ECE291 - LABORATORY VII INPUT IMPEDANCE OF AN OSCILLOSCOPE AND THE SCOPE PROBE
OBJECTIVES Measurement of the oscilloscope input impedance. Principles of operation and usage of the scope probe.
INTRODUCTION After becoming familiar with the frequency response of RC circuits in the previous laboratory, you are ready to learn about the oscilloscope input impedance. This complex impedance, which has resistive and capacitive components, can disturb performance of circuits being measured and limits high frequency performance of oscilloscopes. Fortunately there is a remedy, which you should know about: the scope probe. A scope probe is essentially a 10:1 attenuator with a resistor and a capacitor at the end of the probe cable. The attenuator consists of two impedances in series, one of which is the scope’s own impedance to ground while the other is attached between the point of measurement in a circuit and the scope input. For only dc measurements we could use a resistor as the external impedance. The same would be true also for ac measurements if the scope internal impedance was only resistive. The scope internal impedance, however, has a capacitive component. Note that the capacitance of the scope cable adds to the internal capacitance of the scope input. This not only makes it much larger but also dependent on the length of the cable used. Very bad for high frequency measurements! To make things worse, the load that the scope with its cable presents to a measured circuit depends on frequency and amplitude measurements with the scope are frequency dependent. The scope probe solves this problem not by eliminating this capacitance (which is not possible) but by compensating it with another capacitance. The probe capacitance is chosen so that the measurement is independent of frequency. If the probe impedance is not matched properly to the scope internal impedance the attenuation depends on frequency and thus acts as a filter. Fortunately there is an easy way to adjust the probe impedance with a built-in small trimming capacitor. A filter distorts a square wave so the trimming capacitor can be adjusted by observing a distortion of a square wave on the scope screen. Oscilloscopes are equipped with internal square wave generators for easy probe adjustments. The probe test terminal is provided on the front panel. The probe capacitor is adjusted until there is no distortion of the square wave. A scope probe is a very handy device used by professionals all the time. From now on you should use it too!
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Measuring the impedance of an oscilloscope Measuring the scope internal resistance (RS) is easy; just like a voltmeter resistance, using a DC source. For measurements of the scope internal capacitance AC source must be used, of course. An external resistor R is put in series with the voltage source (instead of the probe in Fig. 5). From the voltage divider formula:
where Z1 is the scope impedance (including the cable capacitance CC), so that Z1 = RS ||CS || CC or
where C = Cs + Cc. Z2 is just the external resistance R, Z2 = R Measuring amplitudes Vo, Vs of Vo and Vs gives us:
where red letters denote complex variables while a and b are the real and the imaginary parts of the impedance ratio in the last equation.
PRELAB If the scope internal resistance is RS, its capacitance is CS, and the cable connecting the probe to the scope has capacitance CC, find the values of the required probe resistance RP and capacitance CP for 10:1 attenuation. Make sure that the same attenuation is also valid for dc measurements. Hint: Consider independently two voltage dividers, one resistive the other capacitive. Note that they are connected in parallel and should give the same attenuation. References: T. C. Hayes and P. Horowitz "Student Manual for The Art of Electronics" pp. 62-632. Cambridge University Press 1989. Also in P. Horowitz and W. Hill "The Art of Electronics" 2-nd edition.
LABORATORY Equipment needed from the stockroom: scope probe, resistance substitution box, parts kit, proto-board, leads. 1. MEASUREMENT OF THE INTERNAL IMPEDANCE OF AN
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scope. Properly adjusted probe should give the same attenuation for all frequencies, which means that it passes a square wave signal without distortion. If you do not see a perfect square wave, using a small screwdriver adjust the probe trimming capacitor which tunes Cp. 2.2. To see the benefits of using the probe make a 2:1 resistive voltage divider, using equal resistors of 50 k to 100k. Particular resistance values are not critical, as long as you know their ratio; check it with the digital ohmmeter. Measure attenuation of a sinewave at two frequencies, in 10 kHz and 100 kHz range using (a) scope without the probe (b) scope with probe.
3. CIRCUIT SIMULATION (at home).
REPORT
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