How to decide VCO Gain for PLL

[coolstuff07]

Hi,

Please any one help me how to decide VCO Gain for PLL with example & on what factors it depends.

Bye.

 

[biff44]

your question can not be answered because you provided no details. what are you trying to control with your phase locked loop. What are the characteristics of the vco, the characteristics of the reference source, the frequencies, the frequency step size.

 

[AdvaRes]

Hi all,
Consider your Vtune ranges between 0.2 V and 2.2 V. Consider the VCO has to be able to work between -20°C and 80°C. So, you have to choose your VCO gain such that you can get the required frequency for any temperature (in the specified range) by choosing a Vtune in 0.2 and 2.2 V.

This is due to the fact that the Freq vs Vtune plots shifts low with temperature.

 

[coolstuff07]

Hi,

Input frequency=20MHz
Output frequency=600MHz
Feedback divider ratio=30

Please reply.

Bye

 

[dick_freebird]

You want an oscillator with a kV as low as possible, yet still
able to cover the pull-range (fixed frequency, this is just temp,
supply, make & drift; for a hopping radio, full band plus these).

Excess kV only brings you noise-gain.

The PLL just works with what it's got.

 

[coolstuff07]

Hello,

Input frequency=100MHz to 266MHz
Output frequency=1066 to 1600 MHz.

What should be VCO Gain.

Bye.

 

[carl]

Hello,

Input frequency=100MHz to 266MHz
Output frequency=1066 to 1600 MHz.

What should be VCO Gain.

Bye.

there is a good example from freescale which covers the gain of VCO.

**broken link removed**

 

[biff44]

assuming we are talking about a discrete design where you have an opamp running off of +/- 15 volt supply rails, you will get better phase noise performance if you have a small vco gain Kvco. That way, any stray noise pickup or ground loop noise will have a reduced effect on output frequency.

If you are using a monolithic approach, with a small supply voltage, like 3.3 volts, then your priorities are different. It is hard to actually tune to either the supply rail, or ground, so you have to guarantee that the vco can be tuned over the entire 1066 to 1600 MHz range with a tuning voltage of perhaps between 0.5 to 2.8 volts. You have to insure this is true over the following conditions:

1) long term aging/drift of the F vs Vtune curve
2) impedance load pulling
3) Power supply frequency pushing
4) operating temperature range.

In that case, you probably want a big vco tuning constant Kvco, and will have to live with the phase noise hit.

Rich