An X-Band Mixer Engineered for 77-K Operation

Summary Demands on the mixer were twofold. First, to optimizereceiversensitivity sincefront-endgainwas limited, themixerAnX-bandSi-diodesinglybalancedmixerdevelopedspecif- was required not only to downconvert the signal withoutically for cryogenic operation is presented. In order to reduce adversely affectingthe pre-amplifierbut also to minimize thethermal demands on a mechanical cooler, the mixer was signal degradation. Hence, low conversion loss, a low noisedesigned to operate with a minimum of local oscillator (LO) figure,andahighport-to-portisolationweredesirable. Second,power. That is, since the LO had to be cooled to reduce phase themixer had to minimize thermal demands on the cooler bynoise,it wasdesirable to minimizetheLO drive.Novelembed- operating witha starvedlocal oscillator (LO).Because theLOding circuit strategy wasresponsible for nearly theoreticalper- was inherently inefficient and had to be cooled in order toformance.The signal-matchingcircuitsimultaneouslyprovided reduce phase noise,minimizing therequired drive level of thea reactive termination to theimage, sum,andfirst, second,and mixer was tantamount to reducing the cooling power require-third LO harmonic frequencies.A conversionloss of 3.2dB at ment.For example,iftheLOis stabilizedwith asuperconduct-77 K with an LO drive of +1 dBm was measured. This loss ing resonator and has an efficiency of I0 percent and if theincluded IF filter, dc block, and hybrid coupler losses. Mixer mixer requires 10dBm of LO drive, then the cryocooler heatconversion loss is shown to be consistent with the theoretical load capacity must be increased by about 100 roW. Coolingperformance limit expected from theintrinsic diode. The rela- themixer increasestheoverall receiversensitivity andpermitstionship amongjunction capacitance, flat-bandpotential, and convenient integration of all the critical front-endfunctions.conversion loss is examined. Manyconventionalmixerswithcompetingperformancewouldseverely tax the cooler. Figure 1 shows the conversion lossperformance of a very good commercially available mixer