCO.

 

[BigBoss]

All VCO's are temperature dependent under the sun and that's why a PLL is used to keep them stable.

 

[brookning]

Thanks to all of you.