Electron Spin Resonance Apparatus and Method

An electron spin resonance (ESR) detecting device having a null-detecting receiver is disclosed. An excitation signal is input to a resonator which holds a sample for ESR detection. A response signal from the resonator is coupled to a balance signal to form a detection signal received by a detector. The detector also receives a reference signal having a phase relationship with the excitation signal, whereby the detector's output is indicative of a difference between the detection signal and the reference signal. The detector's output is used in a feedback loop to generate an adjustment signal which causes the balance signal to substantially cancel the response signal. The adjustment signal can be used to measure resonance, while the input to the detector is minimised to reduce the effect of noise on measurements.