Impedance selection for oscilloscopes

Today, when I was doing a performance test experiment of a solid-state relay (SSR), I gave a square wave to the control end of the SSR and used an oscilloscope to observe the output signal. It should be a good square wave. However, when the oscilloscope selected an input impedance of 1MΩ, the waveform displayed on the oscilloscope screen was very strange. When the input impedance of the oscilloscope was switched to 50Ω, the observed waveform was a good square wave.
There is a signal point on the oscilloscope for calibrating the square wave waveform output. Connect the probe to it and level the square wave. Generally, we think that the impedance of the probe is matched. Of course, other such matches are not complete. But generally, the maximum bandwidth of the passive probe is 5~6 Mbps, which is enough. When it comes to active probes, 50 ohms are generally used, so the impedance matching problem is easy to solve. 

Ideally, testing the instrument under test should not affect its normal operation, and the measured value should be the same as when the test instrument is not connected. When connecting the instrument for measurement, the impact of impedance on the measurement accuracy must be considered. In order to ensure that the maximum power can be transmitted between the instruments, the impedance should be matched. If the impedance is a pure resistor, the input impedance should be equal to the output impedance. If the impedance contains reactance The components should make the input impedance of the load conjugate with the output impedance of the source, so that the maximum power can be transmitted. 

The impedance value of impedance matching is usually consistent with the characteristic impedance value of the transmission line used. For RF systems, 50Ω impedance is generally used. For high-impedance instruments, due to the existence of equivalent parallel capacitance, as the frequency increases, the parallel combination impedance gradually decreases, which will form a load on the circuit under test. For example, with a 1MΩ input impedance, when the frequency reaches 100MHz, the equivalent impedance is only about 100Ω. Therefore, high-bandwidth oscilloscopes generally use 50Ω input impedance, which can ensure the matching of the oscilloscope and the source end. However, when using a 50Ω input impedance, it must be considered that the load effect of the 50Ω input impedance is more obvious. At this time, it is best to use a low-capacitance active probe.