The Introduction of Oscilloscope Probes

Ⅰ. What are oscilloscope probes?

The oscilloscope probe is critical to the accuracy and correctness of the measurement results. It is the electronic component that connects the circuit under test to the input of the oscilloscope. The simplest probe is a wire connecting the circuit under test and the input end of the electronic oscilloscope, while the complex probe consists of RC components and active devices. Simple probes without shielding measures are easily interfered with by external electromagnetic fields, and their equivalent capacitance is large, which increases the load of the circuit under test and distorts the signal under test.

The oscilloscope has expanded the scope of application of the oscilloscope because of the existence of the probe so that the oscilloscope can test and analyze the electronic circuit under test online, as shown in Figure 1.

Figure 1 The Role of the Oscilloscope Probes

Two factors must be considered while choosing and using probes:

Because of the probe's loading impact, the signal under test and the circuit under test will be immediately affected.

Probes are an integral part of an oscilloscope's entire measuring system, and they have a direct impact on the instrument's signal fidelity and test results.

Ⅱ. Load effects of probes

When the probe detects the circuit under test, the probe becomes part of the circuit under test.

Probe loading effects can be divided into three categories:

Resistive loading effects.

Capacitive loading effects.

Inductive load effects.

Figure 2 Loading effect of probe

1 Resistive load

The resistive load is comparable to connecting a resistor in parallel with the circuit under test, which divides the voltage of the signal under test and influences the signal's amplitude and  DC  offset. When the probe is used, the faulty circuit may return to normal. To maintain an amplitude error of less than 10%, it is generally advised that the probe's resistance R be >10 times that of the source under test.

Figure 3 Resistive Loading of Probes

2 Capacitive load

Capacitive load is the impact of connecting a capacitor in parallel with the circuit under test, which filters the signal under test, affects the signal's rise and fall time, affects the transmission delay, and affects the bandwidth of the transmission interconnection channel. When the probe is inserted, the problematic circuit might sometimes become normal, and the capacitive effect plays an important part. To limit the impact on the edge of the signal being monitored, it is generally advised to use a probe with the as little capacitive load as feasible.

Figure 4 Capacitive Loading of Probes

3 Inductive load

The inductive effects of the probe ground lead, which resonate with capacitive and resistive loads, cause ringing on the displayed signal, cause inductive loads. If the displayed signal has noticeable ringing, it is required to determine whether this is a true property of the signal under examination or ringing induced by the ground line. Using the shortest ground wire feasible is a good technique to inspect and confirm. It is typically advised that the ground wire be as short as feasible.

Ground wire inductance=1nH/mm

Figure 5 Inductive Loading of Probes

Ⅲ.  Classification of probes

Oscilloscope probes can be divided into two categories: passive probes and active probes.

Passive probe

Low resistance resistive voltage divider probe, high resistance passive probe with compensation (the most often used passive probe), and high voltage probe are the three types of passive probes.

Resistive divider probe with low resistance:

Low-resistance resistor divider probes have a lower capacitive load (1pf), a higher bandwidth (>1.5GHz), and a lower price, but the resistance load is very large, typically 500ohm or 1Kohm, making them only suitable for testing low source Impedance circuits or circuits that only test time parameters.

Figure 6 Low Input Resistance Probe Structure

High-Z passive probe with compensation:

The most widely used passive probes are high-impedance passive probes with compensation, which are normally ordinary oscilloscope probes  . The high-impedance passive probe with compensation features a high input resistance (usually greater than 1Mohm), an adjustable compensation capacitor  to match the oscilloscope's input, and a huge dynamic range, allowing it to examine large-amplitude signals (tens of amplitudes). The pricing is also reduced (see above). However, the input capacitance is excessively high (usually more than 10pf) and the bandwidth is too narrow (generally within 500MHz).

