[time-nuts] My GPSDO project: OCXO Thermal Oscillation?

Bob Camp lists at rtty.us
Fri Aug 16 14:45:17 EDT 2013


Hi

With the 5335 you have a measurement with dead time. That makes things a bit hard to figure out. A much better way to go is to feed a pair of 1 pps signals into the 5335 and measure their time difference. Unless they are quite close, you can go for a while with no ambiguity to the reading. The effective resolution increases linearly with the time length of the observation. There also are a number of very nice programs that will let you collect the data from the 5335 via GPIB.

Assuming your 5335 works like mine does it's got about a 1 ns resolution at 1 second. It'll give you 1 ppb at a 1 second gate and 1 ppt at a 1,000 second gate. By the time it gets to 1,000 seconds the internal counters have overflowed and the reading is a bit messed up. 

Without some sort of accurate reference, there's really no way to know for sure what's going on with a GPSDO. One solution is to build two or three of them and watch them fight with each other. Another solution is to pick up a Hydrogen Maser. It's always a "what's in your wallet" sort of decision.

Bob

On Aug 16, 2013, at 2:26 PM, Bob Stewart <bob at evoria.net> wrote:

> Hi again Bob,
> 
> D'oh, I think I totally misunderstood your figures in my first response.  The .16ppb is not the frequency accuracy of my GPSDO.  It's the amount that I'm moving the OCXO during a 5 minute timeframe, which is something else entirely.  Like I said I do not have a known good oscillator to compare to.  However, I have a DDS oscillator I made some time ago, and it seems to be pretty stable if I let it be.  So, what I've done is to hook the GPSDO to the clock input of my 5335A.  I've then adjusted the DDS so that it reads near 10.000000 MHz, and watched it over a round-trip 5 minute period several times with a large enough gate that I get 8 decimal points on the counter.  I don't see any relationship between the few milli-Hz movement the counter shows and the changes to the DAC.  During several runs last night, I saw less than 30 mHz of movement, which, if true, would be 3E-9, or 3ppb, right?  Or would that be +/- 1.5ppb?
> 
> Bob
> 
> 
> 
> 
> 
>> ________________________________
>> From: Bob Camp <lists at rtty.us>
>> To: Discussion of precise time and frequency measurement <time-nuts at febo.com> 
>> Sent: Friday, August 16, 2013 10:47 AM
>> Subject: Re: [time-nuts] My GPSDO project: OCXO Thermal Oscillation?
>> 
>> 
>> Hi
>> 
>> Ok, let's try some math and see if I can do it without blinking this time….
>> 
>> +/-4 Hz for 6 volts is 0.66 Hz / V
>> output is 10 MHz so 1 Hz is 0.1 ppm
>> your OCXO is running at 0.066 ppm / V 
>> That's also 66 ppb / V
>> 
>> 0.02 V at 66 ppb / V is 0.0132 ppb or 13.2 ppt
>> 
>> The UT+ has a sawtooth output that's about 45 ns
>> That's 45 ppb at one second
>> 
>> 5 minutes is 300 seconds
>> 
>> so 45 / 300 = 0.15 ppb or 150 ppt
>> 
>> If it's the later clone version it might be  about 1/2 of that. 
>> 
>> Are you doing sawtooth correction?
>> 
>> Bob
>> 
>> On Aug 16, 2013, at 11:09 AM, Bob Stewart <bob at evoria.net> wrote:
>> 
>>> I'm converting the code for the VE2ZAZ FLL to a PLL.  I'm seeing the phase correction change the EFC up and down about .02V to .03V over a period of 5 minutes or so (it varies).  The full range on the OCXO is about +/- 4Hz varied by 0 to +6V, so at least this is a tiny value.  I feel pretty confident with my code at this point.  I'm using a Trimble 34310-T OCXO for which I've been able to find almost no information.  Could this oscillating phase correction be some sort of thermal oscillation?  I've tried two separate 34310s and both act more or less the same.  My GPS device is normally a UT+, but I just now swapped in an "Adafruit Ultimate GPS Breakout" to the same effect.  Is this good, bad, or indifferent for a GPSDO?  I started this project not knowing what to expect, and I still don't.  Experienced help, speculation, or even just kind words at this point would be appreciated!  =)  I don't have a known good/stable reference to compare
> this
>>> to.
>>> 
>>> Bob - AE6RV
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