[time-nuts] Newbie questions

Bob Camp kb8tq at n1k.org
Wed Jan 27 18:09:43 EST 2016


Hi

> On Jan 27, 2016, at 11:08 AM, Jim Harman <j99harman at gmail.com> wrote:
> 
> I am a relative newbie here myself, but at the risk of starting a
> firestorm, I would take issue with some of what Bob says below. See
> comments interspersed.
> 
> 
> On Wed, Jan 27, 2016 at 7:43 AM, Bob Camp <kb8tq at n1k.org> wrote:
> 
>> Hi
>> 
>> Ok, so let me answer the questions you *should* have asked:
>> 
>> (They are in no particular order. Number 3 probably should come first)
>> 
>> 1) Is the gear I have enough to do this project?
>> 
>> No, you will need some sort of frequency  / time standard. An atomic
>> clock of some sort is pretty much a minimum. You probably also need
>> a working GPSDO (or set of them) for comparison as well. You will also
>> need a working / modern precision counter that will give you data down
>> in the < 100 ps range.
>> 
> 
> This depends on your answer to #3 below. For my GPSDO, project, all I have
> is a scope, DMM, and PC. I can't measure ADEV, but by setting the time
> constant of my filter to 1000 sec and monitoring the TIC output I can be
> pretty certain that my local reference is well within 100 nsec of the
> "true" time.

Except you have no idea if you have messed up the performance at < 1000 seconds. Unfortunately 
there are a *lot* of ways to do that .


> 
>> 
>> 2) How will this ultimately be built?
>> 
>> At the very least, you will be building this with surface mount devices.
>> If it’s a scratch build, you will be dealing with fine pitch parts. That
>> gets
>> you into a whole bunch of gear. It also gets you into a very real “is this
>> fun or not” sort of question.
>> 
> 
> For my GPSDO I started with and Arduino board and a solderless breadboard.
> Anything with an SMD is on a purchased breakout board that spreads its pins
> to 0.1" centers. You do have to be careful to keep the wires short when
> working with fast rise times. I migrated this to a solder-type breadboard
> that mimics the layout of the solderless board and it is working fine.

Which is yet another branch to the decision tree. There are literally thousands
of possible branches. We could spend a lot of time enumerating all of them. 


> 
>> 
>> 3) What *is* the goal?
>> 
>> "I’m going to make dinner” is the start of a process. It’s not enough of a
>> goal to accomplish the task. Starting the task with a general objective is
>> fine.
>> It does need to be refined a bit before you go much further.
>> 
> 
> Agreed.
> 
>> 
>> Is this what most of us would call a GPSDO (self contained box) or is it
>> something with a PC in the middle of it?
>> 
> 
> Mine runs either stand-alone or with a PC to monitor it.
> 
>> 
>> Is this an OCXO based “precision” device or is it something more simple?
>> 
> 
> I used a $25.00 surplus OCXO. Eventually I may invest in something better
> but then I would have to get a timing GPS to go with it. Currently I am
> using the $40.00 Adafruit module.

Hopefully you got lucky with your OCXO. Others have had to buy a few dozen
before they got a “good” one. 

> 
>> 
>> Is a pure software solution good enough?
>> 
> 
> Mine is almost all software, but it has a TIC that consists of a 74HC4046
> phase detector chip, a diode, a cap, and 2 resistors, feeding an A/D input
> of the Arduino processor board. This gives a resolution of 1 ns.

….. but can you relate resolution to stability / accuracy / noise and all the other things that
need to be checked. Resolution is normally the easy part….This example gets directly back to
the test gear question. 

> 
>> 
>> Each of those decisions (and that’s by no means a full list) will send you
>> off
>> in a very different direction.
>> 
> For sure!
> 
>> 
>> 4) How long is this likely to take?
>> 
>> Best guess based on the others who have done the same thing - several
>> years.
>> 
> 
> I have been at it on and off for about two years but I have learned a lot
> along the way.
> 
>> 
>> 5) How much is this likely to cost?
>> 
>> If done the way others have done it, several thousand dollars up to
>> quite a bit more than that.
>> 
> 
> My total investment (not including the scope, DMM, and laptop PC) is under
> $200.

….. but you really do not know what it’s doing  in all respects. My example 
was targeted at a more fully worked out solution. Again it’s a matter of “what
is your target"


> 
>> 
>> 6) How much research is involved?
>> 
>> Quite a bit. The information you need is scattered all over the place.
>> Figure
>> that you likely will read at least several hundred papers. There is a whole
>> statistical language that is unique to these gizmos. This is *not* a
>> follow a
>> set recipe sort of project.
>> 
> 
> Again, depends on your answer to #3. I started with a working design and
> code and modified it to suit my fancy. I am pleased with the result. It
> keeps the brain cells firing.
> 
>> 
>> Lots of fun !!!
>> 
> 
> Absolutely!!


Bob

> 
> 
> -- 
> 
> --Jim Harman
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