Question On Fractional-N PLL Test

[bigworm]

hi guys,
I have met with some problems when I tested a fractional-N PLL.
I used 3-wire bus to write control bit into the PLL ic.
I passed division ratio to the PLL with 3order mash delta-sigma modulator in the format: N for integer and 24bit for Fraction.
when only N was set, PLL can lock at some frequency, and a lock indication was locked at 3.3V, which meant the pll locked.
but my problem is when 24bit were set, the PSA didn't show a settled frequency, but showed a moving frequency. the PLL didn't lock
but when I measured the lock indication, it showed a waveform which is high voltage(3.3) for some while and low voltage(0) for a while, and then high, low, high .....
I don't know why. does anybody meet with the same problem?

thank you

 

[orley]

It means your PLL is unlocked. You try to jump some number in fraction and try it again. If your PLL always do wrong , your chip is failed.

Regards!

 

[dushan1]

i think your prescale is wong set.

 

[RFDave]

1-When you program the synth IC, are you using software that you wrote, or software that the manufacturer provided you?

2-When you program it in integer-N mode, how close to the edge of the VCO tuning range are you?

3-Can you program it so that you are close to an integer N frequency? In other words, what happens if you try to lock it at 1 MHz above or below your integer N frequency?

Dave