QUIZZ'AWAY

Measurements using a probe — never a problem! For sure? The use of a probe for measurements with the oscilloscope should be customary to most, if not all, designers of electronic circuits. In particular the switchable 1:1/1:10 probe (Figure 1) is popular. With the probe in ‘1:1’ mode you measure at an impedance of 1 M. and the signal is not attenuated before it reaches the oscilloscope input (switch in Figure 2 closed). If you want to measure with a lighter load attached to the object of your investigations then the probe is usually switched to ‘1:10’ mode (switch in Figure 2 open). The resulting image on the ‘scope will become 10 times smaller because the signal is attenuated ten times in the probe. The trimmer capacitor ensures the voltage divider is as wideband as possible and the division ratio remains as close as possible to 10:1. It’s as simple as that, if only the circuit in Figure 3 did not exist! If you use a 1:1/1:10 probe to measure the signals between points A and B in this circuit, the image on the ‘scope will remain the same if you switch the probe between 1:1 and 1:10 mode. With this simple to build circuit on the bench, most experienced test engineers will be suspicious of their probe. However, it’s working just fine! How do you explain the fact that the scope displays the same signal irrespective of the probe attenuation? The circuit is certainly worth building because seeing an unchanged signal on the ‘scope despite switching the probe is mystifying.