Long Story "Short"
I have long been fascinated by the various methods of accurately measuring the frequencies of HF radio signals. I suppose it all started when I was in high school and my first shortwave radio had a dial that was accurate only to the nearest ten or twenty kilocycles per second (now called kilohertz, kHz).
To make a very long story short, accurate and precise measurements of radio frequency is not difficult and need not be expensive. There are several methods described here and in the references in this article. The method I now use (since 2016) is called AM with external reference and involves these four things :
- a frequency standard that is electronically disciplined to stay close to the UTC frequency scale (in fact, it is a GPS-disciplined oscillator or GPSDO),
- a synthesized signal generator that is referenced to the above frequency standard's 10 MHz output,
- an HF radio receiver in AM mode that I can use as a tunable RF bandpass filter, and
- some FFT spectrum analysis software that will run from my computer's soundcard with audio from the radio receiver as input.
The Frequency Standard (GPSDO)
I have two suitable GPS receivers :
- an old Hewlett-Packard Z3801A, and
- a not-so-old Trimble Thunderbolt.
Both of these receivers provide a suitable 10 MHz reference output that is disciplined to the atomic clock signals of the GPS satellite network, as well as provide GPS data in NMEA format. The 10 MHz reference signal is used to lock the signal generator output to the UTC time/frequency scale, making it very accurate. Before using either GPS receiver, I monitor its performance using the Lady Heather software by John Miles, KE5FX, or the Z38xx software by the late Ulrich Bangert, DF6JB (SK). It is conventional wisdom that, for best results, these receivers should be operated continuously for at least a week or two before serious measurements are made. Both of these GPS receivers, and similar ones, are sometimes available on eBay.
Measuring a Frequency
So, you have an unknown radio signal with a carrier component (an on-off-keyed Morse signal, or an amplitude-modulated broadcast signal, or one of the Mark or Space components of a printer or data signal) and you want to know the exact frequency.
- Tune the radio receiver to the unknown signal and, by whatever means, determine the approximate frequency, say, to within the nearest hundred hertz. This is relatively easy with most modern communications receivers.
- Set the frequency of the signal generator to a convenient frequency about half a kilohertz below the frequency of the radio signal.
- Adjust the amplitude of the signal generator so that its signal is about equal to the amplitude of the radio signal.
- Set the mode of the radio receiver to "AM". What you hear now from the receiver is the audio 'beat note' or difference between the signals from the unknown and the signal generator, and it will be around half a kilohertz in frequency.
- The audio beat note is fed to the sound card of the computer and you can measure it fairly precisely using spectrum analysis software that employs the Fast Fourier Transform (FFT) computing technique. I use the well-known SpectrumLab FFT software by Wolfgang Büscher, DL4YHF.
- Now for a little arithmetic. Add the frequency of the audio beat note that you just found to the frequency of the signal generator. (Remember that we set the signal generator frequency below the unknown radio signal?) Be sure to get the decimal in the right place. The sum of the two is the measured frequency of the radio signal that you tuned to. Le voilà!
There is lots of "fine print" in this, but that is basically how it is done.
In this way, the radio receiver itself has little to do with the precision measurement, acting only as a tunable RF filter to bring in the unknown signal and the signal from the signal generator. The real work is done by the GPS receiver to provide a UTC-derived frequency reference and the FFT spectrum analyzer software that is used to measure the audio frequency difference.
SpectrumLab is powerful and sophisticated software and involves a fair learning curve. I strongly recommend studying the tutorial about SpectrumLab by Jacques Audet, VE2AZX. Jacques has a lot of useful information about precision frequency-measuring techniques and equipment on his Web site.
Also see the famous "FMT" website by Connie Marshall, K5CM, for information about his informal frequency measuring tests (FMTs) where participants often have errors of just a few millihertz, and some of the methods they use to do it. Connie also sponsors and moderates a welcoming and helpful discussion group on Yahoo called FMT-nuts where frequency-measuring enthusiasts discuss their equipment, methods and results, as well as related frequency and time matters.
Steve, WA9VNJ, has a very useful explanation of the main frequency measuring methods on his website. Using his description, the method I use here is the "AM, external BFO" method. As well, a great deal of useful information about precision time and frequency measurements (they are inversely related) can be gained by reading the "Time-Nuts" mailing list where all kinds of technical matters related to time are discussed.
In the ARRL Frequency Measuring Tests (FMT) I have almost always achieved average absolute errors of 1 hertz or less, and with an average of several measurements, sometimes achieving accuracies of a few tens of millihertz (mHz). Indeed, some Amateurs do this routinely, for example, N8UR and VE2IQ Be aware that at this level of precision, Doppler shifting of the signal caused by relative motion in the ionosphere can be a real problem and is often the limiting factor in anyone's practical results.
I hope that some of these ideas will be useful to you. You are welcome to contact me to discuss any of the ideas here, but please be aware that there are many people who are better at this than I am.
Bibliography of Magazine Articles
Over the years I have compiled a bibliography of magazine articles about easily-built Amateur frequency standards and calibrators and applicable measurement techniques.