Signal Generators and Their Uses

   Many of the projects described in the pages of the Amateur Scientist Department make use of sounds or electrical signals which lie in the audio frequency range of 20Hz to 20Khz. In the 50 or so years of the department, such signals have been used for everything from basic experiments on the physics of sound to levitating drops of water on water, oscillating flames, measuring the harmonics of a shower stall, and experimenting with chaos and encryption.

   Just how does one obtain the controlled sounds and other signals? The professional or serious electronics hobbyist will probably already have some sort of  signal generator. Such a device can generate a calibrated sine wave with frequency and amplitude both easily adjusted by the user over a large range. A more generally useful device is a function generator, which is capable of producing not only sine waves but square waves, sawtooths (ramps) and even pulses. These instruments are commonly available from a variety of manufacturers, and there is also a considerable surplus market. While important pieces of equipment to have however, signal and function generators are expensive, and it is often necessary to find other ways to generate the needed signals.

  Depending on the desired frequency, there are a number of interesting ways to generate useful signals. By far the simplest source of a reliable, well known frequency is the AC power lines. In the US, the line frequency is a well regulated 60 Hz and all that is needed to harness the lines is a small AC wall transformer. Stepping the line voltage down to about 6.3 volts will enable the experimenter to use the lines (with care!) as a precise frequency standard. This is precisely how inexpensive digital alarm clocks get their time standard. The line voltage is stepped down and "clipped" with a zener diode to generate a square wave with a 5 volt amplitude, which is then used to clock the digital logic.

   Other frequency standards are also easily available, though they are a bit harder to utilize. WWV, broadcast on shortwave frequencies of 2.5, 5, 10, 15 and 20 MHz, is the radio station of the National Institute of Standards and Technology (http://www.nist.gov/).   The station broadcasts time signals linked to the agency's atomic clocks in Boulder, CO.  In addition to the regular clicks, which serve to synchronize clocks, frequency standard audio tones are also transmitted. CHU, broadcasting from Ottowa at 3.330, 7.335 and 14.670 MHz is the Canadian equivalent of WWV.

   More often than not, the experimenter needs to continuously vary the frequency or amplitude of a signal. For this, some elementary experience with electronic circuits is required. There are a variety of easy to use function generator chips available, such as the venerable ICL8038 (http://www.intersil.com/), or the more modern Maxim MAX038 (http://www.maxim-ic.com/). For generating pulses, square waves, ramps and other waveforms, almost nothing beats the simplicity and ease of use of the 555 timer IC chip. A trip to almost any neighborhood electronics store will turn up this IC and the necessary information to build several useful oscillator circuits. In addition, other more specialized chips can be used to produce quartz crystal oscillators with extremely stable frequencies.

   The availability of cheap IBM compatible computers is also a boon to the experimenter in search of a simple signal generator. While newer operating systems make it hard to access the basic functions of the machine, an old PC running DOS, with QuickBasic installed is a great tool for the adept programmer. One has two options for generating square waves. The easiest is to write a program calling the "sound" function, which causes the speaker to beep for a programmed duration at a known frequency. One can then attach leads to the speaker outputs to obtain the needed signal.

   Another simple method is to program the parallel port of the machine. By alternately driving one bit of the output data high and then low again, one can generate a square wave. In this case, since the rate at which the program runs depends on the speed of the machine, the frequency will have to be measured before use. The advantage is that pulses and other more complicated digital waveforms can also be generated in this fashion. In both cases, the output will have to be properly amplified if any current is to be drawn from the signal, in order to avoid damaging the computer.

   Finally, there is one more simple and innexpensive alternative: an audio CD with the signals one needs already programmed on it. All one needs for a complete signal/function generator is a standard CD player, and some portables can be found at discount electronics stores for under $30. RefCD has been designed as an inexpensive replacement for a number of signal and tone generating devices which operate in the audio frequency range (~20 Hertz to ~20Khz). The Compact Disc contains a collection of tones (pure sine waves) with precisely defined frequency. In addition, there are various more specialized signals and waveforms for use in electronic troubleshooting, repair, testing of electronic/audio equipment and general experimentation. A booklet which completely describes the tracks comes with the CD and also contains suggested methods and simple demonstrations.