T.E. Parker

Characteristics and Sources of Phase Noise in Stable Oscillators

The frequency stability of an oscillator is a very important characteristic for many applications. Yet the causes and sources of some basic types of noise are poorly understood, particularly for close-to-carrier noise. A review is presented in this paper of the present state of knowledge about the sources and characteristics of frequency fluctuations in stable oscillators using quartz acoustic resonators (BAW and SAW) or dielectric resonators as the high Q frequency stabilizing element. A brief discussion of the various parameters used to quantify random frequency fluctuations is presented along with the relative merits of open- and closed-loop phase noise measurements. Phase noise in stable oscillators usually arises from additive voltage fluctuations and direct parameter modulation processes. Additive

Extremely low-phase-noise SAW resonators and oscillators: design and performance

The authors describe prototype low-noise SAW (surface acoustic wave) resonator oscillators which have demonstrated state-of-the-art phase-noise performance not only at their fundamental operating frequencies in the 400- to 600-MHz range but also after 16* frequency multiplication to X-band as well. SAW resonator designs with overmoded cavities, very wide apertures, and dual apertures, as well as modified fabrication techniques, have been used to realize an overall reduction in an oscillators phase-noise spectrum, i.e. white phi M, flicker FM, and random-walk FM. The S resonators can typically handle incident RF power in excess of +20 dBm, a key requirement to achieving an extremely low oscillator-phase-noise floor. A novel burn-in procedure at relatively high incident-RF-power levels (>27 dBm) was used to reduce both the flicker FM and random-walk FM phase-noise levels. Using these various techniques, a 5- to 15-dB improvement in the overall phase-noise spectrum for several prototype oscillators was demonstrated.<<ETX>>

Current Developments in SAW Oscillator Stability

The results reported were obtained with two port delay lines and resonators used in a simple feedback oscillator. The feedback oscillator employed inherently operates with the amplifier in a saturated condition and, therefore, the AM noise is suppressed. Consequently the dominant noise is FM. Generally, it can be concluded that for very narrow-band, or fixed-frequency applications, the resonator-type oscillator will give the best noise performance. For applications where tunability and linearity are important, the delay-line-type oscillator may give the best performance. There have been no significant improvements in oscillator temperature stability. The only two demonstrated materials for temperature stable SAW oscillators are ST-cut quartz and the SiO2/LiTaO3 overlay structure. Aging tests have been going on for the past two-and-a-half years and it has become obvious that the observed aging rates are largely related to cleaning and packaging.

Residual phase noise measurements of VHF, UHF, and microwave components

The results of residual phase noise measurements on a number of VHF, UHF, and microwave amplifiers, both silicon (Si) bipolar junction transistor (BJT) and gallium arsenide (GaAs) field effect transistor (FET) based, electronic phase shifters, frequency dividers and multipliers, etc., which are commonly used in a wide variety of frequency source and synthesizer applications are presented. The measurement technique has also been used to evaluate feedback oscillator components, such as the loop and buffer amplifiers, which can play important roles in determining an oscillators output phase noise spectrum (often in very subtle ways). While some information has previously been published related to component residual phase noise properties, it generally focused on the flicker noise levels of the devices under test, for carrier offset frequencies less than 10 kHz. The work reported herein makes use of an extremely low noise, 500 MHz surface acoustic wave resonator oscillator (SAWRO) test source for residual phase noise measurements, both close-to-and far-from-the-carrier. Using this SAWRO-based test source at 500 MHz, we have been able to achieve a measurement system phase noise floor of -184 dBc/Hz, or better, for carrier offset frequencies greater than 10 kHz, and a system flicker phase noise floor of -150 dBc/Hz, or better, at 1 Hz carrier offset. The paper discusses the results of detailed residual phase noise measurements performed on a number of components using this overall system configuration. Several interesting observations related to the residual phase noise properties of moderate to high power RF amplifiers, i.e., amplifiers with 1 dB gain compression points in the range of +20 to +33 dBm, are highlighted. >

