# [time-nuts] 1PPS for the beginner

Bob kb8tq kb8tq at n1k.org
Tue Aug 14 09:04:20 EDT 2018

```Hi

A normal GPS module generates it’s outputs off of a free running internal clock. Generally
this is a TCXO in a “timing” GPS ( = one that puts out a rational PPS).  This is a bit counter
intuitive, since you would *think* they phase lock the local source in the module. They don’t
mainly because it makes the math easier.

The gotcha with a free running clock is that the device can only generate an edge (like a pps)
when the clock edge(s) allow it to. For simplicity, lets just go with the rising edge and accept that
there could be *double edge* designs as well.  Let’s also assume a 25 MHz clock. That’s in the
general range of what shows up on the surplus GPSDO modules.

The internal fix math in the module comes up with a solution for “when should I send the PPS”.
The clock edges are 40 ns apart. The solution says that the “right time” is 10 ns after an edge.
The module sends out a PPS that is 10 ns early. Next second the math says that the right time
is 30 ns after an edge. The module sends out a PPS that is 10 ns late.

As long as it keeps going early / late / early /late things will average out. What makes it do this
is the local clock on the module being a bit off frequency (modulo 1 Hz). As the clock drifts around
(and they do) you may hit a region where it is relatively stable. It will then send out early /early /early….
( or late / late /late…..)

If the local clock is a TCXO, the “stable points” are likely to also be points of frequency reversal.
The net result is that the early / early /early never gets a corresponding late /late / late to average
against. It will pass right through a PLL and create an offset in the output.

I believe that NIST was the first to spot this and document it with lots of plots. I could be wrong about
that. It was pretty much ignored in the days before SA was turned off. The SA jitter masked out a lot
of issues. Most modern GPSDO’s use sawtooth correction messages to get around the problem. There
may be a few still in production that don’t.

Without full doc’s on a GPSDO, you really don’t *know* where the PPS is set to originate. It may
be coming from the disciplined clock on the board. It also *may* be coming straight from the GPS
module. There are indeed units out there that will let you do it either way under software control.

Bob

> On Aug 14, 2018, at 8:15 AM, Dana Whitlow <k8yumdoober at gmail.com> wrote:
>
> Could someone please define and explain the term 'hanging bridge' in this
> context?
>
> Thanks,
>
> Dana
>
>
> On Tue, Aug 14, 2018 at 7:02 AM, Azelio Boriani <azelio.boriani at gmail.com>
> wrote:
>
>> Hanging bridges out of a GPSDO's PPS? Interesting... time to try to
>> setup a measurement and see the relation between the GPS's PPS hanging
>> bridges and the corresponding DO's ones.
>> On Tue, Aug 14, 2018 at 9:55 AM Mike Cook <michael.cook at sfr.fr> wrote:
>>>
>>>
>>>> Le 14 août 2018 à 09:29, Mike Cook <michael.cook at sfr.fr> a écrit :
>>>>
>>>> Sorry about the previous blank mail. Finger jitter.
>>>>
>>>>
>>>>
>>>>> Le 14 août 2018 à 04:29, Chris Caudle <chris at chriscaudle.org> a
>> écrit :
>>>>>
>>>>> On Mon, August 13, 2018 9:16 pm, Chris Burford wrote:
>>>>>> I have a (generic?) GPSDO which contains an Oscilloquartz STAR 4+
>> OCXO
>>>>>> that I am using to steer a PRS10 RFS. I'm a little confused on where
>> the
>>>>>> 1PPS is coming from with respect to the GPSDO.
>>>>
>>>> As Chris points out the 1PPS from a GPSDO will « generally » be
>> derived from the primary frequency and can show better performance than
>> directly from a GPS receiver.
>>>> However this is becoming less and less true.
>>>> If you look at the Oscilloquarz blurb for the Star 4+ ( I found some
>> here <http://pdf.directindustry.com/pdf/oscilloquartz-sa/
>> star3-4/62169-330779.html#search-en-oscilloquartz-star-4> ) , you will
>> see that the phase stability (jitter) on the 1PPS output is +/- 30ns when
>> locked to GPS, an it has a timing grade GPS receiver. This is not as good
>> as other GPS modules now. 15ns is normal, with some less than half that.
>>>> The PRS10 has outstanding PLL control already. The SRS product doc
>> gives +/- 10ns accuracy with +/-1ns resolution.
>>>> I don’t think that you are buying much with disciplining the PRS10
>> with a GPSDO 1PPS. Do you have any TIC measurements in this config to
>> compare with a direct GPS 1PPS feed?
>>>>
>>>
>>> I forgot to mention one other thing which may be of interest to some.
>> The 1PPS wave form output from the PRS10 is pretty mediocre. I put the
>> details in another post here sometime back.
>>> The Star4 spec is +/- 10ns, something I can only get from my PRS10s with
>> a 74HC7001 shaper.
>>>
>>>
>>>>
>>>>>
>>>>> A GPS disciplined oscillator contains a GPS receiver which outputs
>> 1PPS
>>>>> based on receiving the GPS signals and calculating the position  +
>> time
>>>>> equation. That PPS signal is noisy in time, it jitters around
>> relative to
>>>>> the ideal 1 second period.  The GPSDO implements a long time constant
>> PLL
>>>>> to synchronize the output of the OCXO to the long term average
>> frequency
>>>>> and phase of the GPS PPS, so what you see externally is 10MHz directly
>>>>> from the OCXO, 1 Hz (PPS) which is divided down from the 10MHz OCXO,
>> and
>>>>> those are controlled by a PLL so that long term the phase of the PPS
>>>>> divided down from the OCXO follows the PPS calculated by the GPS
>>>>> but with lower jitter.
>>>>>
>>>>> --
>>>>> Chris Caudle
>>>>>
>>>>>
>>>>>
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>>>>
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```