What is the effect of the active probes of the oscilloscope on the measurements?
The connection section in front of the amplifier is a section of uncontrolled impedance connection line with a lot of equivalent capacitance and equivalent inductance, which has a great impact on the system bandwidth, input impedance at high frequencies, and frequency response characteristics; the back of the amplifier is usually a 50 Ω transmission line, which has a controlled impedance, and has a lesser impact on the system bandwidth.
The easiest way to reduce the impact of the leads on the system bandwidth is to shorten the length of the connection line between the probe and the DUT. An example of this is shown in the diagram below, where a 2GHz single ended active probe was used in the test. The bandwidth of the system is different when using different connecting accessories, the shorter the front-end accessories used, the higher the bandwidth of the system.
Using these different connections for the same 1ns rise time signal, it can be seen that the shorter the connections used, the higher the bandwidth of the system and the steeper the measured rising edge.
However, in some cases, in order to use the convenience of the probe amplifier must be a certain distance from the test point, this section of the connection line is usually shown as inductive, if the inductive effect caused by this lead is not compensated for, this section of the long connection line is very easy to cause signal oscillation. The following two graphs show the results of a 4GHz single-ended active probe with a 2-inch long lead on a 500MHz clock signal with a 100ps rise time. In the figure on the left, the 2-inch long lead is not matched, and the measured clock signal has serious oscillations and distortions; in the figure on the right, the source of the 2-inch long lead is matched by a suitable resistor, and the oscillations and distortions of the signal are significantly reduced.
Therefore, in the probe and the length of the lead can not be shortened, the use of a suitable resistor near the test point of the end of the signal matching can improve the impact of lead inductance, the specific use of the size of the matching resistor should be based on the length of the lead and other characteristics of the simulation and calculation. The figure below shows two differential probes used for differential soldering and point test probes. It can be seen that in the case of high frequency, in order to improve the fidelity of the signal measurement, even for very short leads, it is necessary to carry out suitable matching. One thing to note about resistor matching is that this matching resistor only reduces the oscillation of the signal caused by the long leads, and has limited improvement in bandwidth; if the front-end lead length is too long, the bandwidth of the system will still drop.
As mentioned earlier, to increase the bandwidth of an active probe, in addition to using a high bandwidth amplifier, it is necessary to minimise the length of the uncontrolled impedance transmission line from the test point to the probe amplifier, and to match the resistor at the front end of the connection line. However, high-bandwidth amplifiers require complex shielding, matching, and power supply, and are not particularly small, making them inconvenient to use if they are designed too close to the test point. In order to ensure both ease of use and high measurement bandwidth, many of the high bandwidth probes on the market today have a split structure.
This type of probe consists of two parts, the probe amplifier and the front part of the probe, which are connected by a 50Ω coaxial connector. Usually the impedance of the front part of the probe amplifier is uncontrolled, so the length of this part has a great influence on the signal, while the front of the InfiniiMax probe only has a short section in front (about 5mm or so) that is uncontrolled impedance, which is a very short lead and thus ensures a high measurement bandwidth; and the back part of the front part of the probe (about 10cm) is a 50Ω coaxial transmission line, which has little influence on the bandwidth of the system. The part behind the probe front (approx. 10 cm) is a 50 Ω coaxial transmission line, the length of which has little influence on the system bandwidth. Therefore, with this structure, on the one hand, the probe bandwidth can be made wider, on the other hand, the probe amplifier can be farther away from the test point, so that the size of the probe front end is smaller and thus easier to use. At the same time, this split structure makes it convenient for users to change different test front ends according to different testing needs, such as point measurement, welding, jacks and so on.
