SAW Oscillators Fly On Airborne Radars

June 18, 2010
Surface-acoustic-wave (SAW) oscillators have been available for some time as high-quality reference sources in high-frequency systems, including in ground-based radar and test and measurement equipment. They are capable of relatively ...

Surface-acoustic-wave (SAW) oscillators have been available for some time as high-quality reference sources in high-frequency systems, including in ground-based radar and test and measurement equipment. They are capable of relatively high fundamental frequencies compared to crystal reference oscillators with excellent spectral purity in terms of phase noise, spurious content, and harmonics. For airborne applications, the new ULN line of SAW oscillators from TEMEX offers fundamental output frequencies from 300 to 600 MHz with phase noise as low as -182 dBc/Hz in fundamental-frequency mode.

The ULN sources are actually oven-controlled, voltage-controlled SAW oscillators (OCVCSOs) with SAW resonators housed in all-quartz packaging (AQP) to ensure stable performance and aging characteristics even in the high-gravitation-force (high-g) environments of airborne systems (Fig. 1). The ULN oscillators take advantage of TEMEX's wafer level packaging (WLP) using quartz on quartz with glass frit bonding to create a hermetic enclosure around a high-performance SAW resonator. Figure 2 shows one wafer of SAW resonator die after wafer bonding and before dicing.

The ULN resonators and oscillator circuitry are housed within grade D packaging to withstand the rigors of airborne applications with harsh environmental conditions, including extended operating temperature range and high levels of shock and vibration. For measurement purposes and lab environment like applications (stabilized and conditioned platform), Temex proposes also a "ground-based" version of the oscillator in grade B packaging that exhibits the same electrical performances as grade D but at room temperature and quiet environment only.

The close-in phase noise of the new ULN SAW oscillators is equivalent to the performance of very-high-frequency (VHF) crystal oscillators used with multipliers to achieve the same frequency range (300 to 600 MHz). Although speci fied for fundamental frequencies through 600 MHz, the frequency range of the ULN SAW oscillators can be extended (as an option) to X-band through frequency multiplication. Figure 3 shows typical phase-noise curves for a ULN SAW oscillator with output frequency of 320 MHz. Phase-noise measurements were made with a DCNTS series test system from NoiseXT. The test system includes a noise spectrum analyzer that uses cross-correlation techniques to extract the precise phase noise of an oscillator from measurements on a set of three similar oscillators.

Crystal oscillators have long served as reference sources in commercial and military electronic systems. They are characterized by high-quality-factor (high-Q) performance using bulk-acoustic- wave (BAW) resonators on precision quartz substrates. The typical phase-noise performance of a state-of-the- art crystal oscillator at 100 MHz is shown in Fig. 4. Again, these phase-noise measurements were made with the help of test equipment from NoiseXT. The resonant frequency of a quartz crystal resonator is related to the thickness of the quartz disk (the resonant cavity). Due to manufacturing constraints, the fundamental resonant frequency of a quartz crystal resonator is limited to a maximum frequency of about 150 MHz. Although higher frequencies are possible by means of multiplication, the process significantly degrades phase noise by 20log10(N), with N being the multiplication factor.

To avoid such limitations to phase-noise performance, one solution is to start with a high-Q oscillator at a higher fundamental frequency, so that less multiplication is needed to achieve a desired reference frequency. SAW on quartz technology o ffers this opportunity. In contrast to BAW on quartz, the resonant cavity of a SAW device is not linked to the thickness of the substrate but to the line resolution of the pa ttern formed on the surface of the quartz substrate. With the capability of forming line resolution to 0.3 m, SAW resonator fundamental frequencies to the GHz range are possible.

Figure 5 compares the phase-noise curves from the SAW oscillator of Fig. 3 and the performance of the BAW crystal oscillator of Fig. 4 to highlight the improvements possible with SAW oscillator technology. For a meaningful comparison, the phase-noise curves must be measured at the same carrier frequency. For this comparison, the crystal oscillator output frequency has been multiplied by a factor of 3.2 (so that the phase noise of Fig. 5 has been increased by 10.1 dB due to the multiplication) in order to match the output frequency of the SAW oscillator.