Figure 7 Commonly used Passive Probe Structure

A compensation capacitor is included in the high-impedance passive probe with compensation. When connecting to an oscilloscope, you must adjust the capacitance value to match the oscilloscope input capacitance to remove low or high-frequency gain (you must use the small screwdriver that comes with the probe to adjust, and connect the probe to the oscilloscope compensation output test position when adjusting). The waveform of the modified compensation signal is depicted on the right side of Figure 8, and there is a high frequency or low-frequency gain on the left side of Figure 8.

Figure 8 Compensation of Passive Probes

High voltage probe:

High-voltage probes are based on compensated passive probes, with the input resistance increased to increase attenuation (eg, 100:1 or 1000:1, etc.). High-voltage probes are often larger in size due to the need to use high-voltage components.

Figure 9 Structure of High Voltage Probe

Active probe

Active probe subdivision: single-ended active probe, differential probe, current probe.

Let's look at the results of using a 600MHz passive probe and a 1.5GHz active probe to evaluate a 1ns rise time step signal. After passing through the test fixture, use an SMA cable to connect directly to a 1.5GHz bandwidth oscilloscope, so the oscilloscope will display a waveform (the blue signal in Figure 10) and insert this into the test fixture. As a reference waveform, the waveform is preserved. The signal under examination is then detected using the probe point test fixture. Due to the influence of the probe load, the waveform directly connected through the SMA becomes a yellow waveform, while the probe channel displays a green waveform. The influence of passive and active probes on high-speed signals may then be assessed by measuring the rising time independently.

Figure 10 Influence of Passive Probes and Active Probes on the Measured Signal and Measurement Results

The specific test results are as follows:

Use a crocodile mouth ground lead with a 1165A 600MHz passive probe. The rise time is 1.9ns when the probe load is applied; the waveform exhibited by the probe channel contains ringing, and the rise time is 1.85ns.

With a 5cm ground lead, use a 1156A 1.5GHz active probe. The waveform presented by the probe channel is consistent with the original signal, and the rise time is still: 1ns; the waveform displayed by the probe channel is less affected by the probe load, and the rise time is still: 1ns.

Single-ended active probe:

Figure 11 shows the single-ended active probe's structure, and the amplifier is employed to achieve the impedance transformation goal. The single-ended active probe has a relatively high input impedance (usually more than 100 kohm) and modest input capacitance (generally less than 1pf). The oscilloscope must use 50ohm input impedance after connecting to it through the probe amplifier. Active probes have a large bandwidth (up to 30GHz presently) and a small load, but they are expensive (usually, each probe costs about 10% of the price of an oscilloscope with the same bandwidth), have a short dynamic range, and are delicate, therefore use them with caution. Because a signal that exceeds the dynamic range of the probe cannot be tested effectively, the general dynamic range is around 5V.

Figure 11 Structure of a Single-Ended Active Probe

Differential probe:

Figure 12 shows the differential probe's structure, and a differential amplifier is employed to achieve the impedance transformation goal. The differential probe's input impedance is relatively high (usually greater than 50 kohm), whereas the input capacitance is minimal (generally less than 1pf). The oscilloscope must use 50ohm input impedance after connecting to it via the differential probe amplifier. Differential probes have a large bandwidth (currently up to 30GHz), a tiny load, and a good common-mode rejection ratio, but they are expensive (typically, each probe costs around 10% of an oscilloscope with the same bandwidth), and their dynamic range is limited. (This requires special attention because a signal that exceeds the dynamic range of the probe cannot be properly tested.) It has a rather small dynamic range (about 3V) and must be handled with caution.

Differential probes can be used to evaluate high-speed differential signals (without grounding), amplifiers, power supplies, and virtual ground.

Figure 12 Differential Probe Structure

Current probe:

Current probes are active probes that measure  DC and  AC  currents via Hall sensors and induction coils. The current probe converts the current signal to a voltage signal, which is then collected and shown as a current signal on the oscilloscope. When using current probes, current wires must be drawn out to test currents ranging from tens of milliamps to hundreds of amperes (current probes are tested by sandwiching the wires in the middle, which will not affect the circuit under test).

Passive Probes vs Active Probes