Oscilloscope Probe Complete Details
A probe connects the oscilloscope to the signal you want to measure.
While you could use a simple cable, it doesn’t provide defined resistance and capacitance, which can lead to problems.
At low frequencies, like in the kilohertz range, using a basic cable doesn’t cause much of an issue.
However, at high frequencies, such as in the megahertz range, using a simple cable can distort the signal and give you incorrect results or unexpected outcomes.
That’s why oscilloscope probes were created—to measure voltage under controlled and known conditions.
The probe has a small crocodile clip on the side, which serves as the ground connection.
The tip of the probe head has a metal contact that touches the voltage signal you’re measuring.
This metal contact is linked to the oscilloscope’s inner BNC connector.
Some probes also include a cable clamp at the tip.
When you press down on the tip, the clamp can be extended to create a secure, lasting connection.
Attenuation Ratio
An oscilloscope typically has an input impedance of one Megaohm.
When you use a common passive probe with an input resistance of nine Megaohms, it works together with the oscilloscope to create a voltage divider.
This setup has a total resistance of 10 Megaohms and a divider ratio of 10:1, meaning the voltage measured by the probe is reduced by a factor of 10 before it reaches the oscilloscope.
To find the actual voltage at the tip of the probe, you need to multiply the voltage shown on the oscilloscope by 10.
Different divider ratios, like 10:1, 20:1, or 100:1, are used depending on the probe’s resistance or impedance.
For example, high-frequency oscilloscopes often have an input impedance of 50 Ohms. In this case, a 10:1 probe includes a 450 Ohm resistor, creating a total of 500 Ohms.
This combination results in the same 10:1 divider ratio.
The probe’s defined impedance also sets its capacitance, which is usually a few picofarads.
While the probe’s inductance can affect performance, it’s generally less important at low and medium frequencies.
Active Probe
An active probe contains electronics that boost signals with high bandwidth, which means it needs a power supply to operate.
It uses operational amplifiers that provide high input resistance and low capacitance, allowing it to handle higher-frequency signals effectively.
However, the operational amplifier also restricts the voltage range the probe can detect.
Compared to passive probes, active probes can only handle a smaller maximum voltage swing.
Active probes have extra connections besides the standard BNC connector, and each manufacturer has its way of powering these probes.
This means you need to choose a probe that matches your oscilloscope’s specifications.
You can find more information and simulations about active probes here.
Differential probes are a specific type of active probe.
They measure signals without grounding the oscilloscope, which helps prevent short circuits.
However, they have a limited useful voltage range and generally cost more than passive probes.
Passive Probe
A passive probe is the simplest type of probe available.
It is affordable, doesn’t use any active components, and doesn’t need a power supply.
A passive probe typically has a resistor and a capacitor connected in parallel, and it looks like the examples shown above.
Even though it has a simple design, a passive probe can measure higher frequency signals better than using just BNC cables.
However, passive probes aren’t ideal for very high frequencies.
They usually have a capacitance between 10 and 30 picofarads (pF), which limits the range of frequencies they can measure effectively.
At high megahertz (MHz) or gigahertz (GHz) frequencies, the signal might get weaker, and since you can’t know exactly how much the signal is reduced, it’s hard to tell if you’re seeing measurement errors or the actual signal.
You can find a helpful video on the EEVBlog that explains how probe impedance affects bandwidth, especially with probes that can switch between 10:1 and 1:1 settings.
Probe Compensation
You need to adjust probes for proper compensation because the capacitance in the cable and oscilloscope can change.
Probes usually have a trim capacitor, located at the tip or in the BNC connector, to handle this.
To compensate for the probe, you measure a square wave signal provided by the oscilloscope using the probe.
Then, you turn the trim capacitor knob until the signal looks perfectly square.
This adjustment ensures the probe responds correctly across frequencies and provides accurate, undistorted readings.
You can find more detailed instructions, explanations, and simulations for probe compensation on this page.