Re: Help on PLL
ericz said:
Are you familiar with such component? can you give me datasheet or part number for it?
Thanks,
https://www.fujitsu.com/downloads/MICRO/fme/pll/e421349.pdf
Look at that datasheet ( for instance, you may findyourself many.. ).
In this data sheet the formulae is defined as
fVCO = [(M x N) + A] x fOSC ¸ R (A < N)
Here,
fVCO : Output frequency of external voltage controlled oscillator (VCO)
N : Preset divide ratio of binary 12-bit programmable counter (5 to 4,095)
A : Preset divide ratio of binary 6-bit swallow counter (0 to 63)
fOSC : Output frequency of the reference frequency oscillator
R : Preset divide ratio of binary 14-bit programmable reference counter (5 to 16,383)
M : Preset modulus of dual modulus prescaler (64)
For example. Let your frequency to be synthesized is 74MHz and reference frequency ( at the same time channel spacing ) is 15kHz. If you programme R value as ( for instance ) 512, cyrstal frequency will be 7.68MHz.
R=7.68MHz/15kHz=512
( there will be many possibilities to use different cyristal values))
In that case comparison frequency will be 15kHz and Total division ratio will be
NT= 74MHz/15kHz=4933.333. But this will be exact value of 15kHz so VCO frequency should be 73.995 MHz.
So ,main task is to realize that value.
So, [(M x N) + A]=4933. Since M value is 64 that is constant,
So finally , by taking integer value of N ( 4933/64=77.07813 -->>77 )we find,
N=77
M=64
A=5
values.
I hope that's clear..