In comparing the phase-noise performance of the two different oscillators, it is apparent that the phase-noise levels are equivalent close to the carrier and for offset frequencies of less than 1 kHz. For o ffset frequencies further than 1 kHz from the carrier, the phase noise of the SAW oscillator is lower than that of the crystal oscillator, with the improvement reaching more than 12 dB for offsets extending toward the noise floor. This improvement in phase noise represents a major breakthrough for several different types of airborne radar systems. For Doppler radar, it can mean as much as twice the detection range. For targets with low radar cross sections (RCSs), the low phase noise will translate into improved target sensitivity for such objects as stealth aircraft and missiles.

To support airborne applications, such as radar and avionics systems, grade D packaged SAW oscillators have been developed to withstand the harsh conditions of those environments. For these oscillators, the grade D packaging consists of a suspended core (Fig. 6) for the SAW oscillator printed-circuit board (PCB); an inner metallic enclosure for the oscillator circuitry, mounted on vibration and shock-absorbing material; an additional PCB for functions not sensitive to shock and vibration, such as DC voltage regulation, frequency multiplication, and/or a phase-lock loop (PLL) for locking the SAW oscillator to an external lowfrequency reference oscillator; and an outer waterproof package with input/output feed-through solder pins for DC voltage, builtin- test (BIT) circuitry, and RF signals.

In a ULN SAW oscillator, the oscillation loop is composed of a SAW resonator, an electronic phase shifter, a power spli tter, and a sustaining amplifier (Fig. 7). The phase oscillation condition is adjusted at the factory by means of an electronic delay line. The voltage-controlled tuning range of a ULN oscillator is large enough to compensate for long-term aging (at least one year) and frequency variations due to environmental conditions. The resonator is maintained within an oven, with the temperature of the oven monitored and controlled by an onboard microcontroller. Figure 8 shows a block diagram of a complete grade D packaged ULN SAW oscillator for one possible arrangement, including a PLL for use with an external 10-MHz reference oscillator.

This arrangement, with internal PLL (currently under development), is designed for example for synchronizing more than one more SAW oscillator with a single external reference oscillator. The control input is designed for use with an external 10-MHz reference source, allowing a ULN SAW oscillator to be phase locked to the reference. Within a loop bandwidth of less than 10 Hz, the SAW oscillator will assume the frequency stability of the reference oscillator; outside of that loop bandwidth, the SAW oscillator's spectral purity reverts to its own free-running specifications.

Using the BIT circuitry, the onboard microcontroller monitors the presence of the 10-MHz reference signal, the temperature of the resonator's oven, and the PLL's output signal. It provides an alarm (BIT output signal) if the output frequency is out of the specified control range, due to failure or need of a calibration phase adjustment (because of aging effects). The microcontroller performs an automatic frequency calibration during warm up each time a ULN SAW oscillator is powered on, fine tuning the temperature of the oven. After a BIT alarm, an on-off cycle of the oscillator is needed to perform the self-calibration routine. If the microcontroller does not detect a 10-MHz reference signal, it disconnects the PLL and sets the SAW oscillator in free-running mode or voltage-controlled mode (for example, for phase-noise measurements).

With their higher fundamental frequencies compared to crystal oscillators, SAW oscillators provide the means of dramatically improving the performance of airborne radar systems for earlier detection of stealth targets without degrading the detection performance at low Doppler rates. The use of the higher fundamental frequencies in the ULN SAW oscillators eliminates the need for the frequency multiplication that translates a lower-frequency source to the required frequency range, but also degrades the phase-noise performance of the source in the process. The SAW technology in these ULN series oscillators is mature and proven, and provides the reliability needed to withstand the harsh environments faced by modern fighter aircraft.

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