[time-nuts] TCXO drift - related to TVB's posting

Adrian Godwin artgodwin at gmail.com
Sat Nov 12 19:43:02 EST 2016

What if your shop reference were drifting up ?

On Sat, Nov 12, 2016 at 11:25 PM, Joseph Gray <jgray at zianet.com> wrote:

> TCXO, not OCXO, but related. Sorry, but I have no graphs.
> I work for a municipal radio shop. We service radios that span 20
> years (through acquisitions, it was GE, Ericsson, Com-Net, M/A-COM,
> Tyco, now Harris). There are several different model handhelds and
> mobiles, with different designs and TCXO's. Some are adjusted manualy,
> most via software. I have found that every single TCXO in the various
> model radios drift downward in frequency over time.
> One interesting case was a set of radios that sat on the shelf, unused
> for several years. They were issued to some custodians about a year
> ago. I checked all of them on the service monitor beforehand and they
> were well within spec. All of these radios came back to the shop
> recently. They were 1-3 KHz low in transmit frequency. That is an
> unusual amount of drift in one year. Perhaps it has something to do
> with how long they sat on the shelf.
> I don't have enough history on our newest radios, so I don't know if
> this downward trend will hold true for them.
> Joe Gray
> W5JG
> On Sat, Nov 12, 2016 at 2:54 PM, Tom Van Baak <tvb at leapsecond.com> wrote:
> > There were postings recently about OCXO ageing, or drift rates.
> >
> > I've been testing a batch of TBolts for a couple of months and it
> provides an interesting set of data from which to make visual answers to
> recent questions. Here are three plots.
> >
> >
> > 1) attached plot: TBolt-10day-fit0-e09.gif (
> http://leapsecond.com/pages/tbolt/TBolt-10day-fit0-e09.gif )
> >
> > A bunch of oscillators are measured with a 20-channel system. Each
> frequency plot is a free-running TBolt (no GPS, no disciplining). The
> X-scale is 10 days and the Y-scale is 1 ppb, or 1e-9 per Y-division. What
> you see at this scale is that all the OCXO are quite stable. Also, some of
> them show drift.
> >
> > For example, the OCXO frequency in channel 14 changes by 2e-9 in 10 days
> for a drift rate of 2e-10/day. It looks large in this plot but its well
> under the typical spec, such as 5e-10/day for a 10811A. We see a variety of
> drift rates, including some that appear to be zero: flat line. At this
> scale, CH13, for example, seems to have no drift.
> >
> > But the drift, when present, appears quite linear. So there are two
> things to do. Zoom in and zoom out.
> >
> >
> > 2) attached plot: TBolt-10day-fit0-e10.gif (
> http://leapsecond.com/pages/tbolt/TBolt-10day-fit0-e10.gif )
> >
> > Here we zoom in by changing the Y-scale to 1e-10 per division. The
> X-scale is still 10 days. Now we can see the drift much better. Also at
> this level we can see instability of each OCXO (or the lab environment). At
> this scale, channels CH10 and CH14 are "off the chart". An OCXO like the
> one in CH01 climbs by 2e-10 over 10 days for a drift rate of 2e-11/day.
> This is 25x better than the 10811A spec. CH13, mentioned above, is not zero
> drift after all, but its drift rate is even lower, close to 1e-11/day.
> >
> > For some oscillators the wiggles in the data (frequency instability) are
> large enough that the drift rate is not clearly measurable.
> >
> > The 10-day plots suggests you would not want to try to measure drift
> rate based on just one day of data.
> >
> > The plots also suggest that drift rate is not a hard constant. Look at
> any of the 20 10-day plots. Your eye will tell you that the daily drift
> rate can change significantly from day to day to day.
> >
> > The plots show that an OCXO doesn't necessarily follow strict rules. In
> a sense they each have their own personality. So one needs to be very
> careful about algorithms that assume any sort of constant or consistent
> behavior.
> >
> >
> > 3) attached plot: TBolt-100day-fit0-e08.gif (
> http://leapsecond.com/pages/tbolt/TBolt-100day-fit0-e08.gif )
> >
> > Here we look at 100 days of data instead of just 10 days. To fit, the
> Y-scale is now 1e-8 per division. Once a month I created a temporary
> thermal event in the lab (the little "speed bumps") which we will ignore
> for now.
> >
> > At this long-term scale, OCXO in CH09 has textbook logarithmic drift.
> Also CH14 and CH16. In fact over 100 days most of them are logarithmic but
> the coefficients vary considerably so it's hard to see this at a common
> scale. Note also the logarithmic curve is vastly more apparent in the first
> few days or weeks of operation, but I don't have that data.
> >
> > In general, any exponential or log or parabolic or circular curve looks
> linear if you're looking close enough. A straight highway may look linear
> but the equator is circular. So most OCXO drift (age) with a logarithmic
> curve and this is visible over long enough measurements. But for shorter
> time spans it will appear linear. Or, more likely, internal and external
> stability issues will dominate and this spoils any linear vs. log
> discussion.
> >
> > So is it linear or log? The answer is it depends. Now I sound like Bob
> ;-)
> >
> > /tvb
> >
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