[time-nuts] GPSDO Alternatives

Bob Camp lists at rtty.us
Fri Dec 7 17:15:45 UTC 2012


Without some qualifier on "how good", we all will be talking about
hourglasses and cesium standards at the same time. That makes sorting things
out a bit difficult. It may get you into building a "controlled hourglass"
that's less accurate than a free running $10 wrist watch. 

So far the only suggestion of a "how good" has been TBolt performance. At
least that's how this started out. 

Building a "cpu only" GPSDO can be done with a big enough chip. If you are
limited to clip leads to wire it up, the cost advantage over a TBolt may not
be there. That makes the issue of significant performance loss a very real


Just as surplus deals on TBolts come and go, so do deals on OCXO's and
GPS's. Do I price the OCXO at the $13 that I once saw it for, or the $70
that they sell for today? Is the GPS in the junk box (that I paid $200 for)
free because it's sitting there? Let's not even wonder if any of the stuff

For the sake of moving this forward, I'll suggest that half the fair cost of
a (working) TBolt is the OCXO and the GPS. Put another way, your $200 (or
what ever) is split $100 for the OCXO plus GPS and $100 for:

1) The nice shielded case it's all in
2) The connectors
3) The onboard power regulation and filtering
4) The communications in and out of it
5) The 10 MHz and PPS buffering and generation
6) The PC board it's all built on
7) Putting it all together
8) Programming all the parts in there
9) Testing it and troubleshooting it
10) The smarts to get it to do what it does

I'm sure we could spend at least a year splitting hairs on that division of
costs :)

To "win" the hourglass vs cesium fair trade proposition you need to come up
with a gizmo that: 

1) Does 1-10 above
2) Works with just a GPS and OCXO added.
3) Costs less than $100
4) Works as well as a TBolt
5) Has a Lady Heather like monitor program to go with it

For the sake of eliminating crazy GPS cards. Let's say you also need a
source where we can buy both the OCXO and GPS used for less than $100.

If you want to factor in lower performance, I'll grant a waver for the fact
that you can average multiple gizmos and the result will get better by the
square root of the number of gizmos. If it's half as good as a $200 TBolt it
needs to be 1/4 the price.


-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of Jim Lux
Sent: Friday, December 07, 2012 10:13 AM
To: time-nuts at febo.com
Subject: Re: [time-nuts] GPSDO Alternatives (GLUE)

On 12/6/12 5:01 PM, Bob Camp wrote:
> Hi
> The solution to the problem is well known in several forms. Cost is below
$5 for pretty much all of them. No need to re-invent the wheel. The gotcha
is that you can't do it 100% with internal CPU peripherals. You will need
*some* glue.
I think that's the interesting aspect of this discussion (because it 
mirrors many similar such discussions I've had over the years)..

The challenge is not in building a GPSDO.. there's tons of ways to do 
that at minimal hardware cost..

The challenge is "how do you do it without using any additional glue 
logic or hardware"  That is, given things you can buy off the shelf, and 
no hardware work other than fabricating cables or soldering a wire or 
two, what can you do inexpensively.

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