The input system of an oscilloscope contains a certain
amount of resistance shunted by capacitance. This is called
the input impedance of the oscilloscope. To the input
impedance of the oscilloscope is also added the capacity of
the test leads. This total shunting capacitance and
resistance, when connected across a high impedance, high
frequency, or broad band width circuit, may be sufficient to
detune the resonant circuit or to load the test circuit and
seriously distort the waveforms under observation.
For this reason a low capacity (high impedance)
probe is used to isolate the input of the oscilloscope from
the circuit under test when waveforms are to be observed in
any audio, video, sweep or sync circuit.