Hi

Per the published spec on the part, it should hit 10 MHz at 2.0 V.

A properly working part should move 5 to 10 Hz for a half volt change
(like the one from 1.9942 to 2.6832 V.

With any OCXO, meaningful data can only be taken after the part has
warmed up. For accurate data, something like a 24 hour warmup is
a pretty good idea.

Bob

That's the spec on the new part. These are salvage from decomissioned cell transceivers.They've probably been drifting for 20+ years. Point is they still work. They just need personalized care and feeding.

The tuning voltage is irrelevant so long as you have a low tempco divider and stable voltage reference to provide that.

That is the current task, determine what I need for the voltage dividers and order them. $$$ :-(

Buy some and test them.

Reg

 On Saturday, June 3, 2023 at 08:46:36 PM CDT, Bob Camp via time-nuts <time-nuts@lists.febo.com> wrote:  

Hi

Per the published spec on the part, it should hit 10 MHz at 2.0 V.

A properly working part should move 5 to 10 Hz for a half volt change
(like the one from 1.9942 to 2.6832 V.

With any OCXO, meaningful data can only be taken after the part has
warmed up. For accurate data, something like a 24 hour warmup is
a pretty good idea.

Bob

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Reg,

Thanks for posting your 10 points of raw data. That will help debug the
problem. I made two plots for you.

Your first point (at 1.5819 V) doesn't look correct so I ignored that.
The 8th point (at 23708 V) looks wrong -- 24 kV is kind of high for an
OCXO -- so I changed it to 2.3708 guessing it was just a transcription
error.

Plot reg-1.png -- For EFC testing of a good oscillator the points tend
to fall very consistently along a path and the path is often a line or a
slightly curved line. See [1] for an old example. But your data is quite
scattered. That might indicate that either your voltage source is very
unstable or your oscillator is unstable or even your frequency counter
is unstable. Or something as mundane as loose wires or noise or trigger
error.

I would suggest before investigating EFC further that you strive to get
clean readings. Perhaps set the EFC to a fixed value, maybe use a 1.5 V
battery, and then collect 10 frequency readings in a row. They should
all agree to 0.1 or or 0.01 Hz. If so then your counter is not the
problem. BTW, what is your counter gate time set to?

Plot reg-2.png -- For the 2nd plot I sorted your data and connected the
dots. Again this should be linear or approximately linear. You can see
the reading (1.7740 10.00000050) is completely out of place suggesting a
problem with your setup. And the 2 or 3 points at the top end don't look
right either.

/tvb

[1] http://leapsecond.com/pages/tcxo-efc/

On 6/3/2023 1:40 PM, Reginald Beardsley via time-nuts wrote:

Hi fellow time nuts,

I recently did a design around the CTI OSC5A2B02

For the tuning voltage I use a LT1029 or LM336 Z5, 5 Volt shunt regulator. Both have a nearly flat tempco.

I measured the input current of the Vref pin and it is very very low  < 0.1 μA, so I don't expect any trouble using a potentiometer after the shunt regulator as a means for  tuning the OXCO. I used a 25 stroke Bourns trimmer which is no luxere. While calibrating the OSC5A2B02 with a GPSDO and a a good old fashioned analog HP phase meter, I needed to make really tiny shifts with the trimmer to get the OCXO in phase with the GPSDO. I timed it at more than 100 seconds for a whole 360 degree cycle, which is satisfactory for my needs and more or less the accuracy ;limit of this particular OCXO.

I also did some rudimentary calculations on the heat management of the the two cascaded linear step down voltage convertors I use for the main power supply for the OSC5A2B02 and the shunt regulator.

You can find the whole story of my design on my Ham Wiki: https://www.pa7elf.nl/en:projects:10_MHz_ocxo_frequency_reference

You can find the schematics @:  https://www.pa7elf.nl/_media/nl:projects:PA7ELF_frequency_reference_with_10_mhz_oven_crystal.pdf

Let me know what you think.

Bart
PA7ELF

Tom,

Thanks. Typo is correct. I didn't describe the data properly.

Those are a single measurement on 10 OXCOs to determine the 10 MHz bias voltage requirement for each. I was adjusting the PSU voltage which is rather coarse, to get close to 10 MHz (within 1 Hz) and measuring the actual voltage with a 34401A.

The purpose is to calculate what precision, low temp co resistors I need to set the Vref for each oscillator. With particular care to try to make it neutral at a convenient temperature. Those resistors are seriously not cheap. The Vishay 50 ppm/C precision foil trimmers ar $25.

The 5386A was running on the internal XOCO as I simply wanted order of magnitude values for each OXCO. So an accuracy of ~2 ppb (5386A and DUT) for time and <50 ppm for voltage.

I'll run the same series with a better reference for Vref so all are tested for frequency at 2 V input to Vref.

Reg    On Sunday, June 4, 2023 at 11:35:11 AM CDT, Tom Van Baak via time-nuts time-nuts@lists.febo.com wrote:

Reg,

Thanks for posting your 10 points of raw data. That will help debug the
problem. I made two plots for you.

Your first point (at 1.5819 V) doesn't look correct so I ignored that.
The 8th point (at 23708 V) looks wrong -- 24 kV is kind of high for an
OCXO -- so I changed it to 2.3708 guessing it was just a transcription
error.

Plot reg-1.png -- For EFC testing of a good oscillator the points tend
to fall very consistently along a path and the path is often a line or a
slightly curved line. See [1] for an old example. But your data is quite
scattered. That might indicate that either your voltage source is very
unstable or your oscillator is unstable or even your frequency counter
is unstable. Or something as mundane as loose wires or noise or trigger
error.

