Abstract: An electron paramagnetic resonance (EPR) spectrometer employing a reference arm microwave bridge provides efficient, simultaneous generation of orthogonal I and Q signals. The simultaneous generation is enabled through the use of a field effect transistor (FET) microwave amplifier in the signal arm feeding a balanced quadrature mixer. The I and Q signals are then combined to produce a signal suitable for use as a discriminator in an automatic frequency control (AFC) circuit. An important characteristic of the discriminator signal is that it is independent of the relative phase angle .alpha. between the reference arm and signal arm of the bridge. In a further embodiment, an apparatus is disclosed which computes .alpha. from the I and Q signals, and uses the computed value for .alpha. to extract pure absorption and pure dispersion information from the I and Q signals regardless of the setting of .alpha. in the bridge.