5.2 Oscilloscope probes

How many a man has dated a new era in his life from the reading of a book.

Henry Thoreau

An oscilloscope probe guide (LeCroy 1994) asks whether oscilloscopes should carry a label stating 'connecting your oscilloscope probe to a circuit or device can distort the measured waveforms'. Any measurement requires interaction with the system and distorts the measurement to a greater or lesser extent. The aim is to make the disturbance as small as possible or to know how much it is disturbed and make a correction for it. Loading by the probe is usually decreased by means of a high impedance attenuator so that the improvement is bought at the expense of reduced gain. Figure 5.2.1(a) shows a 10:1 attenuator where R ₂ and C ₂ represent the input of the scope. A variable capacitor C ₁ must be included to allow compensation so the attenuation is independent of frequency. Figure 5.2.2 shows some responses as a function of C ₁.

Figure 5.2.1: ( a) Compensated attenuator. ( b) Probe schematic.

Figure 5.2.2: Response of the simple compensated attenuator of Fig. 5.2.1 ( a) with variation of C _l. The three curves show under- and over-compensation as well as critical compensation, which should occur at C ₁ = 20/9 = 2.22 p as observed.

In practice it is necessary to include a reasonable length of cable to allow flexibility in probing circuits and this introduces additional...