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

[Bob Camp]

Hi

If my numbers are correct, your DAC is moving the OCXO 0.016 ppb or 16 ppt. Your 5335 would read that as a last digit flip with a 100 second gate. 

For really large numbers on the 5335 it's *much* more convenient to use the built in math functions. In this case subtract 10 MHz from the reading before it's displayed. Far easier on the eyes. For even more fun use the multiply function to make it come up in ppb. That makes it a lot easier on the brain….

If the OCXO is at 0.5 ppb stability p-p over 50 degrees C that would be 10 ppt per degree. (It could easily be 5X better than that - who knows…) If your room is moving one or two degrees then the OCXO could easily wander by 10 or 20 ppt. 

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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