If you're familiar with an oscilloscope, you might have wondered: why does it offer input impedance options like 1MΩ and 50Ω? How do you decide which one to use?
Understanding the origin of 50Ω starts with the concept of transmission lines. Electrical signals travel through these lines as electromagnetic waves. When the length of the line becomes comparable to the wavelength of the signal, wave behavior must be considered. For example, when a narrow pulse is sent through a 100-meter coaxial cable and measured at the source, you can clearly see both incident and reflected waves.
When the incident and reflected waves combine, the square wave may appear distorted, as shown in the next image.
So, how do we prevent signal reflection?
Just like light reflects off water, electrical signals reflect when they encounter a change in medium. To avoid this, uniform transmission lines are used—like PCB microstrip lines or coaxial cables. These maintain consistent geometry and material, ensuring no reflections occur along the line.
But what happens at the end of the line? If the signal’s impedance changes, reflection occurs. To prevent that, the load should match the characteristic impedance of the line. This is called terminal matching, where a resistor is placed at the end to absorb the signal and eliminate reflections.
Now, why is 50Ω so common?
The characteristic impedance affects power transfer, loss, and crosstalk. It also influences manufacturing costs. 50Ω was chosen as a balance between performance and cost, making it the standard for high-speed systems like PCIe. Most modern high-speed signals operate on a 50Ω system.
This is why oscilloscopes have a 50Ω setting—to match the transmission line and minimize reflections during measurement.
But then, why is there a 1MΩ option?
This brings us to the load effect. When you connect an oscilloscope, its input impedance is effectively in parallel with the circuit under test. A 1MΩ input is equivalent to a 1MΩ resistor in parallel with a small capacitor (around 10–20 pF). At low frequencies, the resistor dominates, but at higher frequencies, the capacitance becomes significant.
This can cause distortion, especially if the circuit has a low output resistance. For example, at 200 MHz, a 25Ω source with a 1MΩ oscilloscope can result in about -1 dB of attenuation. That’s why high-frequency measurements often use 10:1 passive probes, which have lower capacitance (about 9 pF) compared to 1:1 probes (60 pF).
What if you use the 50Ω setting instead?
In that case, the signal will be attenuated from DC onwards. For a 25Ω source, the attenuation could exceed -3 dB, distorting the signal. So, if your signal source is unbuffered (like a signal generator), and you’re using a 50Ω coaxial cable, the 50Ω setting is necessary to match the line and reduce reflections.
So, when should you use 1MΩ or 50Ω?
Here are some general guidelines:
1. Use 50Ω when measuring unbuffered sources (like a signal generator) with a 50Ω coaxial cable.
2. Use 1MΩ when measuring onboard signals that already have proper termination.
3. Passive probes usually require 1MΩ mode, while active probes may need 50Ω for high frequencies.
4. Be mindful of probe bandwidth—1:1 probes are limited to around 6 MHz, so use 10:1 probes for higher frequencies.
Choosing the right input impedance is key to accurate and reliable oscilloscope measurements. Understanding the trade-offs between 1MΩ and 50Ω helps ensure that your signal remains clean and undistorted.
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