PM6665/PM6669 OCXO upgrade - Page 1
Forum section: electronics / testgear

author: The Bootloader
author_rank: Contributor
timestamp: December 14, 2020, 08:19:51 am
content: Hi,I have designed an OCXO upgrade board for a Philips/Fluke PM6665 / PM6669 frequency counter.I would like to present you the first video about it, which covers the concept and the design process :It's a relatively common mod that a few people have done already, but I did mine a bit differently.In a follow up video, I will cover the testing and calibration.Hope you enjoy it !

author: The Bootloader
author_rank: Contributor
timestamp: December 14, 2020, 08:50:42 am
content: Spoiler :Yes, it works

author: Verticon
author_rank: Contributor
timestamp: December 14, 2020, 09:21:29 pm
content: Hi Bootloader,very nice work and a very clear explanation of the choices you made.It is funny to hear of your project because I am presently in the process of upgrading the reference oscillator of my PM6669, too. But I am some steps behind you, have got already all the neccessary parts and waiting for the PCB from JLCPCB. The idea is also to replace the original board 1:1 with the new one. So same dimensions and same connector. But in contrast to your project I have another rationale for the design. As well as in your case I need more precision as the present oscillator in the PM6669 offer, but not on the level of a OCXO. If very precise measurements are neccessary I can use my GPSDO. Therefore I designed the new board on the basis of a very good VCTCXO. I selected a MEMS oscillator from SiTime with 0.1ppm temperature stability and I think that maximum deviations of 30-50ppb can be expected without special care. This is almost two orders of magnitude better than the original Philips oscillator and sufficient for my tasks. Because this oscillator needs only 50 to 60mA, I don't need a switch mode power supply. I just use an additional linear regulator to reduce the supply voltage to 3.3V.Another thing is the integration of a very high frequency prescaler. My PM6669 has the 1.1 GHz option but I often would like to measure higher frequencies. For that I have a separate homemade battery-operated prescaler. But it would be very nice to have a build in prescaler (as a 3rd input) that can go to 5 or 6 GHz and that don't need taking care of a battery. The biggest challenge that I see is the space on the front panael for the additional input plug. Please let me know if you have found a good solution.

author: The Bootloader
author_rank: Contributor
timestamp: December 15, 2020, 05:35:45 am
content: Quote from: Verticon on December 14, 2020, 09:21:29 pmSuper interesting !In general, there are many very capable instruments still laying around. Because the technology evolved so much, we can often boost their capabilities significantly for relatively cheap.It's nice to see the community around these nice counters is still strong, and that people are working on new upgrades even to this day. I can't wait to hear more about your project !You mention 3.3V. The counter uses 5V logic. Assuming the regular 0.7Vcc Vih, that's 3.5V and 3.3V is a bit short. It might work - might not work - or might be unreliable. If you didn't consider it, I would add a level shifter. Try being careful not to introduce jitter (not that it matters a lot, but still, it could add noise to the readings in certain measurement modes if it is too high).Quote from: Verticon on December 14, 2020, 09:21:29 pmDepends if you want to keep both the 6GHz prescaler and the original 1.1GHz prescaler in your instrument, or replace the 1.1GHz with the 6GHz prescaler.At 6GHz you cannot use BNC connectors. Assuming you want to replace the 1.1GHz prescaler, I would 3D print an adapter piece in 2 parts that you can insert on each side of the existing hole. Then when you tighten the SMA connector around it, it will keep everything in place and won't move.Considering the piece will be super small, and for added durability, I would make a prototype with a regular FDM printer but then have a metal one printed at a company like Shapeways. It should be relatively cheap and will be strong.The hardest part is to make a suitable 6 GHz prescaler. At these frequencies, it is not trivial and there are many things to take into account if you want to have the same flexibility than the original instrument in term of sensitivity and agility.Also the higher you go, the more precise you want the timebase to be. Just sayingBut 0.1ppm is super good ! I frankly didn't realize TCXOs were that good.Cheap OCXOs, super precise TCXOs... at least 2020 is bringing a few good things

author: The Bootloader
author_rank: Contributor
timestamp: December 15, 2020, 05:36:43 am
content: By the way, I just uploaded part 2 !In part 2 I test the OCXO upgrade board in the counter, and show how to precisely calibrate it with commonly available tools.Enjoy !

author: Verticon
author_rank: Contributor
timestamp: December 15, 2020, 08:38:23 pm
content: Many thanks for your hints. I use a 3.3V supply because the VCTCXO is only available in that voltage. The different supply voltage levels should be no problem. The oscillator has an output voltage of 2.6 Vpp and the 74HCT14 Schmitt Trigger IC needs only 1.5Vpp. I think this is a high enough margin. If neccessary I can shift the DC level a bit via an additional resistor to Vdd. Also the jitter should stay low because the oscillator output is rectangular with rise and fall times in the 1 ns regime. In the moment I only can show the schematics but the project is quite simple and I don't expect severe problems.Concerning the integration of a prescaler I am not sure if I ever will start a project because other things are waiting. But if so I will configure it as a 3rd input. We'll see.By the way I watched your 2nd video and I am very impressed by the stability of your oscillator which is of course way better than that what I can achieve with my TCXO but I would be happy to have the next-to-last digit exact.

author: tabajaralabs
author_rank: Regular Contributor
timestamp: January 30, 2021, 02:17:11 am
content: If someone wants to sell the C input option board for the 6665, I'd take it =)

author: The Bootloader
author_rank: Contributor
timestamp: February 07, 2021, 09:54:14 am
content: Hi there !I have made an updated version of the design, which I am describing in the following video :Enjoy !

author: The Bootloader
author_rank: Contributor
timestamp: February 08, 2021, 09:32:42 am
content: Quote from: tabajaralabs on January 30, 2021, 02:17:11 amI think we are all looking for this unicorn

author: Papa_O
author_rank: Newbie
timestamp: March 16, 2022, 11:12:54 am
content: Any chance of you posting the schematic and board of Rev 3? Would like to make my own if possible or buy a spare board from you if you have any left.