an electric circuit testing equipment wherein signals are picked up from a selected point in the circuit by a probe unit capacitively coupled to the selected point and the signals picked up by the probe unit are conditioned by circuits which produce an output signal corresponding only to signals picked up by the unit from the selected point . the use of a capacitively coupled probe unit reduces the possibility of the unit affecting the operation of the circuit under test while the conditioning of the picked up signals enables the pick - up of unwanted signals to be tolerated . designs of probe unit to minimize pick - up of unwanted signals are also described .