how do determine LF bandwidth in fractional-N PLL?

[yylei]

I want design a fractional-N PLL, compare frequency is 2M and channel space is 125K, how to design the bandwidth of loop filter? It is 1/10 reference clock usually,so it should be 200K or 12.5K?

 

[khouly]

you can use the 200 KHz , the nice thing about the fractional N PLL it removed the constaint that the compare frequency should equal the channel spacing

khouly

 

[jecyhale]

Higher bandwidth mean it need as less time to steady-stade, but worse phase noise in band. vice versa!

So it detemined by your system specification.

Added after 1 minutes:

I want design a fractional-N PLL, compare frequency is 2M and channel space is 125K, how to design the bandwidth of loop filter? It is 1/10 reference clock usually,so it should be 200K or 12.5K?

But usually 50k~200k is good for wireless system.

 

[nozone]

1/10 rule is for integer PLL, for fractional-N, the ratio needs to be much higher, e.g. 10MHz reference with 100KHz loop filter bandwidth, but it can still offer you fine resolution, such as 10Hz, of course can be 200KHz too.