How to Create A GPS Disciplined Frequency Counter

Something Old, Something New

A GPS Disciplined Frequency Counter wasn’t on my radar until recently. But a surprise in an eBay purchase and a challenging repair led me to this Gold Standard of frequency measurement. It all started because I’ve been known to cruise eBay from time to time. I especially like mixed lots. You know, collections of stuff that might not be closely related to each other except that they’re all in the same category.

Well, about three years ago, I spotted an interesting lot advertising “21 ham Radio ampres s&o Pro driver Heathkit Weston Digitrex sens & more.” [sic]

There were several panel meters, a Heathkit 50’s-era grid dip meter and resistor substitution box and other items I couldn’t make out. There was also a mysterious box with a window in the front panel. Maybe the box would be useful for a future project.

I was the only bidder, and paid $43 — including tax and shipping.

eBay seller buried the lead

A package arrived a few days later, and I had fun unpacking and inspecting my new treasures. The Tunnel Dippers were in good shape and working. Missions accomplished.

The other Heathkit stuff had the old livery of the 50’s — burgundy and grey/tan. Maybe ok for a Heathkit collector. The resistor substitution box was damaged as the carbon resistor values had drifted too far out of spec to be useful on the bench.

There was a Digitrex pocket size frequency counter with plugin power supply — working! This little baby is certainly good for HF and probably up into the low VHF as it was designed for model airplane hobbyists.

The other junk looked like R/C airplane stuff, but the mystery box was something special.

A classic piece of test gear from the 70’s

I could see right away that it was a really nice anodized aluminum project case with blue front and back panels and black top, bottom and sides. It appeared to be a frequency counter, but no brand, design or model I had ever seen.

There was an attached power cord, so I plugged it in. Flipped the power switch on and the window lit up with 70’s-era LED 7-segment displays. All displaying apparently random numbers. A second or two later, the display cleared, leaving only a single digit “1” and a decimal point. There was some life in this box!

It didn’t take long to connect a signal generator to one of the BNC connectors on the front with a test signal. A little knob twiddling and it was clear this was a functioning frequency counter. Also, labeling on the switch in the upper left corner suggested it would measure up to 900 mHz. Certainly worth the price of admission.

What’s inside?

I removed the four screws in the lid to get a look inside. The back panel was clearly the power supply, and the front panel appeared to be handwired with LED displays, their drivers and supporting circuitry. A single perfboard card was in the middle.

It seemed to be the timebase since a Bliley CK01-1, 2-mHz oscillator can and several TTL ICs were present as well. Most of the ICs were N8280A decade counters.

I had what appeared to be a homebrew frequency counter. Looking around the ‘net, I didn’t find any projects or schematics that were similar, so I’m guessing it was designed by the talented builder. Clearly a lot of time and love had gone into the project.

Updating the Frequency Counter

Since arriving those years ago, I’ve used the frequency counter countless times. I even tested its VHF capabilities, and it will indeed count up to at least 150 mHz.

More recently, I was servicing a Ten Tec Omni VI Plus, and it became clear that more accuracy was needed. As best I could tell with my limited test gear, the 2-mHz timebase was off by as much as 50 hz, and for aligning the Omni VI, I would need more precision.

Several ideas were considered, including the purchase of a CTI OSC5A2BO2 “Oven Controlled Crystal Oscillator” (OCXO) available on eBay for around $5. Why so cheap? Well, this little gem is widely used in the cellular industry as part of a system that provides a super-accurate timebase. These OCXO modules provide an exceptionally clean, low phase noise signal. They can also be adjusted onto frequency with a DC control signal. The idea was to build a frequency reference with that, zero-beat it with WWV, and use that to drive the counter timebase.

A better idea . . .

More looking around eBay yielded something intriguing and new to me: a GPS Disciplined Oscillator or “mini GPSDO.” As advertised in the eBay listing, it uses the same CTI OSC5A2BO2 OCXO mentioned earlier, and compares its frequency with up to five GPS satellites. An Arduino-powered Phase Locked Loop circuit then adjusts the OCXO to match. Accuracy is approximately .001 Hz. For less than $100, no-brainer, I ordered one.

It arrived in just a few days, and included an assembly with a GPS receiver, OCXO, Arduino and a few other parts on a custom PC board. A GPS antenna with a nice, long cable and SMA connector completed the setup.

With the GPS antenna outdoors through a hole in the shop wall, and 9 volts applied to the power connectors, I watched the power-up sequence. A bright blue flashing LED showed it was acquiring satellites. When the blue LED went dark, it had GPS communications, but had not yet achieved phase lock. In about 10 minutes, the blue LED came back on (solid this time) to indicate the OCXO was phase-locked to the GPS system. Accuracy much better than .1 Hz. As good as I’ll ever need.

Frequency counter modifications

The GPSDO provides a 10 MHz signal, and the frequency counter had a 2 mHz timebase. All that was needed to connect them was a divide-by-5 circuit. A 7490 decade counter contains a divide-by-2 and divide-by-5 circuit, so I could use that.

There was plenty of room near the timebase circuit on the middle perforated card in the frequency counter. I dug out my wire-wrapping tools and went to work. Since the GPSDO provides a TTL-compatible signal, no analog circuitry was needed. I simply mounted a BNC-jack on the back panel of the counter, and connected it directly to the 7490 circuit through a short piece of RG-174 miniature coax.

A Laboratory-grade instrument.

This is a game-changer for my bench. Back in the day, we got by with old surplus test gear that while capable, probably hadn’t been calibrated in years. They also tended to be large, heavy and prone to problems.

When working in broadcast television and later on, information technology, I got used to having good instrumentation on the company dime, but I never expected to have GPS disciplined frequency counter in my own shop.

I plan to build a distribution amp for the GPSDO so I can drive several pieces of test gear like a signal generator and the frequency counter with it. With these additions, I will be stepping into the modern age. Who says you can’t teach an old dog new tricks?