Oscilloscope probe (1)

The selection and use of probes need to consider the following two aspects:

First, because the probe has a loading effect, the probe will directly affect the measured signal and the measured circuit;

Second: The probe is part of the entire oscilloscope measurement system and will directly affect the instrument's signal fidelity and test results

1. Probe Load Effect
   When the probe detects the circuit under test, the probe becomes a part of the circuit under test. The probe load effect includes the following three parts:
1. Resistive load effect;
2. Capacitive load effect;
3. Inductive load effect.
The resistive load is equivalent to connecting a resistor in parallel to the measured circuit, which has a voltage-dividing effect on the measured signal, affecting the amplitude and DC bias of the measured signal. Sometimes, when the probe is added, the faulty circuit may become normal. It is generally recommended that the resistance R of the probe be greater than 10 times the resistance of the measured source to maintain an amplitude error of less than 10%.
Capacitive load is equivalent to connecting a capacitor in parallel to the circuit under test, which has a filtering effect on the measured signal, affecting the rise and fall time of the measured signal, affecting the transmission delay, and affecting the bandwidth of the transmission interconnection channel. Sometimes, when the probe is added, the faulty circuit becomes normal, and this capacitive effect plays a key role. It is generally recommended to use a probe with as small a capacitive load as possible to reduce the impact on the edge of the measured signal.
The inductive load comes from the inductive effect of the probe ground wire. This ground wire inductance will resonate with the capacitive load and the resistive load, causing ringing on the displayed signal. If there is obvious ringing on the displayed signal, it is necessary to check whether it is the true characteristic of the measured signal or the ringing caused by the ground wire. The method of checking and confirming is to use the shortest possible ground wire. It is generally recommended to use the shortest possible ground wire, and the general ground wire inductance = 1nH/mm.

2. Types of probes
   Oscilloscope probes can be broadly divided into two categories: passive probes and active probes. Passive and active, as the name implies, refer to whether the probe needs to be powered. Passive
probes are subdivided as follows:
        1. Low-resistance resistor divider probes;
        2. High-resistance passive probes with compensation (the most commonly used passive probes);
        3. High-voltage probes
Active probes are subdivided as follows:
        1. Single-ended active probes;
        2. Differential probes;
        3. Current probes
The most commonly used high-resistance passive probes and active probes are briefly compared as follows:


     Low-resistance resistor divider probes have lower capacitive loads (<1pf), higher bandwidths (>1.5GHz), and lower prices, but their resistance loads are very large, generally only 500ohm or 1Kohm, so they are only suitable for testing circuits with low source impedance, or circuits that only focus on time parameter testing.
    The high-impedance passive probe with compensation is the most commonly used passive probe. The probes that come standard with general oscilloscopes are all of this type. The high-impedance passive probe with compensation has a higher input resistance (generally more than 1Mohm), an adjustable compensation capacitor to match the input of the oscilloscope, a higher dynamic range, and can test larger amplitude signals (more than dozens of amplitudes). The price is also low. However, the unknown problem is that the input capacitance is too large (generally more than 10pf) and the bandwidth is low (generally less than 500MHz).


    The high-impedance passive probe with compensation has a compensation capacitor. When connected to an oscilloscope, the capacitance value generally needs to be adjusted (the small screwdriver that comes with the probe needs to be used for adjustment. When adjusting, connect the probe to the oscilloscope compensation output test position) to match the oscilloscope input capacitance to eliminate low-frequency or high-frequency gain. The left side of the figure below shows high-frequency or low-frequency gain. The adjusted compensation signal display waveform is shown on the right side of the figure below.
   High-voltage probes are passive probes with compensation, but with a larger input resistance to increase attenuation (e.g. 100:1 or 1000:1). Because they require high-voltage resistant components, high-voltage probes are generally larger in size.