de W4AVO 2/28/2008
[ Note- What follows is a beginner's approach to the rather challenging task of off-the-air frequency measurement. The techniques are neither novel nor state-of-the-art. They are, however, succesuful in providing a measurement accuracy in the sub-hertz range with ordinary station equipment. ]
Modern transceivers have ten or one hz readouts and have or can be fitted with OCXOs or TCXOs with typical 1 hour stabilities of 0.1 hz or better. On the air stations such as WWV, WWVB, CHU, and broadcast staions with GPS lock give high accuracy calibration points at several points across the spectrum. Even low end laptops have sound cards that can be configured to do frequency tracking and measurement using DSP and FFT techniques.
With these tools, it is possible to calibrate your own station receiver and measure the frequency of off the air signals. How accurately? Well, not in the same class as laboratory grade setups with local Cesium or Rubidium standards or GPS disciplined XOs but a goal of within 0.25 hz most of the time and within 1.0 hz almost always seemed obtainable.
Can this be done with gear found in most ham stations? Here is my equipment and approach-
The DSP/FFT program of choice of the FMT big guns is DL4YHF's Spectrum Lab. It is flexible and powerful but has a steep learning curve. So far I can't coax it to run reliably on my '90s vintage laptop. Here is a link to a good tutorial on using this program for frequency measurement by VE2AZX. I have downloaded the program and the tutorial. SL is high on my list of things to master. For now, I use Digipan because I have it and it runs on my computer.
Below is a screen shot that shows the mechanics of audio frequency measurement with Digipan 1.7. Frequency in Hz is across (...1000....2000....) and the display moves downward with time ("waterfall"). Noise from the receiver is blue/yellow and the receiver beat note is red. The diamond is placed by the cursor and when clicked locks on and tracks. The frequency readout is in a lower box. In this example, the tracked frequency is 1008.9 hz.
How did it do? In general it met my goals. A lot depended on the quality of the off the air signals that are mostly received via sky wave. The reference is off the air; noise, signal strength, and doppler affect the receiver calibration. These also affect the unknown measurement.
Here are the results of four Frequency Measurement Tests for which I submitted measurements. Not too shabby for a total expenditure of 25 cents.
|3/2008 K5CM **||1846169.245 Hz||1846169.330 Hz||+0.085 Hz||K5CM|
|3569003.235 Hz||3569003.260 Hz||+0.025 Hz|
|3567513.115 Hz||3567513.080 Hz||-0.035 Hz|
|7077111.575 Hz||7077111.530 Hz||-0.045 Hz|
|10115111.284 Hz||10115111.380 Hz||+0.097 Hz|
|14067233.267 Hz||14067233.250 Hz||-0.017 Hz|
|18077634.574 Hz||18077634.960 Hz||+0.386 Hz|
|1/2008 K5CM||3577521.200 Hz||3577521.400 Hz||+0.200 Hz||K5CM|
|3579832.523 Hz||3579832.400 Hz||-0.123 Hz|
|7077075.247 Hz||7077075.300 Hz||+0.053 Hz|
|14057126.252 Hz||14057126.600 Hz||+0.348 Hz|
|4/2007 K5CM||3550451.587 Hz||3550451.420 Hz||-0.167 Hz||K5CM|
|7055802.836 Hz||7055802.950 Hz||+0.114 Hz|
|11/2006 ARRL||1854317.500 Hz||1854317.700 Hz||+0.200 Hz||ARRL|
|3587117.500 Hz||3587117.800 Hz||-0.300 Hz|
|7038804.900 Hz *||7038806.400 Hz||-1.500 Hz|
If you want to pursue this topic, look into these links: