Hi all,
After I got my third OCXO, the CTI OC5SC25, I made some more measurements.
Because my other OCXO, the Isotemp 143-141 had a defective Vref output, I suspected the results a bit.
I didn't need to worry, the results are very close.
To accommodate the CTI, I tried to keep the same VC adjustment as I used with the Isotemp. It turned out that the sweet spot for the CTI is at 2.62V, not quite in the middle of the 4V range, but it allowed me to use the same resistor network as with the Isotemp. The adjustment range is more than enough.
I adopted a different calibration method this time.
Instead of using my counter or DMM to set the midpoint by frequency or voltage, I simply started the program, set the DAC to midrange with h32767 and adjusted the VC trimmer such that diff_ns was around zero. I then switched to H1 and ran that for a minute. I copied the trace results to Excel and used that to calculate the average of diff_ns, and wrote that down. I then switched to H65535 and did the same. The results were +75 and -69, nicely centered, and that resulted in a gain of 455. The trimmer can be substituted by fixed resistors when this has been done, to remove some possible jitter temperature drift as was suggested by imo.
While playing around with different settings, I noticed an unexpected behavior that I want to share here.
I've learned that to get a feel for the whole system, it is best to start with a TC of 4, and the damping at minimum (at 0.5). This allows you to see the almost unfiltered results of the OCXO drift and the response by the program to stay locked.
In the picture below, that is shown by the first 650 samples. The excursions show that every second, the OCXO is nudged rather aggressively into shape.
If you now switch to a TC of 32, the default setting, the combination of the TC setting with the minimal damping is causing oscillations. This is the period from about 730 until 1200. This could be a "trap for young players", I certainly fell for it and I'm no longer young.
If you now add more damping, by setting it to 3.0, the oscillations are almost gone.
At about 1,540, I switched to a TC of 100. At about 2,000 I set the damping to 5.0. You can see the effects.
Note that with higher TC settings, the DAC output is progressively more filtered and so the excursions of the DAC output are limited in time and value. If you have an OCXO that drifts easily, it can get outside of the DAC compensation rate quickly, resulting in the system loosing the lock. The effect of filtering with a higher TC is much more aggressive than the damping factor.
I have also found that before you switch to a higher TC setting, you should have already the corresponding amount of "locked" time sampled by the system that the new setting demands, otherwise you'll loose the lock right away. I found it best to progressively increase the TC setting.
With both the Isotemp and the CTI, I have difficulty getting TC's above 500. With my now dead Bliley, I could get up to 1,000 easily. The Bliley has a much thicker casing then both the CTI and the Isotemp, maybe this is a factor because I have not yet activated the temperature compensation.
Enjoy!