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Questions and Answers on Oscilloscopes |
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 the
horizontal time base and vertical amplitude scales, and how to synchronize
the display with the input signal. Some explanation of how to interpret
periodic and pulse type waveforms is provided as well as tips on
how to avoid overloading and possibly damaging the instrument.Aficionados of vintage oscilloscopes might want to visit the Oscilloscope Museum website, and Simpson Electric fans can visit the Simpson260.com website for just about any Simpson user's manual. Questions and Answers on Oscilloscopes By Robert G. MiddletonChief Field Engineer, Simpson Electric Company  Replies
to the queries the manufacturer most often gets from technicians
on this important instrument. Of the many questions manufacturers of oscilloscopes are asked, the most common is, "Which is the right pattern? I can get nearly any pattern I want on the scope screen." The question is too complex to be answered simply. What is involved here is a basic understanding of the operation and application of the instrument. The "right" pattern, of course, is obtained only when operation and application are correct. Still, the high degree of frequency with which the question is asked, even in this era, justifies a fundamental review of the instrument and the nature of the phenomena which it is used to examine. Basic Scope Display The display of a 60-cycle sine-wave signal is very basic, and an instructive point from which to start. Turn on the scope and adjust the intensity and focus controls to obtain a bright and well-focused horizontal trace (see Fig. 1). The vertical and horizontal centering controls are adjusted, as required, to center the trace on the scope screen. If the vertical-gain controls of the scope are advanced to maximum, a pattern is obtained on the scope screen even though there is no signal input to the scope. This is a very puzzling point to the beginner, and is explained as follows: the scope input system has a very high impedance. For this reason, the exposed input terminals pick up stray 60-cycle fields about the bench, producing substantial vertical deflection. Note that, when a 1-megohm resistor is shunted across the scope input terminals, the pick-up is eliminated. This is the reason that stray fields do not enter the scope in normal circuit testing - the circuit impedance shunts the scope input, so that any stray pick-up is not observable. To display the 60-cycle pattern in proper form, the coarse frequency control is set to a position which includes 60 cycles. For the scope illustrated in Fig. 1, the control would be set to the 14- to 250-cycle position. The fine-frequency control is then adjusted to obtain one, two, or more cycles, as desired, of pattern. To lock the pattern on the screen, so that it does not "run" horizontally, the function switch should be set to a suitable sync position, such as line-sync in the case of this 60-cycle wave, and the sync-amplitude control is advanced just sufficiently to lock the pattern tightly. This 60-cycle sine-wave pattern may appear very elementary, but it has some important properties which are worthy of note.
Why these three values? The first a.c. voltage value which was recognized was the r.m.s. value. This has its origin in power work, and is still used in the measurement of line voltage, transformer voltages, and heater voltages. Power work started out with d.c. power sources (even in some areas today, power lines still supply d.c. voltage). When a.c. power became common, it was desired to measure a.c. voltage in units such that the amount of light, or heat, or power obtained from a 117-volt a.c. line would be the same as that obtained from a 117-volt d.c. line. Thus if a 117-volt (r.m.s.) a.c. line is connected to a soldering iron, just as much heat will be obtained as if a 117-volt d.c. line is connected to that iron. However, the advent of TV brought in a new requirement: vacuum tubes in most cases respond upon the basis of the applied peak-to-peak signal voltage. In some cases, the tube responds to the positive-peak or to the negative-peak voltage which is applied - this depends upon operating bias. In any case, these newer units of voltage are of chief significance in servicing electronic circuits. The peak voltage of a sine wave is equal to 1.414 times its r.m.s. value, and the peak-to-peak voltage of a sine wave is equal to 2.83 times its r.m.s. value. It is evident that the peak-to-peak voltage of a sine wave is double the value of either the positive-peak or of the negative-peak voltage. This is so only because a sine wave is symmetrical. Pulse Voltages
