How oscilloscope probes measure physical quantities such as time, frequency and voltage values
You have to take the oscilloscope probe apart first to see what's inside. On the section that connects to the oscilloscope is a BNC connector, if you don't use the BNC connector but instead use two wires to bring the signal directly into the oscilloscope, you will notice that the signal is distorted, a square wave goes in and shows a sawtooth wave! Why is this?
Oscilloscopes are generally higher input impedance to reduce the effect on the circuit under test. So you'll see a 1M ohm resistor or similar circuit behind the probe's BNC connector. This has both advantages and disadvantages, the smaller external capacitance value can also cause a filter to form at the input, which can distort the measured waveform. How this is resolved depends on how the probe is handled!
Generally, oscilloscope probes use an adjustable capacitor in parallel to cancel out the effect of this part of the cable. Some compensation capacitors allow us to make our own adjustments and select the best results. Oscilloscopes have a square wave source, so we hook the probe to the source and adjust the capacitor so that the square wave displayed on the screen is the most standard "square wave". Too much capacitance will cause the probe to form a low-pass filter, while the opposite will be true for a high-pass filter. Careful adjustment is therefore required.
Square wave measured with an unadjusted probe
The probe also has an attenuator, which attenuates the signal under test. The attenuator is usually a factor of 10. a 1V signal will be displayed as 100mV. some oscilloscopes can automatically recognise the state of the probe and display the correct value.
The probe uses the high impedance characteristic to ensure that the circuit is not disturbed by the measurement part, but there are times when we need to measure certain circuits with a low impedance test. For example, a 50 ohm impedance RF output circuit is a matter of pressing a button for a machine that has a 50 ohm impedance measurement; but for a normal oscilloscope, this is when the probe is not suitable for the measurement. You need to match it with a BNC tee and a 50 ohm end resistor, and connect it directly to the 50 ohm output at the other end.
