[time-nuts] Using CPLD/FPGA or similar for frequency divider

Bob Camp kb8tq at n1k.org
Tue Jun 2 17:49:36 EDT 2015


Hi

A lot depends on exactly which CPLD or which FPGA you are looking at and how 
they put the guts of it together. If you find one that is “just right” it *might* be within
10 db of high speed CMOS. Since there is a 20 db delta between the HC you mention
and the AC that leaves a bit of room. 

If you have a part with a bias generator in it, just forget about using it. You will have all 
sorts of strange spurs that come and go. They will be broadband. They will take the 
noise floor up into the 100 dbc / Hz range in some cases. 

If you are trying to use the internal PLL, it’s phase noise isn’t going to be great. Numbers
like -135 dbc / Hz at 100 KHz offset are not uncommon on an HF output. 

As straight dividers, they might get to -16x dbc/ Hz region. That compares to the -174 dbc / Hz
you could expect under similar conditions with something like AC or faster CMOS. You 
are more likely to get there on a fast CLPD than on an FPGA. Either way you can run
into bum parts. 

Bob

> On Jun 2, 2015, at 9:13 AM, David C. Partridge <david.partridge at perdrix.co.uk> wrote:
> 
> Is this a sensible thing to consider doing?  Or would I be better sticking to AC/HC/AHC/LVC logic?
> 
> Regards,
> David Partridge 
> 
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