Reduction of Close-to-Carrier Phase Noise in Surface Acoustic Wave Resonators

A technique is presented which consistently reduces the close-to-carrier phase noise inherent to SAW resonators. By the application of a high power RF drive signal at the SAWR resonant frequency, phase noise reductions ranging from 0 to 26 dB have been achieved in over 40 resonators of various designs. Using this process. the resulting average SAW resonator close-to-carrier phase noise levels are now comparable to those of the other electronic components used in the construction of SAW resonator stabilized oscillators. The noise reduction technique, its ramifications, and the results of some experiments which were performed in order to characterize this process, are presented. INTRODUCTION Surface acoustic wave resonator (SAWR) oscillators are now accepted as reliable, low-noise signal sources for use in modern radar and communication systems. Recent research indicates that the SAWR is usually the dominant source of close-tocarrier phase noise in both the flicker frequency [1,2] and random walk [31 sections of the noise spectrum, for oscillators utilizing high performance, commercially available silicon bipolar transistor based electronics. This paper presents a technique which has consistently reduced the close-bcarrier phase noise levels in SAW resonators. It involves driving the SAWR with a higher than normal incident RF power level for a short time period. This procedure has produced flicker noise reductions ranging from 0 to 26 dB in over forty SAW resonators of various frequencies, acoustic apertures, and metallization types, with an average noise reduction of 8 dB. Additionally, three 425 MHz SAW resonators exhibited 3 to 15 dB reductions in random walk FM noise, with an average decrease of 10 dB. All noise reductions occur without any significant changes in resonator parameters such as frequency, insertion loss, and quality factor. The measured SAWR open-loop flicker noise levels after this "burn-in'' procedure are now comparable to, or less than the typical close-to-carrier phase noise levels observed in state-of-the-art, commercial silicon bipolar transistor amplifiers. PROCEDURE The typical SAWRs discussed here are 350 to 810 MHz two-port resonators, fabricated on quartz with either aluminum or coppcr-doped aluminum transducers, and acoustic apertures ranging from 130 to 400 wavelengths. Standard in these device designs are 1 to 1.5 pm thick busbar metallizations and long effective cavity lengths (approximately 300-350 wavelengths) [4]. An important parameter to minimize is the phase noise noise inherent to the SAWR device. Due to varations during the fabrication process, there will always be a distribution of SAWR phase noise among like devices. The phase noise of these devices can be evaluated using a Hewlett-Packard 11740A Phase Noise Measurement System in an open-loop, or two-port measurement mode [l]. The distribution of resonator noise levels can be narrowed and the average level reduced significantly by the application of a "burn-in" process to each device. This involves applying a high power RF signal, usually in the range of +24 to +33 dBm, to the device for time periods ranging from 0090-560718710000-0043 $ I . OO 0 I987 IEEE several minutes to hours. The output side of the SAWR is terminated in 50 ohms during the "bum-in" period. The signal frequency is set to the resonant frequency of the device under the incident power conditions. This frequency was determined by connecting a power meter to the output side of the device and adjusting the frequency of the high power signal source for maximum transmission at the burn-in power level. These power levels will produce peak compressional stress levels of 1 to 4x108 N/m2 resulting in non-linear elastic effects within the device [5] and the power dissipated in the device can substantially raise the SAWR substrate temperature. Both of these power induced effects result in a shift of the resonant frequency, and a short time period will occur before this frequency will reach equilibrium. As will be discussed later, maximum noise reduction will only occur when the device is driven as close as possible (well within the 1 dB bandwidth) to its resonant frequency, so that the signal source should be carefully adjusted during the early stages of the run. The choice of maximum incident power is determined by the size of the acoustic aperture and the effective cavity length of the S A W . The level of stress developed within the device is inversely proportional to these parameters [61, and irreversible damage may occur (e.g., increased loss, permanent frequency shifts. and the increase of transverse modes) if the stress levels are set too high. Observations indicate that the minimum power level that can induce noise reduction is that which establishes a peak compressional stress level of about 1x108 N/m2. Stress levels below this value do not generally induce noise reduction in SAWRs whose initial open-loop phase noise levels at 1 Hz offset are in the range of -120 to -130 dBc/Hz, which are typical S A W phase noise levels. We do note that unusually noisy devices have shown improvement at slightly lower strcss levels (-5x10' N/m2). Overall Change in Flicker Noise