I would suggest before investigating EFC further that you strive to get
clean readings. Perhaps set the EFC to a fixed value, maybe use a 1.5 V
battery, and then collect 10 frequency readings in a row. They should
all agree to 0.1 or or 0.01 Hz. If so then your counter is not the
problem. BTW, what is your counter gate time set to?

Plot reg-2.png -- For the 2nd plot I sorted your data and connected the
dots. Again this should be linear or approximately linear. You can see
the reading (1.7740 10.00000050) is completely out of place suggesting a
problem with your setup. And the 2 or 3 points at the top end don't look
right either.

/tvb

[1] http://leapsecond.com/pages/tcxo-efc/

On 6/3/2023 1:40 PM, Reginald Beardsley via time-nuts wrote:

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Reg,

Ah, it wasn't clear the 10 readings were from 10 different oscillators.
Thanks for the clarification. Ok, what you see then is simply that all
ten oscillators can be set on-frequency and still be within your desired
EFC tuning range. That's a good result.

The next step is to determine what your accuracy or stability
requirements are. There's no point in spending extra money for an ultra
stable Vref if all you need is just 7 or 8 digits of accuracy, or if
that's all your 5386A counter can even measure.

I don't quite understand your comment about expensive resistors. Most
instruments with OCXO have an EFC adjustment, usually implemented as a
single 10T or 20T panel- or PCB- mounted pot. To first order tempco
doesn't matter because the pot is being used as a divider across Vref.
Yes, a temperature coefficient in the pot changes the resistance of each
leg but it doesn't change the ratio.

It's a cheap experiment for you to try. Surely you have a ~10k
multi-turn pot lying around. Measure the frequency periodically over a
day and also record temperature. As a baseline do the same with EFC
shorted. The difference between the two sets of data tells you how much
of the tempco you see is in the OCXO and how much is in the Vref /
divider / EFC.

You mentioned you have a hp 34401A as well. You could use that to
directly monitor the EFC voltage and add that to your plots. Best to
automate the data collection; do you have a GPIB adapter for your DMM
and your frequency counter?

/tvb

On 6/4/2023 11:51 AM, Reginald Beardsley via time-nuts wrote:

I was sloppy with my post, but I'd been at it for quite a while and was tired. Next time I'll wait until the next day to post.

OK, time to 'fess up.

I've got a 1990's $500k MSRP T&M lab. Missing the bits to do precision time closer than 1e-10. I was using the 34401A to monitor the voltage.

I have a Tek 11801 sampling scope and a rich suite of sampling heads. I can measure to 20 femptoseconds, though noise becomes a serious issue at delays that small. But I think I see how to use that to compare OXCOs.

I'm already getting better than 1e-10 accuracy (the limit of the 5386A). So the project is to see if I can achieve 1e-11 or better. What I'd really be pleased with would be 1e-12 from a 4 OXCO reference over the course of a year. based on cals at 1,2, 4, 8 and 16 month intervals. Does it make any sense? No. But I've got an idea for solving a difficult problem and those are the things I enjoy playing with. This is a pure aging correction. The shift for an OXCO operating continuously in a stable environment.

The key to this is using basis pursuit to select from say, 1 million possible aging curves, the 2-3 curves whose sum best matches the available data. My testing of numerical models of the heat equation, an infinite sum of exponentials, showed that I could predict future values to with 1-2% for as far into the future as I had data from the past.

To get acceptable adjustments, you can't have a single pot for the entire divider. So you have to have a small pot between a pair of fixed resistors chosen so the temp cos cancel. Vishay is pretty much what you can get and they are not cheap. The voltages I stated were the EFC voltages measured by the 34401A.

I have AR488s for GPIB. No worries there.

The concept I'm working on is to model the aging of N OXCOs, adjust Vref to keep the output on frequency and at octave time intervals recalibrate the unit.

Let's suppose an OXCO with a 365 ppb first year aging rate. If I predict and correct Vref for the aging rate to ~2% let's call the error 7 ppb. That's a 50x improvement in accuracy by using an MCU to adjust Vref over the course of the year. If the 0.02 ppb/s is random, then having 4 OXCOs feeding a common tuned circuit should reduce the random frequency errors from 0.02 ppb/s to 0.01 ppb/s (1/sqrt(N)).

Have Fun!
Reg
On Monday, June 5, 2023 at 03:06:40 PM CDT, Tom Van Baak via time-nuts time-nuts@lists.febo.com wrote:

Reg,

Ah, it wasn't clear the 10 readings were from 10 different oscillators.
Thanks for the clarification. Ok, what you see then is simply that all
ten oscillators can be set on-frequency and still be within your desired
EFC tuning range. That's a good result.

The next step is to determine what your accuracy or stability
requirements are. There's no point in spending extra money for an ultra
stable Vref if all you need is just 7 or 8 digits of accuracy, or if
that's all your 5386A counter can even measure.

I don't quite understand your comment about expensive resistors. Most
instruments with OCXO have an EFC adjustment, usually implemented as a
single 10T or 20T panel- or PCB- mounted pot. To first order tempco
doesn't matter because the pot is being used as a divider across Vref.
Yes, a temperature coefficient in the pot changes the resistance of each
leg but it doesn't change the ratio.

It's a cheap experiment for you to try. Surely you have a ~10k
multi-turn pot lying around. Measure the frequency periodically over a
day and also record temperature. As a baseline do the same with EFC
shorted. The difference between the two sets of data tells you how much
of the tempco you see is in the OCXO and how much is in the Vref /
divider / EFC.

You mentioned you have a hp 34401A as well. You could use that to
directly monitor the EFC voltage and add that to your plots. Best to
automate the data collection; do you have a GPIB adapter for your DMM
and your frequency counter?

/tvb

On 6/4/2023 11:51 AM, Reginald Beardsley via time-nuts wrote:

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