Review of SAW oscillator performance

State-of-the-art phase noise performance has been demonstrated for both surface acoustic wave (SAW) resonator and delay line stabilized oscillators. The same basic feedback-loop oscillator design philosophy was applied in each case in order to achieve these results. This paper reviews the design, fabrication, component selection, and performance for an extremely low-noise SAW resonator hybrid circuit oscillator. Finally, several recent results including the flicker noise of SAW resonator devices and the vibration sensitivity of all quartz package SAW resonators are discussed in the context of potential SAW oscillator performance enhancements

Spectral purity of acoustic resonator oscillators

The spectral purity of state-of-the-art oscillators that use acoustic resonators in the frequency range of 5 to 500 MHz is surveyed, with the primary emphasis placed on oscillators that use bulk acoustic wave and surface acoustic wave devices on single crystal quartz. The phase noise characteristics of low-noise oscillators are analyzed in the context of Leesons model of feedback oscillator phase noise, and particular attention is given to flicker frequency noise levels and white phase noise floors. Procedures and methods for reducing phase noise levels are discussed along with an analysis of reported attempts to obtain improved spectral purity.<<ETX>>

1/f noise in etched groove surface acoustic wave (SAW) resonators

Measurements of 1/f (or flicker) frequency fluctuations in SAW resonators fabricated with etched groove reflectors on single crystal quartz have shown that the observed noise levels vary inversely with device size. These measurements were made on sixteen 450 MHz resonators of four different sizes. The 1/f noise levels were also evaluated on twenty-eight other SAW resonators ranging in frequency from 401 to 915 MHz. This additional data provides valuable information on the dependence of the flicker noise levels on resonator frequency. A model based an localized, independent velocity fluctuations in the quartz is proposed which correctly fits the observed size and frequency dependence of the measured 1/f noise levels. This model suggests that the velocity fluctuations originate in small regions (much less than /spl sim/5 /spl mu/m in diameter) randomly distributed throughout the quartz with an average separation of about 5 /spl mu/m between independent (incoherent) sources. The magnitude of the localized fractional velocity fluctuations, /spl Delta/v/v, averaged over a 5 micron cube is on the order of 1/spl times/10/sup -9/.<<ETX>>

Analysis of Aging Data on SAW Oscillators

Test oscillators including delay lines and resonators in the 300-400 MHz range have been investigated with reference to the long-term stability (aging) characteristics. All devices were fabricated on rotated Y-cut quartz plates (40 deg), and either gold or platinum metallization was used; all packages were high-quality hermetic enclosures, and the mounting was strictly mechanical, with no organics or silicone rubbers used. It is shown that drift of less than 2 ppm in the first year can be obtained on a significant fraction of the devices when reasonably clean packages are used. The data also suggest that the transducer metallization (at least for aluminum) is very likely the source of the relaxation mechanism that causes the frequency drift.

Design techniques for achieving state-of-the-art oscillator performance

The design, fabrication, component selection, and performance of extremely low noise surface acoustic wave (SAW) resonator and delay line oscillators, as well as an extremely low noise L-band dielectric resonator (DR) oscillator are described. To illustrate the design procedure, several specific examples are presented. It is shown that the basic feedback-loop oscillator configuration is well suited to high performance oscillator design in the VHF, U HF and microwave frequency ranges, based upon the incorporation of a two-port frequency stabilizing element. State-of-the-art performance is realized for SAW resonator and delay line based oscillators, as well as for a very simple L-band dielectric resonator oscillator design.<<ETX>>

SAW oscillators with low vibration sensitivity

Surface acoustic wave (SAW) oscillators with vibration sensitivities well below 1*10/sup -9/ fractional change in frequency per g have been demonstrated. Data are presented which indicate the need for a rigid support structure under an inherently low vibration sensitivity all quartz package (AQP) SAW device to achieve low vibration sensitivity at the oscillator level. An alumina stiffener supporting a hybrid circuit SAW oscillator provides this essential rigidity. Vibration sensitivity data for 26 oscillators in a 1.22-inch-square hybrid circuit package mounted on a 0.30-inch-thick alumina stiffener yielded an average value for the magnitude of the vibration sensitivity vector of 3.3*10/sup -10//g at a vibration frequency of 500 Hz. It also appears that some degree of vibration isolation at the AQP SAW device level may be achieved with the proper choice of material used to mount the AQP SAW device into the hybrid circuit package.<<ETX>>