NOISE-ANALYSIS OF EPR SPECTROMETERS WITH CRYOGENIC MICROWAVE PREAMPLIFIERS

Studies are reported on the sensitivity of an EPR spectrometer utilizi ng a cryogenic preamplifier (0.5 dB noise figure, 35 K equivalent nois e temperature). Three cases are considered: (a) resonator matched in a CW experiment, (b) resonator Q spoiled by overcoupling, and (c) reson ator matched in pulse experiments. Contributions to the system noise b ecause of losses in the microwave bridge have been determined quantita tively: the equivalent noise temperature of the receiver chain rises f rom 35 to 100 K. It is suggested that improved system performance can be achieved by cooling major components of the microwave bridge. It is particularly advantageous to cool the load resistor at port 1 of the circulator. Measurements of source noise are reported. This noise cont ribution becomes increasingly apparent as the system noise is decrease d, but can be suppressed by use of loop-gap resonators. Analysis is ca rried out in a mixed engineering vocabulary of output noise temperatur e and equivalent noise temperature, which are temperatures referred to the output and input of the circuit, respectively. It is suggested th at this is an appropriate analytical approach when there are multiple paths that feed noise to the output of the circuit with different phys ical temperatures of the input load resistor for each path. (C) 1995 A cademic Press, Inc.