Oscilloscope Passive Probe (Full Details)
When using an oscilloscope to measure and analyze electronic signals, the probe you choose plays a crucial role in the accuracy and quality of your measurements.
One of the most common types of probes is the passive probe.
These probes are widely used because they are reliable, easy to use, and well-suited for a variety of general-purpose applications.
What is a Passive Probe?
A passive probe is a type of oscilloscope probe that doesn’t require an external power source to function.
It relies on passive components, such as resistors, capacitors, and a simple coaxial cable, to connect the oscilloscope to the circuit you’re testing.
The simplicity of passive probes makes them durable and easy to maintain, which is why they are the go-to choice for many engineers and technicians.
How Does a Passive Probe Work?
A passive probe works by directly connecting the oscilloscope to the point in the circuit where you want to measure the signal.
It consists of a probe tip that you place on the test point, a ground clip that connects to the circuit ground, and a cable that carries the signal to the oscilloscope.
Inside the passive probe, resistors and capacitors are used to match the impedance between the oscilloscope and the circuit.
This matching is crucial because it minimizes signal reflection and distortion, ensuring that the signal you see on the oscilloscope screen is an accurate representation of the actual circuit behavior.
Types of Passive Probes
There are two main types of passive probes:
1x Passive Probes
- Direct Measurement: A 1x passive probe does not attenuate the signal, meaning it directly passes the signal from the circuit to the oscilloscope without any scaling. This type of probe is ideal for low-frequency and low-voltage measurements where signal integrity is critical.
- Low Input Resistance: The 1x probe typically has a low input resistance, usually around 1 MΩ, which can affect the circuit being tested if the probe is not used carefully.
10x Passive Probes
- Signal Attenuation: A 10x passive probe attenuates the signal by a factor of 10, meaning it reduces the signal amplitude before sending it to the oscilloscope. This attenuation helps protect the oscilloscope from high-voltage signals and allows for measurements in higher-voltage circuits.
- High Input Resistance: The 10x probe has a higher input resistance, typically around 10 MΩ, which reduces the load on the circuit and minimizes its impact on the signal.
Oscilloscope Passive Probe: What is it good for?
You can connect signals directly to an oscilloscope using a BNC cable and the right adapters.
But why do you need a probe? To explain, let’s look at a simple circuit diagram involving an oscilloscope, cable, probe, and voltage signal.
This diagram was created using the free simulation software LTSpice, which works on both Windows and MacOS.
The bottom part of the diagram shows the circuit setup for the simulation.
On the right side, the oscilloscope is represented by a 1 Megaohm resistor and a 20 pF capacitor.
A 100 pF capacitor, connected in parallel, represents the BNC cable.
On the left side, there’s a voltage source with a 1V sinusoidal wave and a 1kOhm resistor in series.
This setup limits the current flow to 1mA, simulating a real-world load on the circuit.
In the top part of the diagram, a frequency response is simulated.
It shows the voltage at the oscilloscope’s input, across capacitor C1, as the frequency of the input voltage varies from 1 Hz to 100 MHz.
Up to 100 kHz, the input voltage at the oscilloscope stays constant at 1V.
However, at higher frequencies, the voltage drops quickly.
At 10 MHz, the voltage falls below 0.2V, and by 100 MHz, it becomes nearly undetectable.
The dotted line indicates a phase shift in the signal at 1 MHz, showing a 45° difference from the input phase.
Passive probe with a divider ratio of 10:1
When you use a passive oscilloscope probe, a resistor (R2) and a capacitor (C4) are added to the circuit.
These components, with values of 9 Megaohms and 13.33 pF, represent a properly adjusted probe with a 10:1 divider ratio.
This means the input signal displayed on the oscilloscope is reduced by a factor of 10, so the amplitude appears as approximately 100mV.
When you check the simulation results, you can see the difference in how the signal responds to different frequencies.
The signal amplitude remains the same up to 1 MHz. At 10 MHz, the amplitude drops slightly to about 81mV, and you can still measure frequencies up to 100 MHz.
At 1 MHz, there is only a slight phase shift.
This shows that the passive 10:1 probe has a frequency bandwidth about ten times greater than that of a BNC cable, although this comes at the cost of reducing the signal amplitude by a factor of 10.
Most oscilloscopes are capable of accurately measuring signals in the millivolt and microvolt range, so this reduction in signal amplitude is often a good trade-off for better frequency bandwidth.
Advantages of Passive Probes
Passive probes have several strengths that contribute to their widespread use.
- One of the main advantages is their simplicity. They are easy to use and do not require any external power, making them straightforward for both beginners and experienced users.
- Additionally, passive probes are durable because they lack active components, which reduces the chances of damage and makes them more robust for everyday use.
- Another benefit is their cost-effectiveness. Passive probes are generally less expensive than active probes, making them a budget-friendly choice for many applications.
- Also, passive probes offer wide compatibility with most oscilloscopes, allowing them to be used in a variety of testing scenarios.
Disadvantages of Passive Probes
Despite their many advantages, passive probes also have some limitations.
A. One of the main drawbacks is their lower bandwidth compared to active probes, which can limit their effectiveness in high-frequency applications.
B. Another issue is signal loading, where the input resistance and capacitance of passive probes can load down the circuit, potentially altering the signal being measured, particularly in high-impedance circuits.
C. Additionally, while the attenuation feature in 10x probes is useful for measuring high-voltage signals, it also reduces the signal amplitude, which can be a disadvantage in situations where you need to measure low-level signals.
Conclusion
Oscilloscope passive probes are essential tools in any electronics lab.
They provide a simple, reliable, and cost-effective way to measure and analyze signals in a variety of circuits.
Whether you’re dealing with low-frequency signals, general-purpose measurements, or high-voltage circuits, a passive probe is often the right tool for the job.
Understanding how passive probes work and when to use them can help you make more accurate measurements and get the most out of your oscilloscope.