Optically coupled probe for microwave near-field measurements

A probe capable of measuring both phase and magnitude of an RF field at frequencies up to 10 GHz is described. The probe is linked to its display unit by two optical fibres, thus eliminating field distortion effects that would occur if coaxial cables were used. Because of frequency limitations in optical-fibre components, the probe contains a frequency downconversion circuit powered by a small battery. This produces harmonics (up to the 1000th) of a 10 MHz reference oscillator; an appropriate harmonic produces a beat-frequency signal when mixed with the field signal sampled by a small dipole. The reference- and beat-frequency signals are sent down separate fibres to the display unit, where they are converted to phase and amplitude information