[time-nuts] Another use for a Trimble Thunderbolt
kb8tq at n1k.org
Fri Dec 26 00:06:18 EST 2014
> On Dec 25, 2014, at 8:11 PM, Arthur Dent <golgarfrincham at gmail.com> wrote:
>> ’d say that the plot is telling the truth. It also seems to be giving
>> you information fast enough that thermal drift and barometric pressure
>> is not to big an issue. If you had to wait a day or three for the same
>> data, drift would be a much bigger issue. Yes, when you get to the
>> “close enough” trace, drift may be an issue. (yes close enough is
>> indeed close enough …).
> Keep in mind that I'm talking about using a GPS signal from a Thunderbolt
> to adjust a common rubidium standard that would be used in a telco or
> other piece of general test equipment and thermal drift and barometric
> pressure effects are never an issue for me.
Your 15 ns / hour limit comes out to ~ 4.12 x10^-12.
Your “close enough” comes out (eyeball @ 1/4 slope) to < 1x10^-12.
If your Rb has a tempco of +/- 2x10^-10 over 100 C that *might* be 4x10^-12 / C. If your room moves +/-2 C per hour (as some do) temperature could be an issue. You would be seeing 1.6x10^-11 cycles under those conditions. In order to reasonably “see” your 1x10^-12 limit, the setup you are running would have to be at least 10X better. At that point temperature rather than set point would dominate the plot. In order to measure set point, you would need 100X better.
Are all Rb’s “worst case?”, of course not. Are they all polite and straight line linear over the entire range? - not on the ones I’ve seen. Throw in things like a draft on the heat sink when the HVAC fires up ….
This is *not* in any way a knock on the approach. It seems to work very well for what you are trying to do. It’s well though out and functional. It’s simply a caution that drift can be an issue doing this sort of thing to the 1x10^-12 level on Telco Rb’s.
>> I suspect that if you try the trick with something way far off frequency
>> (many 10’s of ppm), the GPS may not play nice. At any normal tune range
>> on an Rb, it should be fine.
> Actually it does play nice-very nice over any range I'm interested in. Keep
> in mind that I wanted a simple method that would work with a 10 Mhz
> standard to give me closer readings than I could get by watching the scope
> the counter. I can easily use just the counter to check the frequency of a
> less than stellar oscillator so what I'm describing would be used with a
> fairly close 10 Mhz frequency standard and not one that isn't even close.
> The Pendulum CNT-81 frequency counter I have can display a 10 Mhz error to
> decimal places in 10 seconds using the math function and an external time
> Anyone who has used a WWVB comparator remembers the plot zipping back to
> zero position when the plotted frequency difference would exceed the
> maximum deflection. The Thunderbolt's display on Lady Heather works exactly
> the same way. If you look at the plots in the link that follows you will
> see that the 10 Mhz appears very stable but it is actually set by a
> to be 10,000,000.025000 hz in the upper trace and so to keep it in the
> center position on the graph I have an oscillator offset of -2500 PPT in
> Heather. In the lower trace the synthesizer frequency is set to
> 10,000,000.010 hz
> and the offset is -1000 PPT to keep the 10 Mhz trace centered. The
> reference for
> the synthesizer and the Thunderbolt is the GPS signal from the Tbolt so the
> reference is used for everything.
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