[time-nuts] ! PPS Source

[Tom Van Baak]

Hi Ed,

 > I'm also aware that some people have come up with external
 > sawtooth correction, using programmable digital delay lines
 > and special control circuits.

For decades fellow time nut Rick Hambly has done hands-on work with GPS 
receiver timing, including sawtooth correction. Here's a recent version 
of his classic series of presentations:

https://www.cnssys.com/files/TOW/High-accuracy_Time_and_Frequency_in_VLBI_2019_sem.pdf

His "CNS clock" was an example of a GPS/1PPS source that used the delay 
technique to reduce sawtooth effects. The paper is worth a look; lots of 
good info packed along with plenty of plots and photos.

Note that his most recent version of the CNS clock no longer uses the 
delay technique. I can explain more if you want to go down that rabbit hole.

 > But, why do all that if you can just fix the clock instead?

Because it is not possible for you or me to "fix the clock instead". 
That's a complex business and design decision inside the GPS chip 
itself. Almost every DIY or commercial GPSDO uses an off-the-shelf GPS 
receiver chip or PCB, so you work with what you can get. Those GPS 
modules all use some tiny carefully-specified free-running internal 
oscillator for signal sampling, computing, and output pulse generation.

Trimble was in a unique position when the GPSDO era began because they 
made both GPS chips and a GPSDO product. For them the integrated OCXO 
and GPS receiver design was possible. Even hp couldn't do that.

 > 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 sounds right. See http://www.leapsecond.com/pages/vp/sawtooth.htm 
and note Dr Clark's mention of 9.54 MHz, which would be half your 19.096.

 > 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?

Yes. But it would also create a list of other technical concerns. I'd 
probably rather have to deal with sawtooth than having to deal with DDS 
artifacts or phase noise requirements. And now your GPS chip won't even 
boot or work right if the user's 10 MHz isn't there, or isn't accurate 
enough, or is still warming up, or if the user's 10 MHz has too much 
phase noise or instability.

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

Yes, we think so. But remember they had control over everything; the GPS 
chip, the firmware, and the specification of the OCXO.

 > 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.

Yes, very high-grade timing receivers require, or allow, external 
frequency inputs. I've seen some in operation at national timing labs. 
When I heard they cost $20k I decided sawtooth correction wasn't so bad 
after all. ;-)

It seems sawtooth bothers you. There are sawtooth effects in many parts 
of technology; from printer dots to PC clocks; from calendars to leap 
seconds. The goal isn't always to eliminate but rather to fully 
understand the effects. The fact that GPS timing receivers output 
quantization corrections should be seen as a good thing; it gives the 
user maximum information.

/tvb


On 8/13/2020 3:35 PM, ed breya 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. 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?
>
> 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. Maybe a certain amount of dither is 
> necessary for proper operation.
>
> 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?
>
> 2. Does the T-bolt actually do this, and if so, is that all it takes?
>
> 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.
>
> 3. Do any GPS RX modules have such provision?
>
> That's all I can think of for now.
>
> Ed
>
>
>
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