Low Phase Noise Multiple Frequency Microwave Source

Abstract

Use of high overtone bulk acoustic resonators (HBAR) has made possible development of low phase noise multiple frequency microwave sources operating directly at microwave frequencies. Most recently a low noise source has been developed at L-band which provides signals at approximately 5-MHz intervals and achieves phase noise suppression equivalent to sources based on low frequency quartz crystal stabilization and multiplication. The direct microwave HBAR multiple frequency source requires a fraction of the hardware required to achieve the same phase noise suppression for multiple microwave frequency operation by any other means. Stabilization is based on exploiting the high overtone resonance of crystals such as YAG, sapphire, lithium niobate or lithium tantalate for which intrinsic loss indicates a potential Q approximately ten times that of quartz. Resonators of this type have been fabricated as high as 10 GHz. For the 1.5-GHz band compressional mode, loaded Q's of over 50,000 have been achieved in several samples. The resonators consist of deposited film transducers on the resonator crystal. Single reverberation is confined to the region under the transducer. The crystal is mounted rigidly to its housing which minimizes effects of external vibration on frequency stability. Measured vibration sensitivity is described in a companion paper. The source consists of a low noise voltage controlled oscillator (VCO) stabilized in an automatic frequency control (AFC) loop. The HBAR is the frequency determining element in the AFC discriminator. Measured phase noise agrees well with predicted performance. Details of noise performance will be provided in the paper. Multiple frequency operation can easily be obtained with the addition of digital control circuits to preposition the VC0 on any of the HBAR's responses. Thus, a low phase noise electronically controlled multiple frequency source is obtained with minimal hardware.