General Radio 1234 Standing Wave Meter.

Used for 1 kHz modulated standing wave measurements typically on UHF bridges and slotted line instruments. Sensitivity goes well below 1 µV into the handful of Nanovolts. The meter is indicating what the micro volt attenuator is indicating.

This SWR Instrument has +/-5% frequency centering, variable, bandwidth, fast/slow response damping, variable input impedance, expanded scales, and is very useful for attenuator calibration as well as ratio-metric impedance measurements, gain calibrations, and noise measurements.