The oscilloscope is probably the most versatile piece of test equipment you can have on your electronics bench, offering a multitude of possibilities for measuring timing, frequency and voltage as well as subtleties in your circuits revealed by the shape of the waveforms they produce.
On the front of a modern ‘scope is a BNC socket, into which you can feed your signal to be investigated. If however you simply hook up a co-axial BNC lead between source and ‘scope, you’ll immediately notice some problems. Your waveforms will be distorted. In the simplest terms your square waves will no longer be square.
Why is this? Crucial to the operation of an oscilloscope is a very high input impedance, to minimise current draw on the circuit it is investigating. Thus the first thing that you will find behind that BNC socket is a 1 megohm resistor to ground, or at least if not a physical resistor then other circuitry that presents its equivalent. This high resistance does its job of presenting a high impedance to the outside world, but comes with a penalty. Because of its high value, the effects of even a small external capacitance can be enough to create a surprisingly effective low or high pass filter, which in turn can distort the waveform you expect on the screen.
The answer to this problem is to be found in your oscilloscope probe. It might seem that the probe is simply a plug with a bit of wire to a rigid point with an earth clip, but in reality it contains a simple yet clever mitigation of the capacitance problem.
The majority of passive oscilloscope probes contain an attenuator to both isolate the circuit under test from the capacitance of the cable, and compensation capacitors in parallel with each of the resistances to cancel out the effect of the capacitance of the cable. The attenuator is usually chosen to divide the input voltage by ten, hence you will see “10x” probes. The reading on the ‘scope is then a tenth of the voltage at the probe, for example a 1 V level will measure as 100 mV. Many ‘scopes will automatically convert this to the true figure if their settings indicate the probe type.
One of the compensation capacitors will be adjustable, to fine-tune the response. The ‘scope will have a calibrated square wave output, usually at 1 kHz, to which the probe is attached, and then the capacitor is adjusted until the wave displayed on the screen is truly square. Too much capacitance and the probe has the effect of a low-pass filter, while too little and it becomes a high-pass filter. It should become a semi-regular part of bench maintenance to check your probe against the calibration terminal, and to adjust it accordingly if the displayed waveform is not a square wave.
Highs and Lows
The probe is designed to present a high impedance to a circuit in situ, and not to distort the resulting displayed waveform. However there are times when it is necessary to measure an output that expects a low impedance, such as a 50 ohm source. In this situation the source must be terminated with the same impedance, so it should be fed into a 50 ohm resistor.
Some ‘scopes have a 50 ohm input mode, making this a matter of button pressing. Otherwise, a measurement can be made across the termination resistor, but since the impedance has now been reduced to a point at which small capacitances no longer have a significant effect, it becomes unnecessary if the cable to the oscilloscope is reduced to the barest minimum. In these cases you can dispense with the probe entirely if you have a BNC T-piece and a 50 ohm terminator, and put both terminator and T-piece on the end of your cable directly coupled to the BNC socket on the ‘scope.
For many readers this basic primer on the operation of an oscilloscope input and probe will be old hat. But I remember my first oscilloscope, and how it gave me odd-looking results because I was too young to have been taught about probe compensation. When you get your first ‘scope it’s tempting to think that the ‘scope is the main event, without realising that it’s only as good as how you connect it to your circuit under test.