Michael M. Driscoll

Reduction of quartz crystal oscillator flicker-of-frequency and white phase noise (floor) levels and acceleration sensitivity via use of multiple resonators

Through the use of N series-connected quartz crystal resonators in an oscillator circuit, a 10 log N reduction in both flicker-of-frequency noise and white phase-noise (floor) levels has been demonstrated. The reduction in flicker noise occurs as a result of the uncorrelated short-term frequency instability in each of the resonators, and the reduction in noise floor level is a simple result of the increase in net, allowable crystal drive level. This technique has been used in 40-, 80-, and 100-MHz AT-, BT-, and SC-cut crystal oscillators using low flicker-of-phase noise modular amplifier sustaining stages, and four series connected crystals. Total (four crystal) power dissipations of up to 30 mW have been utilized. State-of-the-art, flicker-of-frequency noise levels have been obtained with noise-floor levels (80 MHz) as low as -180 dBc/Hz. Four- to five-fold reduction in acceleration sensitivities has been determined. >

Spectral performance of sapphire dielectric resonator-controlled oscillators operating in the 80 K to 275 K temperature range

This paper reports on the phase noise performance obtained for X-band oscillators using cooled, sapphire dielectric resonators as the frequency-determining element. We report on results obtained using: (1) a TE-cooled, high-order mode resonator purchased from Poseidon Industries exhibiting a loaded Q of 140,000 at an operating temperature of 275 K, and (2) a low-order (TE02) mode resonator fabricated at Westinghouse exhibiting a loaded Q of 350000 at an operating temperature of 77 K. The oscillator sustaining stage designs incorporate GaAs amplifier flicker-of-phase noise feedback reduction techniques as well as a technique that avoids the need for X-band signal amplification altogether. Sustaining stage open loop, flicker of phase noise levels obtained are typically 20 dB below those normally exhibited by GaAs X-band amplifiers.

Extremely low phase noise UHF oscillators utilizing high-overtone, bulk-acoustic resonators

High-overtone, bulk acoustic resonators (HBAR) have been designed that exhibit 9-dB insertion loss and loaded Q values of 80000 at 640 MHz with out-of-phase resonances occurring every 2.5 MHz. These resonators have been used as ovenized frequency-control elements in very low phase noise oscillators. The oscillator sustaining stage circuitry incorporates low-1/f noise modular RF amplifiers, Schottky-diode ALC, and a miniature 2-pole helical filter for suppression of HBAR adjacent resonant responses. Measurement of oscillator output signal flicker-of-frequency noise confirms that state-of-the-art levels of short-term frequency stability have been obtained. Sustaining stage circuit contribution to resulting oscillator flicker-of-frequency noise is 7-10 dB below that due to the resonators themselves. At 16-dBm resonator drive, an oscillator output signal white phase noise floor level of -175 dBc/Hz is achieved. >

Frequency stability of high-overtone bulk-acoustic resonators

The results of phase noise measurement for high-overtone bulk-acoustic resonators (HBARs) for use in high-performance oscillators, operating at 640 MHz with insertion losses of 10-15 dB and unmatched Qs greater than 110 K are reported. Noise measurements made on these resonators with input drive levels of 16 dBm have shown self-noise levels of S/sub y/(f=100 Hz)=8.0*10/sup -26/ for 1/f noise which represents state-of-the-art for a UHF resonator.<<ETX>>

Measured vs. volume model-predicted flicker-of-frequency instability in VHF quartz crystal resonators

Both passive, in-bridge and in-oscillator measurements for AT, SC, and BT-cut, overtone-mode resonators operating at 40, 80, 100, and 160 MHz have been performed. Measurements were made using four-crystal oscillators as low-noise reference signal generators. The data indicate poor agreement between measured and mode-predicted resonator stability, especially for fifth-overtone AT and SC-cut resonators operating at 40 and 160 MHz. Measured data were also compared to predictions using an alternative model proposed by T. Parker (1985). Using the Parker model, relatively poor agreement was found for the case of BT-cut resonators. The data suggest that further refinement of resonator short-term stability models is necessary.<<ETX>>

Two-Stage Self-Limiting Series Mode Type Quartz-Crystal Oscillator Exhibiting Improved Short-Term Frequency Stability

A simplified model of the transistor sustaining stage employed in common quartz-crystal oscillators is presented. Examination of the model, including associated noise sources, provides an explanation for general differences observed in the output-frequency spectra of several types of widely used self-limiting crystal oscillator circuits. A self-limiting quartz-crystal oscillator circuit configuration is described that has been specifically designed to exhibit simultaneously each of the three important circuit characteristics necessary for improved oscillator short-term frequency/phase stability: large value of oscillator resonator loaded Q, adequate suppression of 1/f flicker-of-phase type noise, and improvement in oscillator ultimate signal-to-noise ratio. Several models of the oscillator circuit have been constructed employing high quality third overtone 5-MHz AT- and BT-cut quartz resonators. Measurement of oscillator short-term frequency stability using conventional phase lock and sampling techniques confirm attainment of substantial improvement in oscillator short-term frequency stability when compared to conventional self-limiting oscillator circuits.

