Introduction

An electronic instrument needs to be connected to whatever circuit or system is under test. Very wide bandwidth instruments, those used for signals above 100 MHz, usually have 50 Ω co-axial inputs. Instruments which accept signals of even higher frequency, in the microwave and optical range, will have waveguide or optical fibre inputs. The more conventional laboratory test equipment is usually supplied with some kind of probe input circuit which may be connected to the circuit under test.

The probes and input circuits used with oscilloscopes provide good examples of the techniques employed. These same probes and input circuits may, of course, be used in a variety of instruments: vector voltmeters, network analysers, spectrum analysers, and so on. Very high impedance probes are used for voltage measurement, while very low impedance sensor probes must be inserted for current measurement. These voltage and current probes may be passive or active, and both kinds will be considered in this chapter.

Voltage and current measurements are not the only ones that are called for in electronics. Measurements of incident, transmitted and reflected power may also be required. This is the approach often used for high frequency, wide-band circuits which work as part of a transmission line system; for example, a repeater amplifier in a cable television system. The input circuits needed for these power measurements are particularly interesting in that they may exhibit directional properties.