Microwave Journal

A Mobile Ultra-low Phase Noise Sapphire Oscillator

Introduction to a low noise fixed-frequency X-band sapphire cavity oscillator

January 1, 2002

Product Feature

A Mobile Ultra-low Phase Noise Sapphire Oscillator


Poseidon Scientific Instruments Pty Ltd.
Fremantle, Western Australia

Radar, communications, and test and measurement designers require ever-improving phase noise performance in their master oscillator. For mobile system success, ultra-low phase noise performance must be coupled with exceptional immunity to vibration and in a form suitable for compact environments. A ruggedized sapphire cavity oscillator has been introduced, offering microwave designers and system engineers an exceptionally low noise X-band source in a system-ready package.

The SBO oscillator - nicknamed "the Shoebox" - integrates two patented technologies developed by Poseidon Scientific Instruments and the University of Western Australia: a high Q sapphire loaded "whispering gallery mode" cavity resonator and a noise-reduction circuit. The technique was first perfected and is still used in the company's Sapphire Loaded Cavity Oscillator (SLCO) line of test instruments.

Table 1
Specified Phase Noise Performance

Offset Frequency
(Hz)

Phase Noise
(dBc/Hz)

100

-104

1 k

-138

10 k

-154

100 k

-160

Phase Noise and Jitter Performance

Fig. 1 The SBO-10.240-XPL oscillator's (a) phase noise and (b) amplitude noise.

The SBO oscillator is valuable to microwave engineers working in premium signal generation environments, where phase noise or jitter are critically important. The phase noise of the Shoebox is approximately 25 dB lower than the best reported commercially-available multiplied surface acoustic wave (SAW) oscillators. Its specified phase noise (offset from a 10.24 GHz carrier) is listed in Table 1 .

Figure 1 shows typical noise performance, including a virtually spur-free spectrum. Independent tests confirm that the single-side band phase noise approaches the thermal noise limit approximately 2 MHz from the X-band carrier.

In digital communications terminology, this equates to jitter of better than 6 femtoseconds over an integrated bandwidth of 10 Hz to 10 MHz. The sole contributing factor at larger integration bandwidths is the thermal floor.

Underlying Technologies

The patented resonator and noise-reduction technologies are the key to the Shoebox oscillator's performance. The resonator does not use any piezo-electric principle. Instead, it relies on the internally reflective nature of the sapphire to trap microwave energy and realize an unloaded Q of approximately 200,000 at room temperature. The sapphire is thermo-electrically cooled in order to maintain stability of approximately 0.1 ppm/°C over a 40° range.

The noise-reduction circuit is a real-time phase noise measuring system with good instantaneous bandwidth and noise floor. It optimizes the performance of the oscillator by reducing the phase noise contribution of active components which are otherwise fundamentally noisy.

When integrated in the Shoebox, the two technologies offer microwave engineers a fundamentally new signal generation scheme that allows restraints on traditional technologies to be broken.

Shoebox Features

The Shoebox exhibits exceptionally good vibration immunity of 2 x 10-10 /g in each axis. This performance was measured with no vibration isolation, and has been confirmed by independent laboratories. The immunity to external mechanical forces rivals the very best quartz oscillators.

The oscillator signal may be divided to multiple lower frequencies using the company's proprietary low noise regenerative divider (divide-by-two). This results in a phase noise improvement at each divide stage of almost 6 dB. By incorporating a divider chain, the user has access to multiple outputs simultaneously and can create a multi-frequency reference source with unprecedented phase noise performance.

The Shoebox is a fixed-frequency X-band oscillator that may be disciplined to a system master clock. The disciplining process uses a proprietary phase lock loop circuit that enables the user to lock the unit to the master reference without degrading the close-to-carrier noise.

Weighing less than 6.5 kg (14.5 lbs), the SBO is built from three machined, hermetically-sealed alumin-um modules and has external dimensions of 220 mm x 220 mm x 65 mm (8.7" x 8.7" x 2.6"). Module one is the sapphire loaded cavity resonator. Module two contains the main oscillator circuit in microstrip as well as the electronic servo control boards. Module three contains the phase lock loop circuit. The modules are mounted on an integrated heatsink and options for air or conduction cooling allow easy system integration. Output power is +13 dBm, while the standard operating temperature is 0° to 40°C (extended operating temperature options are available).

Conclusion

In 1996 the Sapphire Loaded Cavity Oscillator was released and introduced a fundamentally new method of signal generation to the microwave market. The Shoebox oscillator integrates those technologies into a package ready for rugged, space-conscious platforms.

More information about the company, its products and technology, including up-to-date data, is available at the company's Web site at www.psi.com.au.

Poseidon Scientific Instruments Pty Ltd., Fremantle, Western Australia, + 618 9430 6639, sales@psi.com.au.
Circle No. 301