In order to consider the influence of the probe on the measurement, the probe model can be simply equivalent to an R, L and C model, which is analyzed together with our measured circuit.
First, the probe itself has an input resistance.The principle of voltage measurement with multimeter is the same. In order to minimize the impact on the circuit under test, the input resistance of the probe itself, Rprobe, should be as large as possible. However, since Rprobe cannot be infinite, it will generate partial voltage with the circuit under test, and the actual measured voltage may not be the real voltage before the probe point, which is often encountered in the test of some power or amplifier circuits. In order to avoid the influence of probe resistive load, it is generally required that Rprobe should be 10 times larger than Rsource and Rload.The input impedance of most probes ranges from tens of k ohms to tens of megohm.
Second, the probe itself has an input capacitance.This capacitor is not deliberately built in, but is a parasitic capacitor of the probe.This parasitic capacitance is also the most important factor affecting the probe bandwidth, because it attenuates the high-frequency component and slows down the rising edge of the signal.In general, the probe parasitic capacitance with high bandwidth is small. Ideally, the Cprobe should be 0, but you can't.Generally, the input capacitance of passive probe ranges from 10pf to several hundred pf, while that of active probe with higher bandwidth ranges from 0.2pf to several pf.
Furthermore, the probe input is affected by inductance.The probe input resistance and capacitance are well understood, but the inductance of the probe input is often overlooked, especially when measuring at high frequencies.Where does the inductance come from?We know that when there is a wire, there is an inductance. There must be a wire connection between the probe and the circuit under test, and the backflow of the signal must pass through the ground wire of the probe. Normally, the ground wire of a 1mm probe will have an inductance of about 1nH. The longer the signal and ground wire are, the greater the inductance value will be.The parasitic inductance and parasitic capacitance of the probe constitute the resonance circuit. When the inductance value is too large, high frequency resonance may be generated under the excitation of the input signal, resulting in the distortion of the signal.Therefore, the length of signal and ground wire should be strictly controlled during high-frequency test, otherwise it is easy to produce ringing.