HP5385A OCXO Timebase

[Miti]

Being obsessed with the stability of the time base in my instruments (does it sound familiar?) I decided to make an OCXO oscillator for my HP5385 frequency counter (the TCXO oscillator that was installed is ridiculous) until I find the time to put together a GPSDO. It is nothing too crazy, a used OSC5A2B02 OCXO from Ebay, a MAX6126AASA21 and a potentiometer. So I made a board, got it done in China by PCBWay and installed it today. All I can say is that the difference is huge. I calibrated it using another 10MHz OCXO manually disciplined to a GPS module using my Tek TDS210 scope and a programmable power supply for VFC.

[metrologist]

Are you offering these populated minus the clock? Do the pins connect to the counter's board as a drop-in replacement?

[Miti]

I'm offering the bare boards only as I don't have the time to populate them and  don't think I'd make any geschaft.
The board is a drop in replacement for option 004, you'll need to do the following beside populating this board:

1. Remove the TCXO.
2. Populate a ferrite bead and a 10 - 100nF capacitor that are left out on the counter board.
3. Move a jumper from one location to another.
4. Solder the new board in the place of OCXO, drop in as I said.
5. Let it run for few hours and then calibrate.

It is a dramatic improvement.

[DC1MC]

Hi Miti, do you still have some boards, how much will cost to post one to Germany in a padded envelope ?

 Cheers,
 DC1MC

[DC1MC]

The board from Miti arrived nicely and the OCXO as well (I've got tow of each type, they are compatible).

Because of a brain fart  , I've posted pictures in this thread:
https://www.eevblog.com/forum/testgear/hp-5385a/

Bit promise that when the board is done, I'll post the final result here  .

  Cheers,
  DC1MC

Edit: Added a picture to the Miti PCB, OCXO and a DIP16 socket for reference

Also a datasheet PDF oft the CTI oscillator.

[Miti]

After more than one month of running continuously, with the OCXO timebase and the 10MHz GPSDO at the input, my frequency counter drifts about +/- 400ppt maximum, probably due to ambient temperature changes. Impressive, eh?

[texaspyro]

An OCXO that has been powered off for a long time can take over a month for the drift to level off and stabilize.  Here is what the 8663 DOCXO did when I put it in an HP58132A counter.

UTC 8663 mounted on Sweeny DOCXO card in a HP 53132A counter.  The 8663 had
not been powered up for over 10 years.  Below is the frequency drift per day
as the 8663 aged in.  THe 8663 is spec'd at 1E-10 (1 mHz) per day aging. From
past experience, they are capable of much better than that (1E-11 to 1E-12 per
day).  They seem to be limited in this application by the EFC DAC / VREF
performance.

The 8663 was mounted on the Gerry Sweeny aftermarket OCXO board for the 531xx
counters.  For the first 30 days, the board was configured for the 0 .. 10V
EFC DAC range.  The Sweeny board powers the DAC with +/- 12V which is a bit
out of the spec'd minimum +/- 13.5V required for optimum performance on
the 0 .. 10V range.

The Sweeny OCXO board is based up on the HP design with a couple of nice mods.
First it has footprints for several common OCXO modules.  Second, it allows
the DAC range to be configured for 0 .. 5V,  0 .. 10V, and +/- 5V. 

The circuit feeds the OCXO output through a differential line driver.  The line
driver sends the OCXO to the HP counter mother board through a ribbon cable.
The motherboard makes the OCXO output available on a BNC on the counter
back panel.  Checking the ADEVs of the raw OCXO outout and the counter output
shows that the 53132A degrades the DOCXO output by around 1.5E-12 (raw DOCXO
value around 5.1E-12 at 200 seconds,  counter 10 MHz output BNC around
6.5E-12)

For the first 30 days the EFC DAC reading was recorded and the HP53132A was
then auto-cal'd and the new EFC DAC output was recorded.  (The DAC readings
for the first 10 days were lost when a momentary power failue occured and the
system was not shut down cleanly).

The initial 30 day run was done with the Sweeny board DAC configured
for the 0 .. 10V range.  Given the DOCXO EFC sensitivity (around 0.75 V / Hz)
and the 12-bit DAC, the minimum auto-cal resolution is 0.002 Hz.  Several
auto-cal cycles were performed each morning until one was within 0.001 Hz.

