[time-nuts] Assistance needed to understand some V_OCXO stability concepts.

[Tom Van Baak]

Joe,

The 00105 oscillator was used in early hp 105 quartz frequency 
standards. So the specifications, theory of operation, photos and 
schematics for the 00105 "brick" oscillator are all in the hp 105 op/svc 
manual (April 1968).

There are two copies on Didier's site. Go to 
http://www.ko4bb.com/manuals/ and type 105ab into the search box.

The EFC+ and EFC- inputs are a little different from a modern OCXO. The 
hp105 provides a rear EFC input that works from -5 V to +5 V and changes 
df/f by "greater than 4e-8" over that range. [1] So that would be 4e9 / 
V which at 5 MHz is 0.02 Hz / V. Check the schematics for how they 
implement it; the manual provides all the details. It may be an example 
for your GPSDO. Your 0.033 number sounds ok to me.

In general, as you have done, it's always best to measure rather than 
read the manual, especially since the manual uses the word "greater 
than", and also since some of these 00105 oscillators are approaching 50 
years old. If you have a frequency counter it should be simple to 
measure the frequency during a voltage sweep, make a plot, and determine 
the range, the slope, and also the linearity. The nice thing about EFC 
experiments is that you don't even have to wait for the oscillator to 
fully stabilize.

/tvb

[1] EFC paragraph from page 2-4 of hp 105 manual (02479-1)


On 10/1/2020 11:53 PM, Joe & Gisela Noci wrote:
> Jim,
> some further info -
> The OCXO I use -  HP_00105-6013 - does not have a lot of info available - I
> have searched all over! I have found no definitive info on its KV, etc, so
> had to determine it myself.
>
> This I did as follows ( the complexity of my process was to try 'really'
> know if it is correct, and if the OCXO behaves according to the correct
> Physics!)
>
> The two jpegs,
> *KV determination @5V and 6V.jpg* and *KV determination @9V and 10V.jpg*
> show the phase detector wrap around during HOLD mode, with the DAC output
> set to specific voltages.
>
> For these tests the OCXO was left running for 24hours to warm up and
> stabilize as best it could.
>
> The 9V_10V and 5V_6V tests differ in the following:
>
> *     For the 9V_10V test:*
>
> The -EFC Voltage was set to -10V
> The +EFC DAC voltage was first set to 8V and the phase detector monitored
> on a 'scope. The coarse adjust trim cap in the OCXO was then adjusted so
> that the phase detector voltage was close to 1volt and 'not' drifting. This
> was monitored and adjusted over a 3 hour period to verify reasonable
> stability.
>
> Then the detector voltage was recorded with DAC set to 9V.
> Then the DAC voltage was set to +10V and the detector voltage recorded
> again.
> The phase detector wrap around time were 620sec @ 9V and 300sec @ 10V
>
> I believe this equates to -
>   @9V t=620sec so 10/620 = 0.0162Hz
>   @10V t=300sec so 10/300 = 0.0333Hz
>     *So KV = 0.0333-0.01612 = 0.017 Hz/V*
>
> *        For the 5V_6V Test:*
>
> The -EFC voltage was set to -7V.
> With the DAC voltage @+4V first, the OCXO coarse adjust was set so the
> phase detector voltage was stable.
> Then the DAC was set to +5V, which gave a wrap-around time of t= 550sec
> Then with the DAC V= +6V, t=200sec.
> *This gives a KV of 0.032 Hz/V*
>
> This all makes sense since the varicap is operating on a different part
> capacitance curve slope - at the lower voltages the capacitance is greater,
> so the delta_V effect will be greater.
>
> This verifies that the KV values are valid and sensible.
>
> I made use of the 0.033Hz/V settings, as that is what my loop gains are
> calculated for.
>

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