Question On Fractional-N PLL Test
时间：04-11
整理：3721RD
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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
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
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!
:D
i think your prescale is wong set.
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