A poor person's frequency measurement
(How to measure frequency without a frequency counter)
 
 
   While working with electronics, one often comes across the need to measure the frequency of a signal precisely. For example, electronic clocks need to run at the proper rate and radios must be tuned to an exact frequency to work properly. To measure frequency precisely, one uses a digital "frequency counter" or frequency meter. But suppose, as is often the case, you don't have a frequency counter and you still need to know the frequency. Can it be done?

   With the proper equipment, the answer is yes. If you have an oscilloscope, the signal to be measured can be synchronized to a known signal and measured by comparison.  It helps here to have a signal generator, capable of producing a precise, known frequency with which to compare the unknown signal. If a signal generator is not available, other sources can often be used (see "Signal Generators and their uses").

   As an example, suppose you want to tune an oscillator to run at 60.0 Hz, to be used to run a clock. The oscillator output is first displayed on the oscilloscope and the trace stabilized. You can then measure the trace on the oscilloscope screen and determine the signal frequency to within a few percent, but a much better measurement is possible. The next step is to trigger the oscilloscope at exactly 60Hz. For this the line frequency is an excellent source. Some oscilloscopes have an internal 60Hz trigger setting. If not, connect a long dangling wire to another input channel or the external trigger, and trigger on that channel. The wire will pick up 60Hz inductively and serves as a trigger signal locked to the power line frequency.

   The displayed trace may drift left or right on the scope because the trigger and display frequencies are not the same. If the trace drifts to the left, the unknown signal is running fast as the trigger signal is arriving later each cycle. If the signal drifts to the right, the unknown signal is slow. By measuring how long it takes to drift one complete cycle on the scope, the frequency of the displayed trace can be determined very precisely. For example, if the trace drifts left by one complete cycle in 45 seconds, the signal is running fast by 1 part in (45 * 60 cycles) = 1 / 2700 = 0.022. The frequency of the signal is thus 60.022Hz. The precision of this measurement can be arbitrarily determined up to the precision of the reference signal itself, just by counting cycles for a longer interval.

   Even without an oscilloscope, all hope is not lost! As long as one has a known frequency standard, such as a tuning fork, the human ear can serve as an excellent instrument. By listening to the reference and signal tones simultaneously, a careful listener will hear "beats" when the two signals are very close in frequency. The beat frequency is caused by the interference of the two signals and is the difference of the two. For example, a signal of 441Hz will beat against a tuning fork of 440Hz (musical note "A") to produce a warbling tone once per second, indicating the signal is 1Hz off. This is exactly how musicians tune their instruments by ear.

   To aid in the measurement of frequencies by the two methods described above, it is helpful to have easy access to a variety of precisely known "reference frequencies", like having a complete set of tuning forks for every note. Most often this is accomplised with high quality signal/function generators or synthesizers. RefCD is a compact disc designed to replace this expensive instrumentation. It is a standard audio disc which contains many tracks spanning the entire audio frequency range. All that is needed to generate presice audio tones is an innexpensive CD player.

For more information, check out the RefCD page.