“Resistive load is equivalent to connecting a resistor in parallel with the circuit under test, which has a partial pressure effect on the signal under test and affects the amplitude and DC bias of the signal under test. Sometimes, when the probe is added, the faulty circuit may become normal. It is generally recommended that the resistance of the probe R>10 times the resistance of the source under test to maintain an amplitude error of less than 10%.
The oscilloscope expands 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 the figure below:
Figure 1 The role of the oscilloscope probe
The selection and use of probes need to consider the following two aspects:
One: Because the probe has a load effect, the probe will directly affect the signal under test and the circuit under test;
Second: The probe is a part of the entire oscilloscope measurement system, which will directly affect the signal fidelity and test results of the instrument
1. The load effect of the probe
When the probe detects the circuit under test, the probe becomes a 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.
Figure 2 Load effect of the probe
Resistive load is equivalent to connecting a resistor in parallel with the circuit under test, which has a partial pressure effect on the signal under test and affects the amplitude and DC bias of the signal under test. Sometimes, when the probe is added, the faulty circuit may become normal. It is generally recommended that the resistance of the probe R>10 times the resistance of the source under test to maintain an amplitude error of less than 10%.
Figure 3 The resistive load of the probe
The capacitive load is equivalent to connecting a capacitor in parallel with the circuit under test, which has a filtering effect on the signal under test, affects the rise and fall time of the signal under test, affects the transmission delay, and affects the bandwidth of the transmission interconnection channel. Sometimes, when the probe is added, the faulty circuit becomes normal, and this capacitance effect plays a key role. It is generally recommended to use a probe with as small a capacitive load as possible to reduce the influence on the edge of the signal under test.
Figure 4 Capacitive load of the probe
The inductive load comes from the inductive effect of the probe ground wire. This ground wire inductance will resonate with the capacitive load and resistive load, causing ringing on the displayed signal. If there is obvious ringing on the displayed signal, you need to check whether it is the true feature of the signal under test or the ringing caused by the grounding wire. The method to check and confirm is to use the shortest possible grounding wire. Generally, it is recommended to use the shortest possible ground wire, and the general ground wire inductance=1nH/mm.
Figure 5 The inductive load of the probe
Two, the type of probe
The major aspects of oscilloscope probes can be divided into two categories: passive probes and active probes. Passive active, as the name implies, is whether or not to supply power to the probe.
The subdivision of passive probes is as follows:
1. Low resistance resistance voltage divider probe;
2. High-impedance passive probe with compensation (the most commonly used passive probe);
3. High voltage probe
The active probe is subdivided as follows:
1. Single-ended active probe;
2. Differential probe;
3. Current Probe
A simple comparison of the most commonly used high-impedance passive probes and active probes is as follows:
Table 1 Comparison of active probe and passive probe
The low-resistance resistance divider probe has a lower capacitive load (1.5GHz) and a lower price, but the resistive load is very large, generally only 500ohm or 1Kohm, so it is only suitable for testing circuits with low source impedance, or only focusing on time parameter testing The circuit.
Figure 6 Low input resistance probe structure
High-impedance passive probes with compensation are the most commonly used passive probes. Generally, the standard probes of the oscilloscope are such probes. The high-impedance passive probe with compensation has high input resistance (generally above 1Mohm) and adjustable compensation capacitance to match the input of the oscilloscope. It has a high dynamic range and can test larger amplitude signals (tens of amplitudes). Above), the price is also lower. But what I don’t know is that the input capacitance is too large (generally more than 10pf) and the bandwidth is lower (generally within 500MHz).
Figure 7 Common passive probe structure
The high-impedance passive probe with compensation has a compensation capacitor. When the oscilloscope is connected, it is generally necessary to adjust the capacitance value (you need to use the small screwdriver that comes with the probe to adjust, and connect the probe to the oscilloscope compensation output test position during adjustment). Match the input capacitance of the oscilloscope to eliminate low-frequency or high-frequency gain. The left side of the figure below shows the presence of high-frequency or low-frequency gain, and the adjusted compensation signal Display waveform is shown on the right side of the figure below.
Figure 8 Compensation of passive probe
High-voltage probes are based on passive probes with compensation. Increase the input resistance to increase the attenuation (such as 100:1 or 1000:1, etc.). Because high-voltage resistant components are required, high-voltage probes generally have a relatively large physical size.
Figure 9 The structure of the high-voltage probe
Three, active probe
Let’s first observe the influence of using a 600MHz passive probe and a 1.5GHz active probe to test the 1ns rise time step signal. Use a pulse generator to generate a 1ns step signal. After passing the test fixture, use an SMA cable to directly connect to a 1.5GHz bandwidth oscilloscope, so that a waveform (the blue signal in the figure below) will be displayed on the oscilloscope. The waveform is saved as a reference waveform. Then use the probe point test fixture to detect the signal under test. 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, you can see the influence of the passive probe and the active probe on the high-speed signal.
