Passive Probe

Oscilloscope Passive Probe: What is it good for?

With a BNC cable and corresponding adapters you can directly connect signals to an oscilloscope. So why do you need a probe? To understand this, have a look at an equivalent circuit diagram for oscilloscope, cable, probe and voltage signal. This schematic has been designed with the simulation software LTSPice, which is available free of charge for Windows and MacOS..

Simulation of passive probe with a divider ratio of 1:1 (=BNC cable only).

The lower part of the picture shows the equivalent circuit diagram of the simulation. Bottom right the oscilloscope is represented by an input resistance of 1 Megaohm and a capacitance of 20 pF. A capacitance of 100pF in parallel models the BNC cable. At the bottom left the voltage source is shown with a sinusoidal amplitude of 1V and in series with a resistor of 1kOhm. This simulates a finite current source load capacity and limits the maximum input current of the circuit to 1mA.

In the upper part of the picture a frequency response is simulated. Shown is the output voltage at capacitor C1, i.e. at the oscilloscope input. The frequency of the input voltage is used in this simulation from 1 Hz up to 100MHz and the corresponding voltage on the oscilloscope input is shown. It can be seen that the input voltage at the oscilloscope up to a frequency of 100kHz remains unchanged at 1V. For higher frequencies, the input voltage drops rapidly. At 10 Mhz the input voltage amplitude at the input is less than 0.2V, at 100MHz you can't measure it anymore. The dotted line shows the phase shift of the signal at 1MHz. At 1MHz, this is already 45° compared to the input phase.

Passive probe with a divider ratio of 10:1

Simulation of passive probe with a divider ratio of 10:1.

If an oscilloscope passive probe is used now, a resistor R2 and the capacity C4 are added to the circuit diagram. The values of 9 Megaohm and 13.33 pF represent a correctly adjusted probe with a divider ratio of 10:1. The input signal on the oscilloscope is now attenuated by a factor of 10, the amplitude is approx. 100mV. If you look at the simulation results, one recognizes clearly the difference in the frequency response. Up to a frequency of 1MHz, the signal amplitude does not change. At 10 MHz the amplitude is around 81mV. Frequencies at 100 MHz can still be measured. The phase shift at 1 MHz is at a few degrees. This means that the passive 10:1 probe has a approx. an order of magnitude higher frequency bandwidth than the BNC cable. The price for this is the signal stroke which is 10 times smaller. Most oscilloscopes can measure reliably in the millivolt and microvolt range. The damping of the signal amplitude by a factor of 10 in favour of the better frequency bandwidth is often advantageous.

The two circuit diagrams for the simulation can be downloaded here .