It should be noted that the average value of a pulse waveform - as of all complex waveforms - is zero. This average level falls along the zero-volt level on the screen; that is, it falls along the resting position of the beam when no input signal is applied to the scope. It is this basic property which is utilized in measuring the positive- and negative-peak voltages of the waveform. Note that, in any pulse or complex waveform, the area of the pattern above the zero-volt axis is exactly equal to the area of the pattern below the zero-volt axis. This is a necessary consequence of the fact that the average value of the waveform is zero. When a pulse waveform is applied to the input of a d.c. voltmeter, the pointer indicates zero volts. Again, this observation is the result of the fact the average value of the waveform is zero. However, when the pulse voltage is applied to the input of an a.c. voltmeter, the indication obtained depends upon several factors. In general, the indication will be largely meaningless. The pulse waveform does have an r.m.s. value, but this is somewhat difficult to determine, and is of little interest to the service technician. The indication obtained will depend upon the frequency characteristics of the test instrument, which way the test leads are applied to the pulse source, and other factors. Of course, some voltmeters have a peak-indication function, or a peak-to-peak indication function. In such case, a useful measurement can be obtained unless the pulse repetition rate is low and the pulse is narrow. Because of these various considerations and reservations, the use of the techniques that do not involve the scope for waveform examination can be seen to have serious shortcomings. Voltage Measurements If the oscilloscope is to be the instrument for reliably measuring waveforms, as well as observing them for appearance, how can dependable measurements be made on an instrument with which, by adjustment, we "can get any pattern we want?" Indeed, these questions pertaining to measurement, and especially to the measurement of peak-to-peak value, fall into the most-frequently-asked category. Most scopes nowadays provide a source of calibrating voltage for reference. The scope shown in Fig. 1, for example, makes an 18-volt peak-to-peak sine-wave voltage available through a binding post on the front panel. (Where a calibrating standard of this kind is not built in, a sine wave of known amplitude may be introduced externally with no change in the remainder of the measurement procedure.) When a lead is connected from this binding post, or other source, to the vertical-input terminal of the scope, a known voltage of 18 peak-to-peak volts develops a sine-wave pattern on the screen. If the vertical-gain controls are adjusted to make the sine wave occupy a total height of 18 squares on the calibrated grid or screen, each square will evidently measure one peak-to-peak volt. Now the calibrating lead can be disconnected and, provided that the vertical-gain controls are left untouched, another signal voltage can be measured. After this unknown signal is applied, a count is made of the squares of vertical deflection it achieves. This is its peak-to-peak voltage.
The continuous, fine vertical-gain control, or the vertical vernier, is still left untouched, but the coarse control is turned in either direction the number of steps required to obtain a satisfactory deflection on the screen for the voltage being measured. If the coarse attenuator has been turned up one step to increase the sensitivity of the scope by ten times as compared to its former position, then each vertical square will represent .1 peak-to-peak volt, instead of 1 volt. On the other hand, if the attenuator has been turned down one step, to reduce an oversize waveform, each square will now represent 10 peak-to-peak volts. If the attenuator has been turned down two steps, each square will represent 100 peak-to-peak volts. Thus the utility of a decimal step attenuator lies in the fact that the basic calibration of the scope is unchanged - only a decimal point is shifted. Some scopes may have a step attenuator that changes sensitivity by some factor other than 10. These can be just as accurate, but they are not quite as convenient, as they may involve a small amount of arithmetic. While a relatively simple pulsed waveform has been chosen to illustrate the technique of measurement involved, the procedure is used unchanged with the most complex wave-shapes. In fact, most technicians prefer, while taking peak-to-peak measurements, to reduce the horizontal gain or width control to zero. This reduces all waveforms, no matter how different or confusing in shape, to a common denominator - a single vertical line. Since the length of that line is the true peak-to-peak value, irrespective of shape, measurement procedure is simplified. Avoid Overload Another question which is often asked concerns distortion of the displayed waveform which results from application of excessive signal voltage to the scope input. It must be recognized that it is quite possible to overdrive a scope amplifier, just as may be done with any other amplifier. When overload occurs, the waveform is clipped on the top, or bottom, or both. The resulting distorted pattern can be very misleading. To avoid scope overload, a simple operating rule should be followed at all times: Adjust the vertical-input controls so that the continuous attenuator is operating on the upper portion of its range. If necessary, the coarse attenuator can always be advanced a step or two, to permit this condition. The reason for this precaution is that the output from the coarse attenuator is generally applied to the grid of the scope input stage, while the continuous attenuator works in the cathode circuit. It is grid overdrive which provides overload and clipping. Posted November 8, 2013 |
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