Doubts about PLL and oscillator in RF transceiver

[fanshuo]

Hello everyone,
There's sth I am not very clear about PLL and oscillator in RF transceiver.

1. PLL is needed because LO frequency should be varied so that different channel could be down-converted. However, it is still possible to change the VCO frequency directly without the PLL. This would normally lead to narrow-range and inaccurate adjusting of the LO frequency.
Is this understanding correct?

2. If LC tank or ring oscillator is used, then it is possible to change its frequency directly. If crystal, BAW or SAW component is used, then the frequency can only be changed by PLL.
Correct?

Thx!

 

[khouly]

PLL&Oscillator

the VCO if not used in PLL its frequency can change due to any thing loading ,temp , process varation

so u need a PLL as a feedback system to make the output frequency is stable , and also have the ability to change it by changing the dividers in the PLL

khouly

 

[fanshuo]

Re: PLL&Oscillator

I can add a buffer at the output of VCO, so the load will not effect the VCO.

how significant would the influence of temp and process variation be?

 

[khouly]

PLL&Oscillator

also don't forget that u use the PLL to clean out the phase noise of the VCO and give u a very good frequcny spacing , if u have just the VCO u will need a very precise DC source , but the PLL will do it all

khouly

 

[fanshuo]

Re: PLL&Oscillator

Thanks khouly.

Well, if the requirement on phase noise is not that high, is it the case that using only VCO instead of PLL will save some power?

 

[khouly]

PLL&Oscillator

no there are alot of PLL's chips with integrated VCO in market with low power

so u can use them easily

khouly

 

[obovolo]

PLL&Oscillator

if u can make sure ur osc work correctly, u don need pll, just like xtal osc.
but most VCO's freq always vary significantly with temp and process variation.
if u use ur scope to c ur osc freq, u can easily see that ur freq is moving.

 

[chenpufeng]

Re: PLL&Oscillator

i had the same problem before

 

[BigBoss]

PLL&Oscillator

The main principal of the PLL is to tune the VCO frequency and to lock the VCO to that frequency with a certain amount of accuracy and to protect it from temperature, components tolerances and noise effects.

 

[RFDave]

Re: PLL&Oscillator

The function of a PLL Frequency synthesizer is to provide a tuneable oscillator that is referenced to a fixed frequency crystal. This gives you the advantage of an adjustable frequency with the stability of a crystal.

You need to figure out how accurate your Oscillator center frequency needs to be. If you can live with 200 kHz or 300 kHz inaccuracy in your center frequency, then you may be able to use a vco without a PLL. If you need a more accurate center frequency, then you will probably need a PLL referenced to a crystal.

SAW Oscillators are more stable than a LC-VCO, but less stable than a PLL.

Dave
www.keystoneradio.com

 

[psmon]

PLL&Oscillator

all VCO drift. In a PLL, a crystal which is highly stable locks the VCO.

 

[m.ghafoori]

Re: PLL&Oscillator

There is one important point here that should not be forgotten : VCO by itself, oscillates as soon as a voltage is applied to the Vtune pin of it, but in such ocasion it is free running, means it does not lock on a certain frequency.
The second point is that the voltage it needs to produce an accurate and precise frequency can not be easily genereated. Imagin that you need a voltage of
3.125 V !! It is a bit difficult to do this.
PLL has other benefits. For example, when a range of frquencies are to be produced with a certain control, it can not be acheived with another way.
If you need more information, please let me know.