Summary of Fluke/Philips PM66xx Frequency Counter OCXO Upgrade
This article details a custom OCXO upgrade board designed for the Fluke/Philips PM66xx frequency counter series, specifically the PM6674. The project addresses the limitations of the original XO timebase by integrating a more stable OCXO module. Key challenges included adapting to the counter's 24V standby rail and high voltage ripple, which were resolved using a Recom 78C5.0 DC-DC converter to generate a stable 5V supply. The resulting board mimics the original PM9691 module specifications.
Parts used in the PM6674 OCXO Upgrade:
- Recom 78C5.0 DC-DC converter
- Electrolytic capacitors
- Ceramic capacitors
- Custom OCXO upgrade board
In this post I will document an OCXO upgrade board I designed for the Fluke/Philips PM66xx line of frequency counters.
Introduction
A few months ago I purchased a Philips PM6674 frequency counter on eBay. It’s an older 9 digit counter with two channels that has a maximum input frequency of 550MHz. The design feels dated compared to more modern counters, such as my Agilent 53131A. However, it is still a fully functional piece of lab equipment with a simple user interface and compact design. I often prefer older counters for day-to-day use because I don’t have to fuss with complicated menu-based interfaces and features that I don’t need. (Set the gate time on a 53131A and count how many button presses it takes).
My counter came with the standard XO timebase option, which has fairly poor specs for stability and drift. It is difficult to trim precisely with the single-turn trimmer capacitor on the board. For most testing in my lab I use an external reference from a GPSDO, but it is still nice to have an accurate timebase available in the counter if I need to take it somewhere and do testing away from the bench.
Previously I posted about an OCXO upgrade I made for my Racal-Dana 1992. The fun of designing a similar upgrade for the Philips counter was one of my motivations for purchasing it. My upgrade board is roughly equivalent to the original PM9691 OCXO module, and it should be compatible with any Fluke/Philips counter that is capable of using that option.
Designing an OCXO Upgrade
Creating a timebase upgrade board for the PM6674 was a bit more involved than the process for designing the Racal-Dana upgrade. That counter had a clean 5V supply available on the header where the timebase board connected, which was exactly what I needed for the OCXO. The header for the OCXO module in the Philips counter also has a 5V rail. However, that rail does not stay active when the counter is in standby. The original Philips OCXO modules operated from the 24V rail, and I had to use that for my own upgrade. In my counter, the “24V” rail actually runs at about 27V, and drifts up to 30V when the counter is in standby. There is also a substantial amount of ripple.
I opted for a Recom 78C5.0 DC-DC converter to get the 5V I needed to power my board. This is a nice little module with a pinout that mimics the 7805 linear regulator. It has good specs for efficiency (as high as 96%) and was very easy to implement on my board. I used a combination of electrolytic and ceramic capacitors on the input and output for filtering.
- Why was an OCXO upgrade necessary for the Philips PM6674?
The standard XO timebase option has poor stability and drift, making precise trimming difficult. - How did the designer power the new upgrade board?
A Recom 78C5.0 DC-DC converter was used to convert the 24V rail to a stable 5V supply. - What voltage does the original Philips OCXO module operate from?
The original modules operated from the 24V rail. - What specific issue existed with the 24V rail in this counter?
The rail runs at about 27V normally but can drift up to 30V in standby with substantial ripple. - Does the 5V rail on the PM6674 header stay active in standby?
No, the 5V rail does not stay active when the counter is in standby. - Is this upgrade board compatible with other counters?
Yes, it should be compatible with any Fluke/Philips counter capable of using the PM9691 option. - What type of capacitors were used for filtering?
A combination of electrolytic and ceramic capacitors was used on the input and output. - What is the maximum efficiency of the Recom 78C5.0 converter mentioned?
The converter has good efficiency specs as high as 96%.
