Precise frequency and phase control of high efficiency LSA oscillators

Gallium Arsenide LSA devices will be useful in phased arrays only after precise and reliable control can be obtained on both the output frequency and phase of the device. In this work, detailed measurements have been made of the frequency and phase locking properties of pulsed LSA S-band oscillators in a variety of circuit configurations. LSA diode parameters, along with circuit and bias pulse considerations are presented for optimum locking performance. Data on best locking gain, electronic phase control, and optimum spectral output are given. Previous LSA spectra have shown large amounts of both AM and FM variations. This is due mainly to the significant electronic tuning of these devices with bias voltage. Typical side lobe reduction is often only three to six DB below the main lobe, with noticeable asymmetry. The main lobe spectral width of a 400 nanosecond pulse was sometimes as much as 30 megahertz. With proper circuit and pulse design, symmetrical, 5 megahertz wide spectra were possible, with the side lobe reduction down to -13 DB of the main lobe, the theoretical limit. Well-behaved locking gains of over 20 DB have been observed, and the effects of circuit Q and pulse impedance on the gain have been measured. A cascaded locking experiment was performed, whereby a small amplitude CW signal was used to frequency lock a medium power LSA device, which was then used to lock a high power (100-200 watt) pulsed LSA oscillator. A novel multi-axis radial circuit, with control of the load on the device at both the fundamental and second harmonic, was used in these experiments to optimize power and efficiency to values up to 0.9 of that predicted by theory. This for the first time demonstrates the feasibility of utilizing LSA devices in a phased-array scheme.