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.
Super 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).
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.
Depends 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 saying
But 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
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