In article <1139426514.841712.37900@f14g2000cwb.googlegroups.com>,
Ron N. <rhnlogic@yahoo.com> wrote:
>Mark wrote:
>
>> A software PLL is based on an NCO and an NCO unlike a VCO has a minimum
>> step size so it can only achieve a number of discrete frequencies, i.e.
>> the output frequency is quantized.
>
>Why do you say this?
Perhaps because this is what he has seen.
> An NCO is, of course based on some digital
>number representation; but if you use IEEE doubles, the quantization
>error of the phase is much smaller than the thermal noise of any
>analog oscillator, maybe even smaller than the quantum limits
>physics allows any analog VCO.
IEEE doubles requires some serious amount of electronics and is not really
the best way to go. You do better with a 64 bit fixed point system.
I've done quite a few digital PLL systems and have never needed to go
beyond about 32 bits to get good results.
Still if you are making a square wave on the output you have 1 clocks
worth of jitter.
--
--
kensmith@rahul.net forging knowledge