Upgrading a Philips-Fluke PM6665 Frequency Counter with an Oven Controlled Crystal Oscillator – Up and Running!
I needed a frequency counter for my little lab at home. Naturally, I wanted an HP Agilent 53132A or similar, but it’s way too expensive for my modest use. So I browsed around eBay and found an old Philips-Fluke PM6665 frequency counter with the optional 1.3GHz add-on board. With a 10 second gate time, I can get down to 0.1 Hz at 10 MHz, so that’s more than I need. It didn’t have the option of a better crystal, unfortunately. When I tested the PM6665 against my Trimble 10 MHz GPSDO reference, the unit was off by 200.1 Hz. Without a calibration sticker on the instrument, I’d say that’s pretty good for a device manufactured in the late 80s. I tried to trim it, but it uses an adjustable capacitor, and you only needed to look at it for it to change settings.
The other problem was the temperature difference when having the unit open for calibration. As soon as I closed it, it started to drift. There is an input on the back of the unit for a lab-reference 10 MHz signal so I could connect my GPSDO to it, but I wanted to see if I could get good precision on it as a standalone unit. There are a lot of salvaged components available on eBay from old cell-towers and other equipment that need precision timekeeping. So for around $15 you can get a 10 MHz crystal ovenized oscillator. They look like they’ve been removed with a crowbar. But the two I bought has been working flawlessly. And just like with old wine, a crystal gets better with age.
One of my favorite YouTubers, Gerry Sweeney, go and subscribe! Upgraded his old Racal-Dana 199x with a 5V ISOTemp OCXO131–100.
So I modified his circuit to work with my old Philips PM6665. It uses a MAX6198A as a voltage reference for calibration. It has low drift from temperature differences, 5ppm/°C (max) Temperature Coefficient and ±2mV (max) Initial Accuracy, so It’s a perfect fit for using as a calibrator for the voltage controller pin on the OCXO. The trimpot also has drift due to temperature, but when the unit warmed up in about 15–20 minutes its super stable. There is a built-in calibration in the frequency counter, but I couldn’t be bothered trying to decode the serial traffic between an old 8-bit processor and the board.
I designed the PCB using the (not so well scanned) service manual as a reference because I still hadn’t received the unit sent to me from the seller in Canada. A weird side story. My counter turned out to have been manufactured just a couple of kilometers from where I live here in Sweden. So in a way, it finally came home again. (Ah, the good old days when we manufactured electronic equipment.)
After reading the schematic, I was confident of the placement of the board. But I was wrong. My design was counting on that the oscillator board would be horizontal, but naturally, it was vertically mounted. That was a stupid mistake. No problem, I just had to bend the connector socket, installing it at an angle and everything was hunky dory. Well, one of the mistakes. I forgot that it was mirrored when making the package for the ISOTemp unit in Eagle PCB, Easily fixed, just solder it on the backside! (fixed in the new version below).
The PM6665 has an LCD screen that isn’t backlit, so I added two white LEDs. That turned out quite well. Now I can see even in low light. There’s a connector to the front panel where pin 9 is +5 volt and pin 1 & 10 are GND. I was a little bit worried that the weak old 7805 DC regulator would go berserk with all the extra current being drawn, both from the OCXO (600 mA on turn on, stabilizes to around 250 mA when up and running) and from the two LEDs, but It seems to work correctly. I added an extra 5 Volt supply connector to the PCB if there would be a problem. While at it, I also added a place for an SMA connector for the 10 MHz signals if I want to use the card for something else.
Here’s the schematic with the 10-pin female connector (I desoldered the original and put it on my new PCB). As you can see, there’s a header for choosing where to get the power. Either from the main board or your supply. I use the +5 Volt from the connector without a problem (so far). If you want to go fancy-schmancy, you could build a 5-volt regulator with filtering, etc., but I’ve been running the counter for days, and after calibrating the OCXO using an oscilloscope (more comfortable and quicker), it’s been rock solid.
A New Schematic and PCB
I’ve designed a single sided PCB in Eagle PCB with the correct size and orientation. Download it and go nuts. It’s pretty Schmick to have an old instrument perform like it’s 1999!
The warmup time is about 10–15 minutes, but if you want to be sure, give it 30 minutes (or leave it on, it has no fan, so it’s dead silent).
Here’s the device connected to my Trimble GPSDO and this is after two days, and not a flicker of deviation from the calibration. When feeding the counter from my Siglent SDG2122X, it shows an error of 2 Hz. Until I enabled the external GPSDO reference on the Siglent signal generator. Then it’s dead on from 1–120 MHz. I’m pretty impressed with the Siglent SDG2122X, 2 Hz off with its internal non-ovenized crystal is pretty good.
(Ohh! Look at that lovely backlight!)
You need the ISOTemp 131–100 5 volt OCXO version!
Download Eagle PCB Schematic and PCB Design Here
And here’s a tip, just upload the board file to DirtyPCBs. They automatically make the Gerber files needed. You get 8–12 boards for about $25 including shipping, and they haven’t failed me yet. You can even select the PCB color!