[time-nuts] Generating a solid PPS from 10Mhz source

Bob Camp kb8tq at n1k.org
Thu Jan 21 12:27:15 UTC 2016


The PLL cocks in MCU’s are abut the worst jitter monsters out there. It’s clear they only make
them as good as they need to be for the internal MCU logic. The PLL’s in the FPGA’s I’ve tested are much
better jitter wise. Part of the issue is that the FPGA VCO’s will “pull” a few percent. Some of the MCU VCO’s are
good over a decade. 


> On Jan 20, 2016, at 4:32 PM, Bob Stewart <bob at evoria.net> wrote:
> Hi Poul,
> Your comment about on-chip PLLs got me to wondering about the dsPIC33FJ128MC804 I'm using in my GPSDO.  So, I put it on the 5370A to see what's up.  Input to the gate port is the drive signal for an LED that I flash at 1PPS (controlled by an on-chip timer).  I delay that with a 10ft piece of RG-58 to drive the start gate.  The stop gate is driven by the OCXO output from the GPSDO.  Internally, the chip's PLL is running at 160MHz  (10MHz OCXO / 2 * 32), which is divided down to 80MHz for FOSC and then by 2 again for FCY, if you know these dsPICs.
> The ADEV at 1s is about 1.7E-10, declining at about a decade per decade.  I take this to represent the jitter of the PIC's onboard RC oscillator as driven by the OCXO.
> Bob
> --------------------------------------------
> On Wed, 1/20/16, Poul-Henning Kamp <phk at phk.freebsd.dk> wrote:
> Subject: Re: [time-nuts] Generating a solid PPS from 10Mhz source
> To: "Discussion of precise time and frequency measurement" <time-nuts at febo.com>, "Attila Kinali" <attila at kinali.ch>
> Date: Wednesday, January 20, 2016, 8:13 AM
> --------
> In message <20160120122824.39fb655285dd0e68c3884835 at kinali.ch>,
> Attila Kinali w
> rites:
>> The test results showed a quite more
>> detailed structure with few delays over
> 100ps and most being between
>> 20ps and
> 80ps. Interestingly, some were close to 0ps, for which
>> we have no explanation good explanation.
> Any on-chip PLL's with
> "spread-spectrum" to fudge EMI tests ?
> -- 
> Poul-Henning Kamp       | UNIX
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