Untitled Document

16 May 2003

Experimenting with Circular Sweep, Continued

By Craig Kendrick Sellen
Birch Hills Residence
25 Reservoir Street
Simpson PA, 18407-1300

Uses

Some familiar waveforms displayed with the circular sweep converter are shown in Fig.7. In each case, the waveform frequency was adjusted to give a pattern with a whole number of cycles. The waveforms are sine (Fig.7A), triangle (Fig.7B), sawtooth (Fig.7C), and square (Fig.7D). As the amplitude of the waveform is increased, the inside of the trace will meet at a point in the center (if the converter has been adjusted properly), as illustrated in Fig. 8A for the triangle waveform. Increasing the amplitude further causes the trace to go through the center and come out the opposite side as shown in Fig. 8B (even number of cycles) and Fig. 8C (odd number of cycles).

The pinwheel pattern in Fig. 9A and the spiral in Fig. 9B are both made with sawtooth waveforms. In Fig. 9A, the waveform amplitude is adjusted so that the traces meet in the center. In Fig. 9B, a low-frequency sawtooth is used. All the patterns illustrated in this article were made using a 6.3-vac filament transformer to supply the 60-Hz sweep. The waveforms were obtained from a 8038 waveform generator IC. Hundreds of other patterns can be produced with these basic waveforms. If you exhaust those possibilities, try mixing the outputs of two (or more) waveform generators.

One of the most fascinating displays is that made by music waveforms. What-ever else you do with the converter, be sure to try this out. Simply connect the audio from a radio, tuner, phone, tape recorder, etc. to the signal in jack of the converter. The result is a kaleidoscopic succession of patterns synchronized to the music. No examples are shown because the patterns and effects cannot be satisfactorily captured by still, photography. If you use an FM station as the source you may need to insert a low-pass filter (Fig. 11) between the source and signal in to eliminate the multiplex and SCA subcarriers. Speech also makes an interesting display.

Frequency Comparison

Using an oscilloscope in the conventional manner, the frequencies of two waveforms can be compared with lissajous figures. In an analogous way, frequencies can be compared using circular sweep. For example, the traces in Fig. 7 all show eight complete cycles of the waveform, which means that the signal goes through eight cycles while the sweep goes through one cycle. Since a 60-Hz sweep was used, the signal frequency must be 8 times 60-Hz, or 480-Hz. Fig. 9B shows almost the opposite situation. Here the sweep goes through seven cycles while the signal goes through only one cycle. The signal frequency is thus 60-Hz divided by 7, or about 8.43-Hz

Sometimes the pattern will be more complicated, like the one shown in Fig. 12. It is still relatively easy to determine the frequency as illustrated by the following analysis of the pattern. Starting at one peak on the waveform and following the trace, the next peak that we come to is the fourth one over from the starting point. This means that the sweep goes around four times to make one complete pattern. Note also that there are 11 peaks in all, which means that there are 11 cycles of the triangle waveform in the pattern. Thus, the sweep-to-signal frequency ratio is 4:11. Since a 60-Hz sweep was used, this gives a signal frequency of (11/4) x 60 =165Hz.

The frequencies thus, determined are exact only if the pattern is stationary. A rotating pattern indicates a slightly higher or lower frequency, depending on the direction of rotation.

Besides circular sweep, the converter can be used for other types of displays which may be less practical and more difficult to analyze, but are just as interesting. For example, you can adjust P4 the vertical signal offset or P6 the horizontal signal offset to the opposite end of its range to get the diamond-shaped display mentioned earlier (Fig. 6). For even more variety, all seven controls on the converter can be varied. Combine this with all the different waveforms and combinations which can be used as the signal or sweep and you should be kept busy for a while. Figure 13 illustrates a few possibilities. But be warned--you may become so engrossed that you abandon your color organ, computer graphics, and even your television!