A high-frequency calibration method and circuit including a dual path step attenuator. A calibration system is provided having a switch between the user signal input and the instrument input and an amplifier between the calibration signal input and the instrument input. The amplifier provides signal conditioning. The output of the amplifier is connected to the instrument input through a switch and a resistor, the resistor isolating the switch from the instrument input so as effectively to prevent degradation of the user input signal. A sense amplifier provides a calibration signal output indicative of the input impedance of the instrument input in response to a known stimulus. The input of the sense amplifier is isolated from the instrument input by a resistor. A current source provides a known stimulus to the instrument input to measure input impedance. A step attenuator is provided having an attenuated path and an unattenuated path. The signal input may be switched from the attenuated path to the unattenuated path. A compensation circuit is provided in the attenuated path to compensate for the impedances of switches used to select the amount of attenuation. The calibration input amplifier and sense amplifier are connected to the instrument input via the unattenuated path of the step attenuator. When the signal input is switched to the unattenuated path, the calibration input is switched to the attenuated path, and when the signal input is switched to the attenuated path, the calibration input is switched to the unattenuated path.