DAC readings were made by a Tektronix DMM914 4.5 digit DMM, not the best tool
for the task, but it was available. 


        drift (Hz/day)    DAC (after cal -> 24 hrs later)
day 1:    .298000 Hz       
day 2:    .057000 Hz       
day 3:    .033000 Hz       
day 4:    .022000 Hz       
day 5:    .017000 Hz       
day 6:    .011800 Hz       
day 7:    .009640 Hz      4.471V
day 8:    .008400 Hz      4.458V
day 9:    .006380 Hz      4.451V
day 10:   .003880 Hz      4.443V
day 11:   .002840 Hz      4.446V -> 4.438V  // power glitch caused reset
day 12:   .003470 Hz      4.437V -> 4.437V
day 13:   .002100 Hz      4.432V -> 4.433V
day 14:   .001590 Hz      4.430V -> 4.430V
day 15:   .001380 Hz      4.426V -> 4.428V
day 16:   .001660 Hz      4.424V -> 4.426V
day 17:   .000526 Hz      4.422V -> 4.423V
day 19:   .000284 Hz      4.421V -> 4.423V
day 20:   .000038 Hz      4.423V -> 4.424V
day 21:   .001019 Hz      4.420V -> 4.420V
day 22:   .001350 Hz      4.417V -> 4.418V
day 23:   .000402 Hz      4.417V -> 4.417V
day 24:   .000953 Hz      4.418V -> 4.419V
day 25:   .000304 Hz      4.417V -> 4.416V
day 26:  -.000441 Hz      4.417V -> 4.418V
day 27:   .000214 Hz      4.415V -> 4.415V
day 28:   .000517 Hz      4.415V -> 4.416V
day 29:  -.000998 Hz      4.415V -> 4.415V
day 30:   .001300 Hz      4.415V -> 4.414V
day 31:   .000423 Hz


Powered down, replaced 53132A fan, changed DAC range to 0 .. 5V, and
the 53132A was put back into case (hence no more DAC readings).  The 0 .. 5V
DAC range reduced the auto-cal minimum step size to 0.001 Hz.   Hopefully
it should also improve the DAC stability.

Warmup frequency error after power up:
   1 Hz     - 3m     
   0.1 Hz   - 3m 25s
   0.01 Hz  - 5m 45s
   0.001 Hz - 6m 15s

After 24 hours the unit was auto-cal'd once and left alone.  The frequency
error for the next week:

day 32:   .000152 Hz  (0.00009 Hz/day for last 18 hours)
day 33:  -.000825 Hz
day 34:  -.000730 Hz
day 35:  -.000287 Hz
day 36:  -.000144 Hz
day 37:  -.000086 Hz
day 38:  -.000083 Hz

[cncjerry]

Mark, I sent you a note the other day asking how you are using Lady Heather to do these plots. I was thinking that once I had my Rb standard tuning word set, couldn't I use another 1PPS signal against the now stable Rb 1PPS to do ADEV in LH?  How are you reading both the Tek meter and frequency counter in LH?   Or am I missing something simple?

Thanks

Jerry

[texaspyro]

No Tek meter involved...

I was using a TAPR TICC dual channel interval counter clocked by an HP-5071A.   For the HP counter, I connected the counter freq ref output to the TICC via a TAPR TADD2-Mini divider (divides 10 MHz down to 1Hz).  Heather is configured with the TICC as the input device.

[cncjerry]

That TICC shows up a lot in time nut projects and I assume heather supports it. Isn't the sampling rate limited to >1sec though?  I have to go read the manual.  I've been looking for a new counter and have been considering the PM6681 or a TICC.  I have a TADD2 and I guess you use that to make up for the sampling rate in the TICC, correct? I've been reading up on the Tight PLL method of measuring ADEV and was planning to start there and maybe build the mini Dual Mixer Time Difference setup.   But if you get acceptable results using the TICC maybe I should just buy one already and try it out.

Thanks

Jerry

[texaspyro]

The TICC can handle over 100 Hz.  With some firmware mods the hardware can probably do 1 kHz... hence the use of input dividers.  The serial port data rate is one limitation.

If you want to do independent two-channel measurements on the TICC you need to have a good input clock source.   If you are doing differential measurements the TICC input clock does not need to be as good because it is basically being used as a short-term time-transfer reference.

Both Heather and Timelab support the TICC.

