[time-nuts] To improve a Tbolt?

Tom Van Baak tvb at LeapSecond.com
Fri Nov 21 21:33:17 UTC 2008


> I've upgraded the graph on the http://www.ke5fx.com/tbolt.htm page to show
> the performance of my Thunderbolt from the TAPR group buy (in orange) next
> to the traces for my older unit before and after the 10811 upgrade.
> 
> There is no upside to tinkering with the OCXO on the TAPR units, unless you
> have something that can beat a 10811.  You could replace it with a rubidium
> source and get better short-term stability, but I don't think swapping one
> quartz OCXO for another would be a useful thing to do.
> 
> -- john, KE5FX

Hi John,

Very nice work and I mostly agree with your conclusions. The
OCXO inside the TAPR TBolts is really quite good. And those
that aren't that good are not sent to TAPR for sale (my pile of
TBolt rejects is growing; more on that later).

So, yes, the typical TBolt OCXO is in the same ballpark as many
10811, which is why replacing one with the other may be futile.

A problem here is that 10811 are all over the map. There are
variants of 10811; and even those with the same part number or
part number suffix will vary. Some 10811 get way down in the
-13's and are thus much better than any TBolt I've seen (which
is also one reason that some HP 58503* or Z38* series GPSDO
command such a price). Also, some 10811 have much better
phase noise specs than other. Some people are after better PN
rather than better ADEV.

So if you have the ability to measure each 10811, or measure
an assortment of good OCXO, clearly you can pick a superior
one and thus improve your TBolt performance.

On your rubidium comment, one needs to be a little careful about
expectations. For short tau (say 0.1 to 10 seconds) your average
cheap eBay surplus telecom Rb will have far less performance
than a good OCXO. Yet mid-term (say 10 to 10^4 seconds), a
Rb-GPSDO will win. However, long-term the LO makes much less
difference since GPS always wins. And GPS aside, clearly the
holdover performance of Rb will blow away OCXO.

So it all depends on ones need. I guess my main point is that a
typical rubidium-based TBolt is not necessarily, just because it's
"atomic", automatically better than a stock TBolt at every point.

If you'd like to test an FRS or LPRO version of a TBolt for me let
me know. I'd rather see your plots than my words.

Thanks,
/tvb





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