PLL malfunctioning [hlp]

[No one]

Hello Everyone;
We have designed a PLL; but it malfunctions. It don't lock and VCO sweet from start frequency of itself and free runs to high (that is the tuning voltage is a ramp). How we can use it?

 

[khouly]

u mean that the VCO control voltage is ramping up and hold on this state , and the PLL is not locked ?

is ur VCO gain "Kvco" is a negative or positive , i mean as u increase the VCO control voltage the frquency increase or decrease ?

khouly

 

[No one]

Hello;
Kvco is positive.

 

[khouly]

did u measure the frequency at the input of the phase detector ? ,

khouly

 

[rogerynt]

Are you programming the PLL correctly ? The vco will normally work ( lock the frequency ) even the loop filter has not added to the circuit. Assuming the following parameters are correct.

For example

1.) VCO tuning range covers your operation frequency range

2.) Correct calculation e.g. divide ratio, N, A B factor, KVCO etc.

Rogerynt

 

[khouly]

also the filter design , and stabilty considration must be reviewed

khouly

 

[biff44]

By "ramp" what do you mean. Does it start at some low voltage and ramp up to the power supply voltage and stay there forever, or does the tuning voltage have a periodic "sawtooth" voltage that repeats itself at perhaps a 20 KHz rate?

 

[eecly]

You must make sure all part can work properly.e.g.

1. Make sure the VCO can work. Try to apply a voltage with different voltage. See how frequenc changw during voltage change
2. Make sure the phase detect can work. Try to apply a puse in ref input and feedback input. Capture the PD output. Use scope to capture all signal. Make sure it work properly.
3. Make sure the loop filter is design well. Use software tools like PLL sim to find the capacitance value of loop filter. Make sure you know the loop BW