Looking around blindly? STOP! (Part 1)

        As a part of the test system, the importance of the probe is often overlooked. When matching a probe for an oscilloscope, some people even just pick the closest probe from the drawer. So what are the categories of oscilloscope probes? What are the differences between these categories? How should we choose? This article will introduce the types of probes, their advantages and disadvantages, and discuss the factors to pay attention to when choosing a probe.

Factors to consider when selecting a probe

        First, the "probe + oscilloscope" constitutes an overall test system, which will work together to affect the test results. In fact, a single oscilloscope and a single probe each have their own bandwidth and rise time indicators, but there is no simple formula to calculate the indicators of this system. Usually, we will recommend a specific probe for the oscilloscope and indicate the indicators of the entire system. Therefore, when you choose a non-standard or recommended probe, you need to consider that there may be unexpected test results. Secondly, to ensure that the probe has the least impact on the probed circuit, an ideal probe should provide absolute signal fidelity, have no effect on the original signal, and be very easy to connect to the circuit under test. But the reality is that the probe will definitely affect your test. But by choosing the right probe and carefully considering your test application, you can minimize the impact.

         Usually you need to consider the type of signal. Is it voltage or current? Is it single-ended or differential?

       1. Secondly, you also need to understand the signal frequency components of the test signal. In addition to testing the fundamental frequency, do you also need to test harmonics and spurious signals? You must ensure that your oscilloscope and probe have sufficient bandwidth to observe all signal energy.

        2. The rise time of the test system must also be considered as it will affect the measurement of the rise time of the signal being measured. The rise time of the test system needs to be 5 times faster than the rise time of the signal being measured.

        3. In addition, the resistance and capacitance of the probe are also important factors that must be considered, and their importance depends on the impedance of the measured signal. The geometric dimensions of the test point will also affect the choice of probe and connection method.

         There are various types of probes on the market to meet different test applications. They are mainly divided into four categories: voltage, logic, current and optical probes, which can be further subdivided. (See the figure below)

 

Passive voltage probes

        Passive probes are made of wires and connectors, and when compensation or attenuation is required, resistors and capacitors are also included. There are no active devices (transistors or amplifiers) in the probe, so the probe does not need to be powered. Because of their relative simplicity, passive probes are generally the most rugged and economical probes. They are easy to use and are also the most widely used probes.

        Passive voltage probes offer various attenuation factors (1X, 10X, and 100X) for different voltage ranges. Among these passive probes, 10X passive voltage probes are the most commonly used probes and are also the type of probes that are usually provided as standard accessories for oscilloscopes. Tektronix has a wide range of passive probes and is the only manufacturer that matches the probe bandwidth to the oscilloscope probe bandwidth at 1 GHz (the first passive 1GHz probe TPP1000 can be found in the previous blog post).

        1. New passive probe - TPP0X0X series. This series is a cost-effective 100MHZ to 200MHz passive probe with smaller probe tip size and lower input capacitance.

        2. Upgraded passive voltage probes - TPP0850 and TPP0502. TPP0502 uses 2x attenuation based on 500MHz bandwidth to help customers capture ripple signals; TPP0850 is also used for single-ended high-voltage signal measurement, DC+PK is 2500V, and the bandwidth is up to 800MHz. TPP0850 and TPP0502 series passive probes are suitable for MSO/DPO5000 series and MSO/DPO4000B series oscilloscopes.

Active voltage probes

        Active probes contain or rely on active devices, such as transistors. Most commonly, the active device is a field effect transistor (FET). The advantage of a FET input is that it offers very low input capacitance, typically a few picofarads and as low as less than 1 picofarad. This ultra-low capacitance allows for a variety of effects that users desire—active FET probes are typically specified with bandwidths between 500 MHz and 4 GHz. In addition to higher bandwidth, the high input impedance of active FET probes allows measurements to be made on test points with unknown impedances with much less risk of loading effects. Also, because the low capacitance reduces ground lead effects, longer ground leads can be used. But most importantly, FET probes offer very low loading, so they can be used on high impedance circuits that would be severely loaded by passive probes.

         Given these advantages, why would you use a passive probe? This is because active FET probes do not have the voltage range of passive probes. Active probes typically have a linear dynamic range of ± 0.6V to ± 10V. In addition, the maximum voltage they can withstand is ± 40V (DC + peak AC).

 

Differential probes

        In many cases, the signal voltage is a differential voltage, that is, the signal exists in two points or two lines, and neither of these two points or two signals is at ground or common potential. This differential signal is very common in telephone voice circuits, computer disk read channels, and multi-phase power supply circuits, such as hard disk drive read channel signals, multi-phase power supply systems, etc. These signals are essentially "floating" above the ground. At this time, a differential probe is needed.

         Tektronix's new high-voltage differential probe P5201 provides 25MHz bandwidth, 1400V differential dynamic range and 1000V common mode withstand voltage at a very shocking price, which is convenient for application in various motors, large backlights and new energy industries such as wind power. Two power supply methods, battery and adapter, greatly facilitate user use.

       

         The P5201 can be used with any oscilloscope, allowing users to safely measure floating circuits when the oscilloscope is grounded. The P5201 high-voltage differential probe converts floating signals into low-voltage reference ground level signals, which can then be safely and easily displayed on any ground-referenced oscilloscope. (To be continued)