1.25-MHz attenuation measurement system

A system has been developed to make highly accurate measurements of nominally 6-dB increments of attenuation at 1.25 MHz. Initial experiments indicate a typical systematic error of 5 μB (1 μB = 0.00001 dB) with a resolution of 1 μB. A special linearity measurement system (LMS) using NBS-constructed linear tuned hybrids and power detectors has been used to determine the nonlinearity of a tuned 1.25-MHz power detector. This detector utilizes a single thermistor bead design with thermal isolation to obtain nearly linear tracking of a 4:1 change in input power. The nonlinear correction for this change, determined by the LMS, is on the order of 13 μB for the detector presently in use. This calibrated detector and another of similar design are used in the attenuation measurement system (AMS) to make power ratio measurements to determine the change in attenuation of the device under test (DUT). It is anticipated that changes of approximately 6 dB can be measured with initial insertion loss of up to 100 dB with an accuracy of 0.001 dB. A special design consideration will be required for units used for calibration in order to keep mismatch errors from significantly degrading the accuracies estimated above.