[time-nuts] ! PPS Source

[Bob kb8tq]

Hi

> On Aug 13, 2020, at 6:35 PM, ed breya <eb at telight.com> wrote:
> 
> I have often wondered about all this sawtooth correction stuff, and I think I've asked here too, but never got a definitive answer. Every time this comes up, there are all sorts of explanations of the characteristic, and inevitably someone mentions the T-bolt having its internal GPS clock synchronized with the desired ideal 10 MHz output, either eliminating or greatly reducing the sawtooth effect. But, to my knowledge, nobody has said for sure if this is indeed the case, or whether that's all it takes to achieve perfection, or if more magic is needed besides synchronizing.

Properly steering the clock that supplies the pps divider is what is needed. There is no *direct* need to use that clock for the GPS. A GPSDO *is* a device 
that steers the clock that supplies the pps divider. (Yes, there are other ways to get it done)


> I'm also aware that some people have come up with external sawtooth correction, using programmable digital delay lines and special control circuits. But, why do all that if you can just fix the clock instead?

Because you can’t “fix the clock” on a GPS module. That’s what’ this is all about. The GPS *modules* ( = little PCB GPS receivers ) don’t steer their clock.

> 
> Many years ago I looked at my Motorola Oncore VP (or whatever model is used in the HPZ3801A), to see what its clock was, and if it was reasonably possible to synthesize it from the 10 MHz. I think the clock is a Motorola brand odd looking TCXO, labeled "19096" or something like that, probably 19.096 MHz, as I recall. That's about as far as I got. Since then I've just wondered what would happen if it was synthesized from the 10 MHz, or if it was even worth trying, or if it would result in other problems.

If you multiply 10 MHz directly to GPS band you may have issues ….

> Maybe a certain amount of dither is necessary for proper operation.

It’s more an issue of spurs. 

> 
> So, here are some questions, that if answered, may go a long way toward possible improvements in our GPS stuff.
> 
> 1. If the GPS RX module's internal clock is synthesized (to the same nominal frequency) from the 10 MHz output of the GPSDO, can that alone eliminate or substantially reduce the sawtooth effect?

How are you doing the “synthesized”? Indeed pulse dropping *is* a version of synthesis. If you phase lock the local clock to the GPS
code clock, that will duplicate what the TBolt does. 

> 
> 2. Does the T-bolt actually do this, and if so, is that all it takes?

They phase lock the local 10 MHz OCXO to the GPS code clock. 

> 
> If either answer is yes, then I would think the GPS RX makers would have provisions for external clock reference, at least for certain high-grade timing type models.m

That sounds wonderful. It turns out to cost money. Since you can get the job done just fine with the software correction, why double (or whatever) the 
price of the module? If I was designing a device (like a GPSDO) the lowest cost device with the best accuracy will be the winner Needless 
to say, anything that pumps up the price is a major downer. (at least that’s how it worked for the decades I did do this for a living ….).

> 
> 3. Do any GPS RX modules have such provision?

No not on the low cost parts. That’s  for the simple reason that OEM users of these modules are quite happy doing it with the sawtooth message. 
It’s dirt cheap to implement and has very few downside issues. 

Bob

> 
> That's all I can think of for now.
> 
> Ed
> 
> 
> 
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