PM6665/PM6669 OCXO upgrade : concept and design process (part 1)

Today, I guide you through some of the design process of a more accurate replacement timebase board for Philips PM6665 and PM6669 counters.
Related work cited :
- github.com/LieBtrau/pm6665-re...
- www.jackenhack.com/philips-fl...
OCXO source :
- www.ebay.com/itm/1-CETC-CTI-O...

[@gijsderidder8103]

Very nice! I'm looking to upgrade my PM6669 as well, would you consider sharing the schematic and parts list?

[@the_bootloader]

I have published a new video showing the latest version of the design. Send me a message (see the about page), I have a few spare PCBs

[@poul-erikjensen508]

The voltage supply to the oscillator should be linearly regulated (L7805 for example) instead of a switch regulator which makes a lot of switc noise ?

[@the_bootloader]

It is true that switching power supplies tend to be noisy. However, most modern high-end spectrum analyzers, oscilloscopes, and similar instruments are full of them. So they are not necessarily a problem. In this case, this is a pretty low tech instrument by todays standards. It is not very sensitive. Additionally, the OCXO itself has a pretty high power-supply noise rejection ration (per the spec +-2ppb for +-5% Vcc change). It is true that it is possible that the power supply could, in theory, inject noise into the instrument, and then conduct that noise into the device under test. However, one should also take into account various other noise sources in the lab, including noise coupled into the AC from other stuff plugged into the AC power, and then into the instrument itself. I have seen multiple labs with cheap LED drivers that were noisy as hell for instance. So in a nutshell... I believe it is a non issue in this application, but, without specific measurements, I am just guessing. Maybe this is a good idea for another video :-) If it is indeed an issue, it is possible to use a linear regulator. However I am concerned about the spare capacity of the built-in transformer to supply enough power when the OCXO warms up as it uses a significant current. One option to explore would be to use a shieled DC/DC module and proper filtering at its inputs and outputs. Best of both worlds. Maybe for a next revision of the board, if it turns out to indeed be an issue.

[@stargazer7644]

Just plug a GPSDO into the external ref jack. Far easier and more accurate.

[@the_bootloader]

Depends of the situation. Right now, in my lab, I can't have satellite reception, and I can't install an antenna on the roof either. Having the capability to have an OCXO in my instruments, that I can calibrate once a year or so, is perfect. It gives me more precision than I realistically need. Of course, everyone's situation and needs are different. Thanks for watching!

[@stargazer7644]

@@the_bootloader While an outside antenna is always best, once a GPSDO's location is set, it only needs one satellite to stay phase locked. GPS receivers are very sensitive now, and some will track multiple GNSS constellations. Mine works just fine inside the house through the floor above and the roof. It looks like your lab has a window. As long as there isn't metal film on the glass, a GPS receiver will usually work fine looking out the window. If you're in a multi-story concrete building with tinted windows and no balcony, you may have a problem. Good luck!

[@the_bootloader]

@@stargazer7644 Thanks! All good points. Unfortunately I am in a multistory townhome, and the lab is on the ground floor