[time-nuts] Newbie questions

Jim Harman j99harman at gmail.com
Wed Jan 27 11:08:13 EST 2016


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.

>
> 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.

>
> 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.

>
> 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.

>
> 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.

>
> 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!!


-- 

--Jim Harman



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