[time-nuts] Re: Current-day GPS timing receivers

[Bob kb8tq]

HI

Here’s the basic problem:

If you don’t already know that everything you are doing is perfect, you need
something better than what you GPSDO does to compare to. If you are happy
with the performance of an Rb then … why not just use the Rb? The grubby issues
are all out past a number of hours and stop at however long you decide to look. 

Why “how ever long?”. There are a *ton* of corner cases. They all pop up eventually. 
For a design that you can trust ( one up or whatever ), you need to handle whatever 
comes along. Yes, this assumes that a design that works “most of the time” is not
adequate for the task.

Since this is a software rather than a hardware project, debugging does matter. If 
you write bug free code every time you do anything, good for you. For the other 
99.999% of the group, there will be “stuff” to track down. The problems are not all
from some weird GPS issue. There will indeed be self inflicted wounds to be dealt
with. Some of them are very hard to spot ….

No that is not to suggest in any way that you can ignore the “shorter term” stuff. 
You most certainly can not. It’s simply to say that the things that happen quicker 
are more easily troubleshot. You still need to spot them, the debug is easier.

Doing the comparison requires that you have something good enough to measure
what’s going on. There are a *ton* of post from folks who really don’t understand
just how difficult this actually is. Without something like a TimePod, this is going 
to be tough. 

Can you build this or that from scratch? Sure you can. Being sure that it does indeed
work correctly .. not so easy. 

Bob

> On Feb 19, 2022, at 8:02 PM, Attila Kinali <attila at kinali.ch> wrote:
> 
> On Fri, 18 Feb 2022 19:48:21 -0500
> Bob kb8tq <kb8tq at n1k.org> wrote:
> 
> I would not be so negative about doing your own GPSDO.
> While it is not something the unintiated can just simply conjure
> out of thin air, it isn't difficult either...
> 
>> By far, the most likely thing to buy as part of your GPSDO design project is
>> an atomic clock. Not an Rb mind you , but a properly working Cs or something 
>> more exotic. ( like a maser ). The bill for them is way past (>10X)  the price of the
>> GPSDO. 
> 
> Depends on what you are looking for. If you trust your GPS receiver to
> do the right thing, than you only have to measure out to a day or so.
> And for that, a decent Rb is good enough. Maybe get two or three for
> a three/four cornered hat.
> 
> Sure, if I'd be thinking about selling a GPSDO as a commercial product,
> then I'd have to measure it against a H-maser /and/ a Cs beam standard.
> But for a hobbyist? A stable Rb vapor cell standard is good enough.
> 
> 
>> On a GPSDO, there are no books. There are no magazine articles. The folks who 
>> design GPSDO’s don’t talk about what’s inside. It’s not so much the individuals, 
>> it’s how the companies operate. IP matters and it matters a lot. Practical stuff gets
>> buried as a result. You are off on a “invent it from scratch” expedition (more or less).
> 
> While the companies who build GPSDO mostly keep quiet about what they are
> doing, the scientific community doesn't. There are plenty of publications
> that deal with the inner working of GPSDO. But you will have to piece it
> together on your own, as there is no step-by-step guide available.
> 
> If you start with a commercial receiver module, like most hobbyist would,
> then a GPSDO becomes a control theoretical problem. I.e., how to design
> the control loop to get most out of the stability of the local oscillator,
> while using the noisy signal of GPS receiver as a long term reference.
> Early GPSDO and what most of the hobby GPSDO that I mentioned do, is a
> simple PI or PID loop. If you are a little bit more fancy, you can do a
> PII²D loop that compensates for the drift of the local oscillator.
> A bit more fancy would be using some adaptive control system, like
> the often misunderstood Kalman filter and compensate for the environmental
> effects on the local oscillator.
> 
> So far, none of this that make a good GPSDO is any secret. It is even well
> documented and there are plenty of textbook that explain how to do it.
> But, it's not the stuff one ever sees unless one specializes into control
> systems (or has a keen interest in them). Another difficulty is that one
> needs to model the behaviour of an OCXO. Especially one needs to do
> system identification on it and find the proper model to then build the
> adaptive control loop on it. Again, the data needed for this is easily
> available, but it is not something one casually stumbles upon unless
> one specifically goes looking. And it's this combination of two areas
> of specialized knowledge that make it difficult for the casual engineer
> in a company to build a GPSDO that performs well.
> 
> But then, this is time-nuts. Not only do we have plenty of people who
> know these things and can point someone interested to the right literature,
> the average time-nuts are also the people who spend an exorbitant time in
> reading up on all the intricate details of this and that (it's a hobby after
> all!) and will be ableo to gather all the knowledge one needs to do it correctly.
> 
> So, my advice to anyone who wants to build a GPSDO is the following:
> 
> There are 3 key areas that are difficult in a GPSDO design:
> 1) The receiver and the measurement of the PPS pulse
> 2) The oscillator and controlling its freequency
> 3) The control loop binding the above together
> 
> For the receiver, I recommend using an available GNSS receiver module.
> Don't go the route of running your own (unless you have lots of time).
> Measuring the PPS pulse relative to the local oscillator is a bit
> tricky, but not too difficult. You only need better than 0.5ns resolution,
> which can be done easily in various ways. The most instructive would probably
> be to build your own time-to-amplitude converter. Bruce Griffiths published
> a nice design years ago that works well. The PICTIC-2 has another one that
> gives decent results. And then there is Nick Sayer's super simple one.
> (Challenge for those who are so inclined: Come up with a way to do what
> Bruce's design does, but only using a single, low voltage power supply.
> Yes, it is possible and quite easy once you see it. Hint: Use modern opamps
> instead of discrete transistors)
> 
> The oscillator is, for our needs, usually an OCXO. Most people use it's EFC
> to control the frequency. Which in turn requires something around 20-24bit
> of DAC resolution. While not difficult, it's not easy either. The easiest
> way to achieve this is to use a 16-20bit DAC and use a 2nd-4th order sigma-
> delta modulator to get the remaining bits. I would not go higher than 4th
> order unless you are confident in your skills in designing the modulator
> correctly. I also recommend using either a DAC with a load data word pin
> to have the correct timing or use a CMOS switch to achieve the same.
> If you don't use a CMOS switch, make sure you use a DAC with low glitch power.
> Alternatively, it's possible to fix the output of the OCXO (bind the EFC
> to GND) and use DDS to control the frequency going into the system. There
> are multiple ways to do that, starting fromt he obvious way to use the DDS
> to generate the frequency, over the FE-405 like PLL based offset generator,
> to a full fledged multi-stage direct offset generator like Rick Karlquist did.
> Advantage of the latter approach is that you can use any frequency source,
> even those that cannot be tuned, like an Rb standard.
> 
> For the control loop, I would start with a simple PI loop, extend it to a
> PII², then start adding a model that predicts the oscillators drift and
> environmental parameters. Start with an as simple model as you can think
> of (e.g. linear temperature dependence) and refine from there. Hint: do
> not add too many model paramters. The more you have, the harder they get
> to estimate. And with that, the chances that the control loop becomes 
> unstable increase. Also verify after each change that the control loop
> is still stable under varying conditions.
> 
> And that's about what you need to know about how to design a GPSDO.
> Sounds easy, doesn't it? ;-)
> 
> 			Attila Kinali
> -- 
> Science is made up of so many things that appear obvious 
> after they are explained. -- Pardot Kynes
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