Figure 10 The influence of passive probes and active probes on the measured signal and measurement results
The specific test results are as follows:
Use 1165A 600MHz passive probe, use alligator mouth ground wire: Affected by the probe load, the rise time becomes: 1.9ns; the waveform displayed by the probe channel has ringing, and the rise time is: 1.85ns;
Use 1156A 1.5GHz active probe, use 5cm ground wire: less affected by the probe load, 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, using an amplifier to achieve the purpose of impedance transformation. The input impedance of the single-ended active probe is relatively high (generally more than 100Kohm), and the input capacitance is relatively small (generally less than 1pf). After connecting to the oscilloscope through the probe amplifier, the oscilloscope must use 50 ohm input impedance. Active probes have wide bandwidth (up to 30GHz now), and low load, but the price is relatively high (generally each probe reaches about 10% of the price of the same bandwidth oscilloscope), and the dynamic range is small (this needs attention, because it exceeds the probe dynamics The signal of the range cannot be tested correctly. Generally, the dynamic range is about 5V), which is relatively fragile, so please be careful when using it.
Figure 11 Active probe structure
The structure diagram of the differential probe is as follows, using a differential amplifier to achieve the purpose of impedance transformation. The input impedance of the differential probe is relatively high (generally up to 50Kohm or more), and the input capacitance is small (generally less than 1pf). After connecting to the oscilloscope through the differential probe amplifier, the oscilloscope must use 50 ohm input impedance. The differential probe has a very wide bandwidth (up to 30GHz now), a very small load, and a high common-mode rejection ratio, but the price is relatively high (generally each probe reaches about 10% of the price of an oscilloscope with the same bandwidth), and the dynamic range is also small (This needs attention, because the signal that exceeds the dynamic range of the probe cannot be tested correctly. Generally, the dynamic range is about 3V), it is relatively fragile, and you need to be careful when using it.
Differential probes are suitable for testing high-speed differential signals (without grounding during testing), suitable for amplifier testing, power supply testing, and virtual ground testing.
Figure 12 Differential probe structure
Current probes are also active probes, which use Hall sensors and induction coils to measure DC and AC currents. The current probe converts the current signal into a voltage signal, and the oscilloscope collects the voltage signal, and then displays it as a current signal. The current probe can test currents from tens of milliamps to hundreds of amperes, and the current wire needs to be drawn during use (the current probe is tested by sandwiching the wire in the middle, and will not affect the circuit under test).
The working principle of the current probe when testing DC and low-frequency AC:
When the current clamp is closed and a current-carrying conductor is enclosed in the center, a magnetic field will appear accordingly. These magnetic fields deflect the electrons in the Hall sensor and generate an electromotive force at the output of the Hall sensor. The current probe generates a reverse (compensation) current according to this electromotive force and sends it to the coil of the current probe, so that the magnetic field in the current clamp is zero to prevent saturation. The current probe measures the actual current value according to the reverse current. With this method, large currents can be measured very linearly, including mixed AC and DC currents.
Figure 13 The working principle of the current probe when testing DC and low frequency
The working principle of the current probe when testing high frequency:
As the frequency of the measured current increases, the Hall effect gradually weakens. When measuring a high-frequency AC current that does not contain a DC component, most of it is directly induced to the coil of the current probe through the strength of the magnetic field. At this time, the probe is like a current transformer. The current probe directly measures the induced current instead of the compensation current. The output of the power amplifier provides a low-impedance ground loop for the coil.
Figure 14 The working principle of the current probe when testing high frequency
The working principle of the current probe in the cross area:
When the current probe is working in the high and low frequency crossing area of 20KHz, part of the measurement is realized by the Hall sensor, and the other part is realized by the coil.
Figure 15 The working principle of the current probe crossing area
Four, active probe accessories
Modern high-bandwidth active probes adopt a separate design method, that is, the probe amplifier is separated from the probe accessories. The advantages of this design are:
1. Support more probe accessories, making detection more flexible;
2. To protect investment, the most expensive one is the probe amplifier (a probe amplifier can support multiple detection methods, which previously required several probes to achieve); at the same time, the probe accessory protects the probe amplifier (even if the probe accessory is damaged, the price is relatively cheap);
3. This design method is easy to achieve high bandwidth.
Figure 16 Probe accessories
These probe accessories mainly include the following:
1. Point measuring probe accessories (including: single end point test and differential point test);
2. Welding probe accessories (including: single-ended welding and differential welding, separate ZIF welding);
3. Accessories for jack probes;
4. Differential SMA probe accessory (the oscilloscope generally directly supports SMA connection, but if the signal under test needs to be pulled up such as HDMI, you must use the SMA probe accessory).
The circuit structure of the probe accessory is shown in the figure below:
1. There will be a pair of damping resistors (usually 82ohm) at the tip of the probe accessory. The effect of the damping resistor is to eliminate the resonance effect of the inductance of the tip of the probe accessory;
2. Behind the tip of the probe is a resistance of 25Kohm, this resistance determines the input impedance of the probe (DC input impedance is resistance: single-ended 25Kohm, differential 50Kohm), this resistance makes the measured signal transmitted to the probe amplifier part of the power is very Small, it will not have a big impact on the measured signal.
