Comprehensive understanding of oscilloscope probes

I. Introduction

The existence of the probe has expanded the application range of the oscilloscope, allowing the oscilloscope to be used for online testing.

And analyze the electronic circuit under test, as shown below:

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 measurement accuracy.

Test results.

2. Loading effect of probe

When the probe detects the circuit under test, the probe becomes part of the circuit under test. The loading effect of the probe includes

The following 3 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 circuit being measured, which has a voltage dividing effect on the measured signal.

Amplitude and DC offset of the signal being measured. Sometimes, a faulty circuit may appear normal when the probe is applied.

It is generally recommended that the probe resistance R>10 times the measured source resistance to maintain an amplitude error of less than 10%.

Capacitive load is equivalent to connecting a capacitor in parallel with the circuit under test, which has a filtering effect on the measured signal and affects

The rise and fall time of the measured signal affects the transmission delay and the bandwidth of the transmission interconnection channel.

When the probe is installed, the faulty circuit becomes normal. This capacitance effect plays a key role.

It is 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 inductance effect of the probe ground wire, which forms a

resonance, which causes ringing to appear on the displayed signal. If obvious ringing appears on the displayed signal,

Check to confirm whether it is a true characteristic of the measured signal or ringing caused by the ground wire. The method to check is

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.

3. Types of Probes

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

Is it necessary to power the probe?

Passive probes are subdivided as follows:

1. Low resistance voltage divider probe;

2. High-impedance passive probe with compensation (the most commonly used passive probe);

3. High voltage probe

Active probes are subdivided as follows:

1. Single-ended active probe;

2. Differential probe;

3. Current probe

A brief comparison of the most commonly used high-impedance passive probes and active probes is as follows:

The low-resistance resistor divider probe has low capacitive loading (<1pf), high bandwidth (>1.5GHz), and low

The price is very high, but the resistance load is very large, usually only 500ohm or 1Kohm, so it is only suitable for testing low

Circuits with limited source impedance, or circuits that only focus on timing 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 of this type.

The high-impedance passive probe with compensation has a higher input resistance (usually more than 1Mohm) and an adjustable compensation

Compensation capacitor to match the input of the oscilloscope, with a higher dynamic range, can test larger amplitude signals

(more than dozens of pieces), the price is also relatively low. But the unknown point is that the input capacitance is too large (usually more than 10pf),

The bandwidth is low (generally within 500MHz).

A high-impedance passive probe with compensation has a compensation capacitor. When connected to an oscilloscope, the capacitance value generally needs to be adjusted.

(You need to use the small screwdriver that comes with the probe to adjust it. When adjusting, connect the probe to the oscilloscope compensation output test

test position) to match the oscilloscope input capacitance to eliminate low-frequency or high-frequency gain.

At high or low frequency gain, the adjusted compensation signal displays the waveform as shown on the right side of the figure below.

The high voltage probe is based on the passive probe with compensation. The input resistance is increased to increase the attenuation (e.g. 100:1

or 1000:1, etc.). Because high voltage resistant components are required, high voltage probes are generally physically larger.

4. Active probe

Let's first observe the impact of using a 600MHz passive probe and a 1.5GHz active probe to test a 1ns rise time step signal. Use a pulse generator to generate a 1ns step signal. After passing through the test fixture, use an SMA cable to directly connect to a 1.5GHz bandwidth oscilloscope. In this way, a waveform will be displayed on the oscilloscope (the blue signal in the figure below). Save this waveform as a reference waveform. Then use the probe point test fixture to detect the measured signal. The waveform directly connected through the SMA becomes a yellow waveform due to the influence of the probe load, and the probe channel displays a green waveform. Then test the rise time separately, and you can see the impact of passive and active probes on high-speed signals.

The specific test results are as follows:

Using 1165A 600MHz passive probe and crocodile mouth ground lead: Affected by probe loading, the rise time

The rise time is changed to: 1.9ns; the waveform displayed by the probe channel has ringing, and the rise time is: 1.85ns; using 1156A 1.5GHz active probe, using 5cm ground wire: the influence of probe load is small, and the rise time is still: 1ns; the waveform displayed by the probe channel is consistent with the original signal, and the rise time is still: 1ns. The structure diagram of the single-ended active probe is as follows, and the amplifier is used to achieve the purpose of impedance transformation. The input impedance of the single-ended active probe is high (generally more than 100Kohm), and the input capacitance is small (generally less than 1pf). After connecting to the oscilloscope through the probe amplifier, the oscilloscope must use 50ohm input impedance. The active probe has a wide bandwidth (now up to 30GHz) and a small load, but the price is relatively high (generally each probe reaches about 10% of the price of the oscilloscope with the same bandwidth), the dynamic range is small (this needs to be noted, because the signal exceeding the dynamic range of the probe cannot be tested correctly. The general dynamic range is about 5V), it is relatively fragile, and it needs to be used with caution.

The structure of the differential probe is shown below. A differential amplifier is used to achieve impedance transformation.

The impedance is high (usually more than 50Kohm), while the input capacitance is small (usually less than 1pf).

The differential probe is connected to the oscilloscope after the head amplifier. The oscilloscope must use a 50ohm input impedance.

Very wide (now up to 30GHz), very small load, high common mode rejection ratio, but relatively expensive

The price of each probe is usually about 10% of the price of an oscilloscope with the same bandwidth, and the dynamic range is also small (this

This needs attention, because the signal that exceeds the dynamic range of the probe cannot be tested correctly. The general dynamic range is 3V

It is relatively fragile and needs to be used with care.

Differential probes are suitable for testing high-speed differential signals (no grounding required during testing), amplifier testing, power supply testing,

It is suitable for virtual ground test and other applications.

The current probe is also an active probe that uses a Hall sensor and an induction coil to measure DC and AC current. The current probe converts the current signal into a voltage signal. The oscilloscope collects the voltage signal and displays it as a current signal. The current probe can test currents ranging from tens of milliamperes to hundreds of amperes. When using it, you need to lead out the current line (the current probe is tested by clamping the wire in the middle, which will not affect the circuit being tested).

Working principle of current probe when testing DC and low frequency AC:

When the current clamp is closed and surrounds a conductor with current flowing through it, a magnetic field will appear in response.

The magnetic field deflects the electrons in the Hall sensor, generating an electromotive force at the output of the Hall sensor.

The current probe generates a reverse (compensation) current based on this electromotive force and sends it to the coil of the current probe, making the current

The magnetic field in the clamp is zero to prevent saturation. The current probe measures the actual current value based on the reverse current.

This method can measure large currents very linearly, including mixed AC and DC currents.

Working principle of current probe when testing high frequency:

As the frequency of the measured current increases, the Hall effect gradually weakens.

When the current is alternating, the strength of the magnetic field is directly sensed by the coil of the current probe.

Like a current transformer, the current probe directly measures the induced current, not the compensation current.

The output of the MOSFET provides a low impedance return path to ground for the coil.

Working principle of current probe in the crossover area:

When the current probe operates in the high-low frequency crossover region of 20KHz, part of the measurement is achieved through the Hall sensor and the other part is achieved through the coil.

5. Active probe accessories

Modern high-bandwidth active probes are designed in a separate way: the probe amplifier and the probe accessories are separated.