Insertion Loss Test Sets using Square Law Detectors

This paper discusses theory and errors, when measuring radio frequency attenuation, using imperfect square law detectors. The audio modulation of a radio frequency source is detected by the resistance modulation of a bolometer producing an audio frequency voltage. For ideal square law detectors, the variation of the audio frequency level in decibels, using a constant current source, is twice the decibel change of the radio frequency level. A theoretical analysis of the error due to deviation from square law is given. Experimental data regarding this error based on the static characteristic and independent experimental data relying upon differential rf measuements are furnished and are in agreement with the theory. Applying this error analysis to typical barretters, accuracies of .01 db are feasible for a measurment of a 20 db loss. A simple test set which has been reduced to practice is discussed. It permits rf loss measurements of 30 db with an accuracy of .1 db.