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.
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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