From the voltage measurement perspective, to minimize the impact of to be measured circuit, the oscilloscope can use a high input impedance of 1M ohm. However, the bandwidth of the high impedance circuit is susceptible to parasitic capacitance, so 1M ohm input impedance is widely used with measurements below 500M bandwidth. For higher frequency measurements, a 50 ohm transmission lead is usually used, so the 50 ohm match of the oscilloscope is mainly used for high frequency measurements.
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