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

jerry shirᴀr radio.n9xr at gmail.com
Wed Jun 3 10:02:44 EDT 2015


Thanks Bruce.  That is an excellent option.

I did a paper over a decade ago on the jitter and phase noise for Actel
(Now Microsemi) comparing their eX device to the Xilinx CPLD.  It was
intended to show the eX device was preferable to the Xilinx CPLD.  It makes
a difference as to what device is selected.

Jerry



On Tue, Jun 2, 2015 at 11:17 PM, Bruce Griffiths <bruce.griffiths at xtra.co.nz
> wrote:

> You can always cleanup the outputs of the CPLD or FPGA by resynchronising
> the outputs to the input clock using a dedicated D flipflop for each output.
>
> Bruce
>
>
>
>      On Wednesday, 3 June 2015 3:22 PM, Bob Camp <kb8tq at n1k.org> wrote:
>
>
>  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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