Oscilloscopes are powerful tools essential in any electrician’s toolbox for finding faults in electronic circuits, for developing electronic parts, and diagnosing and repairing electronics.
The Oscilloscope, or more commonly known as just a scope, is able to detect the waveform signal measurements and display it over time in a graphical format onthe screen.
To be able to accomplish this, the scope will need probes to connect the scope to the electronic device. When selecting the best probe for your scope, you’ll want to check the scope’s manual to see what types it recommends that you use.
You can also check online in case you no longer have the user manual as most companies host these on their websites now.
Choosing the right probe is important because the probes themselves affect the signal being measured in different ways. The input’s resistance, capacitance, and bandwidth affect the signal being transmitted to the scope. It can lessen the current, increase it, and modify it in different ways that will affect your calculations and measurements.
Scope Connector Types
The first thing you’ll need to identify is that the connector type matches that of your scope. The primary type of connector that scopes have are “BNC” type. You may also see SMA connector types, but they are rarer.
You’ll want to ensure you’ve checked if yours is BNC, SMA or possibly another type so that when you purchase your probes they’ll connect to the scope.
Additionally, you’ll want to ensure your probe connectors type’s resistance and capacitance match the resistance and capacitance of the scope you’re using.
This is because you typically don’t want your probes to have any effect on the measurements you’re making. If you can keep the resistance and capacitance equal on the probe and scope, then you won’t see any issues.
Most scopes have two choices for resistance either in 50Ω or 1MΩ with 1MΩ being used for most general testing and the 50Ω used for circuit/logic boards using high-speed signals.
Understanding the schematics of how probes work is important but quite complex at first. Passive probes are one of the most commonly used probes for taking basic measurements.
They are made using wires, connectors, a box for resistors and capacitors but no active components like amplifiers. Most passive probes are very affordable and durable.
You’ll typically find passive probes in 4 different configurations with different attenuation (reducer in the amplitude of a signal) ranging from none and then up from 10, to 100, to 1000. You’ll usually see these shown like below:
- 1×: no attenuation
- 10×: factor-of-10 attenuation
- 100×: factor-of-100 attenuation
- 1000×: factor-of-1000 attenuation
If the scope used has a 1MΩ, so if the probe is a 10× probe, it’ll house an internal resistor of 9MΩ which creates a 10:1 attenuation for your scope’s input.
The reason these calculations are important is because they can allow you to measure signals that exceed your scope’s limit by reducing the signal coming in which is important especially in high-frequency measurements.
The active probes on the other hand are different as they do contain active components like amplifiers and are most commonly used for taking high speed measurements.
The internal components on an active probe have a very high input resistance and are typically externally powered which will allow you to amplify the signal.
The active probes normally have bandwidths ranging from 500 MHz to 4 GHz.
There are certain times that you’ll need to measure low level audio signals or disk drive signals and, in this case, you’ll need a probe that can measure these called differential probes. To achieve this you’ll need two probes lined up to each signal and then use the oscilloscope to subtract one from the other to provide the difference.
Using two scope probes like this typically causes a bunch of problems like excess noise, the length of the probes may be different which skews the results, etc.
Another type of probe is the current probe which is a less invasive way to measure electrical current by using a sensor that measures the magnetic field generated by the DC current as it passes through the probe’s core. Current probes that measure AC and DC current are quite common and is similar to that of a Clamp Meter (like a multimeter) but with an oscilloscope, you can measure it over time.
Here are a few extra tips when selecting probes:
- Always try to keep your leads short. Unnecessary long probe leads may make a significant signal distortion to the measurement. Also keeping the ground wires short is very important for high-speed signals as the ground lead can behave as an antenna and may pick up noise that disrupts the signal.
- Use the appropriate probe adaptors when applicable. Always refer to the user manual to ensure you have the right connection types when attaching probes.
- The probe’s tip is also an important factor as it must be a reliable connection to the tested object. Make sure to always select well-known and respectable brands to ensure the material is made up to industry standards. Fluke is the best known company on the market now for both scopes and meter probes.
- When troubleshooting a circuit, it’s much easier for you to use a sharp probe tip so you can accurately position the probe tip on top of the small fine tip components.
You can find more detailed information on this Physics guide walkthrough here: https://www.physics.wisc.edu/undergrads/courses/fall2018/623/lab_equipment/MSO2014_scope_tutorials/Probes/Probes-Lab.pdf
There are many different types of probes available no matter what your measurement is, but to avoid getting confused just stick to the basics of measuring bandwidth and loading.
Starting with those will help you understand the basics before you move into the more complex probes and measurements.