The output noise of a granular NbN microbridge has been measured at frequencies near 25 MHz as a function of bias voltage, temperature, and rf bias at 70 GHz. It is generally greater than predicted by the resistively shunted junction model, but is significantly lower than has previously been observed in clean superconducting microbridges with comparableRIcproducts. An estimate of the performance of the device as a Josephson mixer at 70 GHz is made on the basis of these measurements. At an operating temperature of 10 K a double sideband mixer noise temperature sim;200 K, with conversion loss of sim;3 dB should be attainable. Using the noise as a probe of the average temperature within the microbridge, it is found that simple selfhyphen;heating is not a likely explanation for the disappearance of Josephson effects at high dc or microwave bias.
展开▼