Oscillator AM-to-FM noise conversion due to the dynamic frequency-drive sensitivity of the crystal resonator

An analysis has been made of the potential effects of AM-to-FM noise conversion in quartz crystal oscillators as a result of the dynamic frequency-drive sensitivity of the crystal resonator. The analysis indicates that it is quite possible for the FM noise resulting from AM-to-FM conversion in the crystal resonator to equal or exceed that due to the conversion of oscillator open loop phase noise to closed loop frequency noise. This is especially true in oscillators designed to exhibit low white phase noise (floor) levels by operating the crystal resonator at relatively high drive level. In addition, the analysis indicates that, for the same drive sensitivity, the relative degradation in FM noise level due to this effect is more severe at lower relative oscillator operating frequencies.

Vibration-Induced Phase Noise: It Isn't Just About the Oscillator

Very often, actual, measured levels of vibration-induced phase noise in signal generation equipment are significantly higher than predicted. In many cases, this is because the frequency sensitivity of the oscillator or oscillator resonator to vibration was assumed to be the sole source of the degradation. As improvements in oscillator output signal static phase noise and vibration sensitivity have been realized, the effects of vibration in non-oscillator components and assemblies have become more dominant and cannot be ignored. Primary contributors to vibration-induced signal phase modulation include coaxial cables and cable connectors, narrowband filters, and enclosure mechanical resonances and non-linearities. Accurate measurement of vibration-induced, signal spectral degradation is often difficult due to the influence of both the measurement environment and test apparatus. In addition, isolating and eliminating the cause of out-of-spec hardware performance is time consuming and expensive. This paper will describe potential sources of vibration-induced signal spectral degradation and methods for obtaining and verifying adequately low vibration sensitivity in non-oscillator hardware.

High-overtone, bulk acoustic resonator frequency stability improvements

In an effort to counter the effects of both moderate-term and long-term frequency instability associated with the (-30 PPM//spl deg/C) frequency-temperature coefficient in high overtone, bulk acoustic resonators (HBAR), AFC loop stabilization of low-noise, 640 MHz HBAR oscillators has been demonstrated. A thermoelectric (TE) cooler is used as the resonator frequency tuning element. The TE cooler approach provides a means of obtaining near-equal device heating and cooling time-response characteristics, allowing stable loop operation. Using this technique, long-term, fractional frequency stability improvement to five parts in 10/sup 7/ has been demonstrated. Prototype resonators have been fabricated on thicker (700 ns delay) YAG crystals and evaluated in oscillators operating at both 320 MHz and 640 MHz that exhibit flicker-of-frequency noise comparable, on a Sy(f) basis, to that reported for ultra-low noise, multiple crystal oscillators operating in the 80-100 MHz range.<<ETX>>

A SAWR oscillator vibration sensitivity and phase noise reduction technique using multiple resonators and RF outputs

A technique previously described by the author for reduction of quartz crystal oscillator vibration sensitivity and phase noise has been modified and applied to the design of a UHF, SAWR oscillator. Use of four SAW resonators mounted in an in-plane orientation provides 3-axis vibration sensitivity cancellation. Because of variations in individual SAWR vibration sensitivity vector magnitude and direction, the cancellation is non-exact. However, a 14:1 reduction in vibration sensitivity has been achieved using resonators having fairly uniform individual sensitivities in the range 44/spl times/10/sup -9/ to 55/spl times/10/sup -9/ per g. The novel oscillator loop circuitry consists of cascaded stages of SAWR/modular amplifier/power divider combinations, providing multiple, per-stage RF output signals whose noise phase floors are uncorrelated as a result of individual SAW resonator frequency selectivity. Thus, output signal amplitude (power combiner) or frequency (mixer) summation of M oscillator outputs yields a 10logM noise floor improvement. A minimum, 10logN improvement in flicker-of-frequency noise is obtained via use of N resonators due to the uncorrelated nature of the individual resonator frequency instabilities and N times increase in oscillator loop group delay. Phase noise floor levels on the order of -185 dBc/Hz and flicker-of-frequency noise levels given by f(100 Hz)=-124 dBc/Hz have been demonstrated in an oscillator using 320 MHz SAW resonators manufactured by SAWTEK, Inc., using the technique (with M=N=4), which is also applicable to oscillators incorporating other types of resonators.