Wednesday, April 13, 2016

Fluke/Philips PM66xx Frequency Counter OCXO Upgrade

In this post I will document an OCXO upgrade board I designed for the Fluke/Philips PM66xx line of frequency counters.

Custom OCXO upgrade installed in my Philips PM6674.


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).

A nice old frequency counter: The Philips PM6674.

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.

With power taken care of, the remainder of the board is very simple. I used the CTS 10MHz OCXO that I did a teardown on and reverse-engineered the schematic for. This OCXO has a 4V reference available on one of the pins, so I did not need to add a separate reference IC to the design. Additionally, the CTS OCXO has a ~1.4Vpp sine wave output, which is quite compatible with the Philips counters. The input spec for the internal reference signal is 1Vpp into 1kohm.

My take on a modern replacement for the PM969x series of OCXO modules.

The form-factor of my upgrade board mimics the original OCXO module as closely as possible. I found the exact header I needed on Digikey for the mounting arrangement used in the Philips counter, and there is a single hole for a securing screw. The module mounts nicely in my counter and looks pretty snazzy. The performance of the CTS OCXO is sufficient for the 9-digit resolution of the counter. It warms up quickly and hugely improves the accuracy and usefulness of the counter when it is away from a proper 10MHz external reference.

The CTS OCXO warms up very quickly.

Shared as Open Source Hardware (OSHW)

Fluke/Philips PM66xx OCXO Upgrade: Order PCBs on OSHPark!

Eagle Files: Shared on Github

Schematic of the PM66xx OCXO upgrade board.

Parts List

OCXO: CTS model 1960017 (available on eBay) or compatible substitute
DC-DC Converter: Recom 78C5.0-1.0
C1: 100uF / 50V, 105*C rated electrolytic (8mm diameter)
C2: 2.2uF X7R 0805 50V rated ceramic
C3: 220uF / 35V, 105*C rated electrolytic (8mm diameter)
C4: 10uF X5R 0805 25V rated ceramic
C5, C6: 100nF 0603 ceramic
R1: 51 ohm 1206
VR1: Vishay M64Y103KB40 10k 21 turn pot
Header: Molex 22-14-2104

The ceramic capacitors install on the bottom side of the board.

Installation Note: On some counter models, such as the PM6674, you need to remove the stock 10MHz crystal when you install this board. I simply cut the two legs of the crystal from the top of the board and left it glued in place, as you can see in the first picture in this article. Other counter models such as the PM6680B have jumpers near the crystal to easily disconnect it and select the OCXO reference.

12V/24V Rail: Some frequency counters in the series, such as the Fluke PM6680B, have a 12V rail available for the OCXO instead of 24V. This upgrade board is still compatible with those models, as 12V is sufficient to operate the DC-DC converter.

Rev A?

Sharp-eyed readers will note that this shared version of the board is Revision B. Well, that begs the question: what happened to Revision A? The original version of the board used a different OCXO, the 1" square NDK unit that I incorporated into my Racal-Dana upgrade. Due to the lack of a voltage reference on that OCXO, I had to incorporate an LM4140 into the design. The square wave output of that unit was also not ideal for the Philips counter. The whole design seemed needlessly complicated to me, especially when I knew I could make a more elegant solution based around the CTS OCXO.

Revision A of the PM66xx OCXO upgrade board. This one didn't make the cut.

Wrap Up

Please let me know if you assemble one of these boards for your counter! I have also provided the Eagle files, so you can modify the board if you like to use a different OCXO footprint. I have an entire case of the CTS OCXOs to use up, so expect some more shared projects that use them in the future.

Thanks for reading!

- Dan W.


  1. Very nice, Dan. If you buy test equipment to use those CTS OCXOs, you will have one impressive lab by the time you use them all. ;-)

  2. This will work on the PM6654c ?
    and how much for one of this units?

  3. Hello Dan, I just ordered a used PM6671 on Ebay, and when I started reading the description properly I found it was the entry level device with just a crystal in it. I found it has the optional upgrade to an oven based oscillator, so this is where you come in!

    My counter will arrive somewhere next week,so I started looking for the parts you used on your board. Somehow I cannot find the OCXO on Ebay?

    Can you help me to find that part please?

    I love your upgrade,

    Un saludo,

    Leo Kroonenburg
    (Retired Dutch engineer, living in Spain)

    1. Hi, the original US seller sold out, but there are still some left here:

      I have purchased from this seller before with no issues. Good luck with your upgrade and thanks for your comments!

  4. mhm the molex 22142104 sees to be hard to find in Belgium ..