Characterisation of a GaAs MESFET oscillator at 4.2 K

A stripline X-band oscillator for use in Josephson voltage standards and potentiometers has been designed and constructed. The oscillator, comprising a packaged GaAs MESFET, has been tested at temperatures of 300 K and 4.2 K. The conversion efficiency, the power leveling capability, the frequency tuning, and the phase noise have been investigated. The power level can be varied over a range of 35 dB by tuning the drain-source voltage. A hysteretic behavior in the frequency versus the drain-source voltage characteristic has been observed when the drain-source voltage exceeds a device-dependent level. This effect is attributed to the trapping of hot electrons in the drain region. By varying the gate-source voltage, the frequency can be only tuned over a range of 0.8 MHz at 4.2 K. At a gate-source voltage of 0 V, higher harmonics are generated. A phase noise reduction of 22 dB at a frequency offset of 50 kHz from the carrier can be obtained by cooling to 4.2 K and a feedback network consisting of copper layers.<>