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

Fri Oct 2 11:44:15 UTC 2020

Hello Tom.
Thanks for your info.

My oscillator comes from an HP-5061a, which I obtained from a company as
the tube was dead.
The log book indicates a start date of 1971, so yes, not a youngster!
I have the full set of original manuals for this 5061A , but the oscillator
specs are not detailed to the KV level, nor is the actual voltage level
range of EFC indicated in the manual.
The full circuits are there, etc.
In the 5061A , the EFC was a +ve voltage from an accurate reference, via a
front panel multiturn pot, onto the +EFC input, and then the loop control
voltage from a op-amp/hybrid, which is fed from a split 30volt supply.
So it was not possible to discover what range the control voltage out of
the op amp covered ( not in the circuit descriptions) . I finally
discovered in the calibration descriptions that the front panel pot had to
be at midrange ( -7.5v) and the coarse
control trimcap in the OCXO adjusted to set the control voltage (+EFC
voltage) to +5V....

Thanks again!

I will persevere!

Kind Regards
Joe

On Fri, Oct 2, 2020 at 12:41 PM Tom Van Baak <tvb at leapsecond.com> wrote:

> 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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