[Miti]

An OCXO that has been powered off for a long time can take over a month for the drift to level off and stabilize.  Here is what the 8663 DOCXO did when I put it in an HP58132A counter.

Yes, I was surprised to see both OCXOs, the one in the GPSDO and the one in the frequency counter, drifting slowly up, the GPSDO adjusting the DAC lower and lower, and I having to adjust the potentiometer every few days. They seem to have slowed down but still drifting up super slow.

I thought that them drifting was the reason why they were removed from the original equipment but now I understand it is normal and I can expect them to level off by now.
On top of that, I had to fix the OCXO in the GPSDO, the heater transistor was disconnected from the crystal so I know at least why that one was removed.

[metrologist]

After more than one month of running continuously, with the OCXO timebase and the 10MHz GPSDO at the input, my frequency counter drifts about +/- 400ppt maximum, probably due to ambient temperature changes. Impressive, eh?

That is just within spec. It will probably get better as it runs. My Wenzel timekeeper has increased 0.593mHz since September. Not as impressive as texaspyro's...

[mikepi]

Hi, I'm a new member are the boards still available?

[Miti]

I've sent you a PM.

[Miti]

I'm adding the BOM as some people are asking. It is in .csv format, just rename it.

[ChrisJordan]

Hi Miti,  have you got one of these OCXO boards left? How muchwould it be posted to the UK?
I've got a Grundig UZ2400 counter/timer and it's OCXO has died, no circuit diagram seems to be available so I think it's easier to replace it with an OSC5A2B02 off eBay!
Regards... Chris

[aquaman8]

Hi Miti,

Any boards left? If yes, how much for board and postage to Ontario L3X1R9?

Thanks,

Mitch

[fornext]

Sorry to be late to the party... I would buy a board if you have any left.  I'm new here, how would I go about it?

Thanks for the post.

[pizzigri]

Updating this thread as I am building the timebase! Therefore, I have 8 5 boards just in from JLCPCB (keeping a spare just in case) that I have very little use for.
So for 5 Euro each, plus postage (your choice of delivery... i.e. registered mail in EU with certified delivery is going to be around 2.65 Euro for a parcel up to 50g), you can have one of the boards!
I asked Miti for permission, and this is obviously not done for commercial reasons whatsoever.
Anyone interested, just PM me.

Best,

Franco

ETA: the scratches are in the plastic envelope, not on the boards. Just FYI.

[Miti]

Sorry I missed all these requests for boards. I don’t have any board left.

[pizzigri]

Hello Miti,
I am building the board, however there are a few things that are unclear to me in actually mounting the OCXO on the meter.
First off, the jumper you refer to are W2 and W3, correct? Meaning I remove the 0R part from W2 and solder it to W3 to switch to oven XO.
Plus, I removed the original TCXO, added the ferrite inductor and I have a 70nf ceramic disc capacitor salvaged from old equipment (forgot to order it in the Mouser BOM... crap), would that be OK?
Do I need to do other things to make the transition?
Also, I can not find replacements for the three big caps in the power section, especially the big ones as they have three leads.... any suggestions? It’s not that they don’t work, but the smaller one is suspicious and since the meter is open on the bench I thought to recap it.

[Miti]

That sounds right to me. I did not recap my counter, it may not be necessary.

[pizzigri]

Ok, so to update this thread with a happy ending....
I built the board and installed it. The OCXO I bought from Ebay continued drifting for almost two weeks, I actually thought it was defective. However, in the end it settled and I managed to calibrate it against the GPSDO to a couple millihertz. Extremely difficult to since the freaking OCXO can feel the heat coming off my hand when I get near to it. The Morion MV85 I have is not so critical. Mah? In any case, once done it’s over a week Miti’s board is stable and not drifts at all. I am enclosing a couple pictures, the first one is at turn on of the meter, the second one is after 20 minutes and is spot on. I am grateful for Miti for the project and help in building it and also to every one of the EEV Blog members that also helped and posted in this thread. The HP 5385A is a fantastic counter, no fancy stuff but a basic solid meter that just works and has great resolution, I mean there aren’t all that many 11 digit counters around at less than 200$, granted not on all ranges....

[pizzigri]

So, since people have asked...
Miti's OCXO board is a direct replacement for Option 004 Oven Timebase, mfr 28480, code 0960-0636.
And, is therefore compatible with the following HP meters:

5384A
5385A
5386A