3. Behind the 25Kohm resistor is the coaxial transmission line, which is responsible for transmitting the small signal to the amplifier. The length of this transmission line can be very long or very short, and an attenuator or a coupling capacitor can be added in the middle.
4. The coaxial transmission line is connected to the amplifier, and the amplifier is 50ohm matched (differential 100ohm matched).
Figure 17 The structure of the active probe accessory
In order to maintain the accuracy of the probe, the active probe needs to work at a constant temperature, so the probe amplifier cannot be placed in a high and low temperature box to test the circuit board under test in a high and low temperature environment. It can be seen from the probe accessory structure that the length of the 50ohm transmission line in the middle does not affect the detection, so a very long coaxial cable or extended coaxial cable can be used, and the coaxial cable can be extended into the high and low temperature box for high and low temperature to be tested. Circuit board testing. The following figure shows the N5450A extension cable. Using the N5381A welding probe accessory, it can work in the temperature range of -55° to 150°.
Figure 18 High and low temperature probe structure principle
Using N5450A extension cable and N5381A probe accessory, using 1169A 12GHz probe amplifier, the frequency response curve under -55° and 150° environment is shown in the figure below, which shows that it can meet the requirements of high-speed signal testing.
Figure 19 Frequency response of high and low temperature probe at high and low temperature
V. Verification of the accuracy of probes and accessories
The following figure is an example: the measured signal is a clock signal with a frequency of 456MHz and an edge time of about 65ps. The test results of different types of probes and probe accessories are used.
Picture A is the test result of using 12GHz 1169A differential probe and N5381A 12GHz welding probe accessory, which almost completely reproduces the signal under test;
Picture B is the test result using a 500MHz passive probe, and the displayed signal is completely distorted;
Figure C is the test result using a 12GHz 1169A differential probe and a longer test lead. The displayed signal has a large overshoot;
Figure D is the test result using a 4GHz 1158A single-ended probe and a longer test lead. The displayed signal is almost a sine wave with greater distortion.
Figure 20 Comparison of test results of different probe accessories
It can be seen from the figure that the probe and probe accessories have a very large impact on the test accuracy, and it is one of the contents that we should pay attention to when testing high-speed signals. So how should we verify the probe and probe accessories?
To verify the probe and probe accessories, you need to use a pulse pattern generator (such as 81134A, 3.35GHz rate, 60ps edge pulse pattern generator). If the oscilloscope has its own high-speed signal output function, you can also use the auxiliary output of the oscilloscope Port instead of pulse pattern generator (for example: Infiniium oscilloscope AUX OUT port can send a high-speed clock: 456MHz frequency, about 65ps edge). In addition, a coaxial cable and a test fixture are required (the probe calibration fixture configured with the Infiniium oscilloscope can be used as the probe and probe accessory verification test fixture). The outer surface of the test fixture is ground, and the inner wiring is signal, as shown in the figure below. When in use, connect one end to the AUX OUT port of the pulse pattern generator or the auxiliary output of the oscilloscope through a coaxial cable, and connect the other end to channel 1 of the oscilloscope through an adapter.
Figure 21 Probe Verification Fixture
Then connect the verified probe to channel 2. The probe can contact the signal and ground of the test fixture through the probe accessory (if it is a differential probe, then connect the + terminal to the signal line of the test fixture, and connect the-terminal to the test fixture On the ground).
1. If the probe does not touch the signal line, an original waveform will appear on the screen, which is saved as a reference waveform;
2. When the probe is used to probe the signal line, the waveform of channel 1 will change. The changed waveform is the signal under test after being affected by the probe and the probe accessories;
3. At this time, a waveform will appear on channel 2 connected to the probe, which is the waveform tested by the probe;
4. By comparing the reference waveform, the waveform of channel 1, and the waveform of channel 2 connected to the probe, the difference between the three can be seen intuitively or read through the test parameters, and the influence of the probe and the probe accessories can be verified.
Figure 22 Probe verification connection and principle
The following figure is an example of actual verification. In figure A, the AUX OUT of the oscilloscope is connected to the test fixture through a coaxial cable, and the other end of the test fixture is connected to a channel of the oscilloscope through an SMA-PBNC adapter (in this example, it is connected to channel 3) , Connect the probe to channel 1, adjust the waveform on the screen at this time, so that an edge step waveform appears, as shown in Figure C, and save this waveform as a reference waveform. As shown in Figure B, the verified probe and accessories are measured on the test fixture. As shown in Figure D, 3 waveforms appear on the screen. The blue one is the reference waveform, the green one is the measured waveform after being affected by the probe, and the yellow one. It is the waveform displayed by the probe. By testing the rise time parameters, overshoot parameters, etc., the performance of the probe and the probe accessories can be confirmed.
Figure 23 Probe verification example