Frequency Measurement for the WW0WWV Crowd
Learning advanced nerdism in some number of easy lessons
We’ve been promising this for a while. The NIST Time and Frequency Division gets to define time through its cesium clocks and all the rest. It gets that information out by WWV, GPS satellites, and other means. The Frequency Measuring Test exercises our skills in applying those standards from NIST to measurements of signals’ frequencies in real-world noise. Those skills, in turn, permit us to calibrate our equipment accurately, promote minimal bit-error-rates in our communications, and brag quite loudly to one another about who, exactly, is nerdo di tutti nerdi. Try some of these experiments. The next ham with the bragging rights could be you.
A first experiment: Audio frequencies experienced acoustically
Acoustic waves have some similarities to electromagnetic waves. Our system of ear, acoustic nerve, and auditory cortex provides an available transduction system for them, at least within the range that is broadly called “audio frequencies.” Pitch is the psychologic experience of frequency; the words are not quite synonymous. Acoustic waves at frequencies below about 20 hertz are more felt than heard if they are loud enough. Mid-range sounds of about 300 to 3000 Hz provide speech information adequate for telephone communication, and that factor of ten in frequencies permits easy filtering and phase-shift networks. Wider ranges of 20 to 20,000 Hz are used in music reproduction, with three factors of ten representing a good challenge in circuit and transducer (loudspeaker and microphone) design.
Consider measuring the frequency of an acoustic signal that is known ahead of time to be of only a single frequency. Sonically, such a signal is rather boring, as it contains no overtones, no amplitude or frequency variance, nothing that a music composer might use to express much beyond a science fiction sort of ambiance. Such sounds were possibly unknown before the late 19th century and the advent of tuning forks and other simple harmonic oscillators at frequencies above those of pendula. Radio amateurs certainly know them from hearing the heterodyne of a receiver’s local oscillator with an AM station’s carrier.
Several web sites provide musical frequency detection for musical instrument tuning.
This web site uses a computer’s microphone for acoustic-to-electric transduction, and the computer’s soundcard for electric analog to digital conversion. The soundcard’s oscillator is the time standard for the measurement, and while some are more accurate than others, they’re all within about 10 parts per million. Start the page and trying playing a musical instrument, singing, striking a tuning fork, or doing something else that will generate a mostly-pure acoustic tone. A slide whistle is especially useful for some of these experiments.
Don't have a tuning fork or a slide whistle? Singing skills a little off? You can download a tone generator for cell phones at your devices app store. For the Droid user there's Tone and Tone Gen is a simple to use app for iPhone. The nice thing about a tone generator is it puts out a steady note.
The video instrument reads out the signal’s main frequency in two different measures. The numbers above the pitch wheel are in hertz. Note that International Standards Organization Standard 16 puts A4, or piano’s A above middle-C, at 440 Hz plus or minus 0.5 Hz (i.e., about 0.1%). That’s not an SI standard and it is not universally observed.
Get a feel for what low frequencies look like on musical instruments and what they sound like to your ear. Also get some experience hearing what frequency relationships sound like. Doublings of frequency are musical “octaves” and get the same note name (with different subscript numbers). They sound similar in a musical sense.
Now tune up WWV on a frequency with good signal strength. Use your receiver’s AM detector, not the amateur’s usual upper or lower sideband.
In any short time interval, you’ll hear one of the following:
A “second tick”
A voice announcement
The WWV Scientific Modulation Group’s audio study signals
A tone, either 440, 500, 600, 1000, 1200, or 1500 Hz.
We’ll concentrate on those tones. Wait for a minute that has one or another of them.
With WWV playing on your receiver’s speaker and Tuner Ninja running on your computer with the microphone on, you should see the computer’s frequency dial rotated to some note and the frequency readout corresponding to the frequency you’re hearing. 440 Hz shows up only once each hour, and most other minutes have either 500 and 600 Hz, with the two alternating minute-by-minute.
Give this a try, see how well calibrated your computer sound card is and how well it can distinguish a dominant signal in noise--and let us know!
Now on to the next lesson...