Oscilloscope 1MOhm VS. 50Ohm input resistance

[bittware]

oscilloscope input impedance

Hello experts,
Why are there two input resistance configs in oscilloscope? How to determine where I should apply which config? What are the points should be taken into consideration?

Thanks in advance!

Best regards,
bittware

 

[IanP]

50 ohm oscilloscope

Passive Probe for 50 Ohm Oscilloscopes
A 50 Ohm passive probe with an attenuation factor of X10 is useful for measuring high frequency signals generated by a relatively low source impedance. A 50 Ohm passive probe will load the circuit being measured with a 500 Ohm resistance and therefore should not be used to measure voltages with source impedance higher than a few ohms. Measuring digital signals and wide bandwidth analog signals from amplifiers and DACs are examples of situations where a 50 Ohm passive probe is appropriate.

Quote from:
"Oscilloscope probes"
https://www.pcinstruments.com/products/Probes.html

Regards,
IanP

 

[Kral]

oscilloscope 50 ohm

bittware,
In RF work, many circuits are designed to operate with transmission lines with a 50 Ohm characteristic impedence. Idealy this impedence is purely resistive. Providing a 50 Ohm input impedance at a 'scope input allows measurements without disturbing the impedance matching of the circuit under test. For example, passive filters that are designed for 50 Ohm source and load impedances will diplay drastically different characteristics when loaded with anything other than 50 Ohms.
Regards,
Kral

 

[analogocean]

oscilloscope input resistance

50 ohm is the one to use. Since its a purely real load it prevents any reflection thats possible. But this is a fact assuming that your output load is purely real too

 

[raybo]

impedance matching oscilloscope

you guys miss the boat 50 ohms is not resistance but the inpedance of the connector usualy A BNC therefore the probe is designed for inpedance matching of 50 ohms and a good probe is adjustable to achieve this a 1 megaohms is the dc resistance of the probe.

 

[Kral]

input resistance oscilloscope

It is true that most 'scope probes have an input resistance of either 1Meg or 10 Meg, depending on whether the probe is 1:1 or 10:1. However, there are probes available with 50 Ohms input resistance. The idea is to provide a probe that will load (terminate) a 50 Ohm transmission line with an impedance that is equal to the characteristic impeqance of the transmission line. With a properly terminated line (at both ends) there will be no waveform distortion; only a delay from input to output.
.
You mentioned the BNC connector. This connector is commonly used because it is designed to mechanically fit the typical coaxial cables that are used for 50 Ohm transmission lines. However, it should be noted that the SWR of the BNC connector is typically quite high, which means that its characteristic impedance can vary considerably from the ideal 50 Ohm value. For high quality termination, other connectors are available that more closely match the 50 Ohm characteristic impedance.
Regards,
Kral