Phase noise
is critical in many communications applications, especially those
employing high-data-rate digital modulation schemes. In a digital
radio employing quadrature-amplitude modulation (QAM), for example,
different phase states represent digital bits. Excessive phase
noise in the system can obscure these bits, resulting in an excessively
high bit-error rate (BER) and possible lost data. Fortunately,
a line of voltage-controlled surface-acoustic-wave oscillators
(VCSOs) developed by Synergy Microwave Corp. (Paterson, NJ) provides
extremely low phase noise at high fundamental output frequencies,
making them suitable for digital microwave radios, optical communications,
and other phase-critical systems. The first several models are
available at fixed frequencies of 622 MHz and 2.488 GHz.

Oscillators based on surface-acoustic-wave (SAW) technology offer several advantages compared to other source types, including high fundamental-frequency operation without subharmonic content, very-low noise floor, and excellent immunity to vibration and microphonics. The use of high fundamental frequencies helps to minimize the number of multiplication stages required to achieve a particular output frequency. Since phase noise will degrade with multiplication by a factor of 20logN, where N = the multiplication factor (for example, N = 2 for doubling), a simple doubling of a signal source will result in a degradation of 6 dB in phase noise. Quadrupling the output frequency will result in a penalty of 12 dB in phase noise, and so on. Thus, ideally, a low-phase-noise oscillator should be specified at the highest-possible fundamental frequencies to minimize the number of multiplication steps required.

The
first series of Synergy SAW oscillators **(Fig.
1)** is at
optical-carrier (OC) frequencies of OC-12 (622.08 MHz) and OC-48
(2.488 GHz), models VCSO-OC12 and VCSO-OC48, respectively. (Phased-locked
versions are also available as models PLL-OC12 and PLL- OC48,
respectively.) The sources are based on the use of a fundamental-tone
SAW resonator in a feedback-loop circuit **(Fig. 2)**. The
phaseshift circuitry, which is realized with inductive-capacitive
(LC) lumped or distributed circuit elements, provides the tuning
mechanism using varactor diodes through applied voltage, typically
from +0.5 to +4.5 VDC. It essentially pulls the resonator across
its frequency range of adjustment, typically ±100 PPM
of the center frequency. Energy is coupled out of the oscillator
through the small-signal amplifier, which is maintained in its
linear region to minimize harmonic contributions. The amplifier
delivers output signals at +10 dBm typical

Compared to a coaxial or ceramic resonator (CR), the SAW resonators make it possible to create extremely small fundamental-frequency oscillators that can fit within standard surface-mount packages. The 2.488-GHz model, for example, uses a resonator measuring only 3.8 X 3.8 mm. Of course, at higher OC frequencies, such as OC-192 (10 GHz), the small size of the SAW resonator required becomes a practical limit to achieving OC-192 sources using this technology, although some work is being performed on practical 5-GHz resonators which can be doubled in frequency to achieve the desired OC-192 frequencies. Presently, higher OC rates are typically achieved by multiplying high-fundamental-frequency sources. For example, an OC-48 source at 2.488 GHz can be multiplied by four to achieve the 10-GHz OC-192 frequency, although with a phase-noise penalty of 20log4 = 12 dB.

Fig 2. The basic architecture of the SAW sources consists of active circuitry, a resonator, an LC phase shifter, and a buffer amplifier

The SAW oscillators are designed for phase-locked-loop (PLL) synthesizer applications, providing enough of a tuning range (±100 PPM) to accommodate oscillator-frequency variations due to temperature, load pull, and voltage pushing. High-quality-factor (Q) resonators are used in these sources, typically 700 to 800 for the 2.488 GHz OC-48 oscillators and greater than 1200 for the OC-12 (622-MHz) oscillators. The temperature coefficient for the higher-frequency resonator is typically less than 150 PPM from - 40 to +85 ºC.

Phase-locked oscillators (PLOs) basically consist of a phase detector, a voltage-controlled oscillator (VCO), an integrator (in the form of a loop filter), and frequency divider(s) connected in a feedback loop. The phase-noise performance of the new VCSOs is good enough to forgive a degradation of 12 dB in phase noise due to a quadrupling of the carrier frequency. Since it is the phase noise of the oscillator that essentially dominates the noise in the loop, especially at wider loop bandwidths, the noise floor of a VCSO will basically set the BER for an optical- or digital-communications system. Designers working with PLLs should keep in mind that the loop bandwidth should be kept as narrow as possible to take advantage of the low phase noise of the VCSOs, although some consideration should also be given to the fact that the design may become susceptible to microphonics when using a narrow loop filter. This can be explained by the fact that the VCSO phase noise dominates the noise performance outside the loop bandwidth.

The VCSOs are suitable for use in phase-locked converters or multipliers for optical-communications networks. These devices have an integer relationship with the system reference frequency. For example, the most common reference frequency, 155.52 MHz or OC-3, is four times the frequency of OC-12 (622.08 MHz) and 16 times the frequency of OC-48 (2488.32 MHz). Ideally, a multiplication factor of 4 is used, so that the phase-noise degradation with multiplication is held to 20log4 = 12 dB. Unfortunately, the phase-noise limitation comes from the usable frequency of the phase detector. The noise floor of the detector degrades with increasing frequency. Without overly degrading the noise performance, the practical high end of the phase-detector frequency band is 20 to 30 MHz. In the case of an OC-3/OC-12 converter, the reference signal is divided by 6 (for a frequency of 25.92 MHz), while the OC-12 VCSO is divided by 24 to achieve the same 25.92-MHz frequency at the phase detector. The multiplication factor with respect to the phase-detector frequency will be 24, with degradation due to multiplication of 20log24 = 27.6 dB. Assuming phase noise of -140 dBc/Hz for the phase detector, the phase noise within the loop will be -112 dBc/Hz.

Depending upon system specifications, a PLL loop filter can be adjusted for optimum performance. For wider loop bandwidths, the phase noise within the loop is limited by -112 dBc/Hz. If a design requires better phase noise at some desired offset frequency, then the loop bandwidth must be kept narrow so that the VCSO becomes the dominant source of phase noise. Even if the phase noise of the VCSO is better than the phase noise within the loop, the noise of the loop will dominate. The choice of loop bandwidth is then dictated by the system switching-speed and phase-noise requirements. Better immunity to vibration and microphonics is achieved through the use of wider loop bandwidths. The integrated phase noise over a particular bandwidth depends on the levels of spurious products and the phase noise at different offset frequencies within the band.

Fig 3. The single-sideband (SSB) phase noise of a 622-MHz VCSO reaches a noise floor of -165 dBc/Hz after dropping from a close-in level of -80 dBc/Hz.

Since specifications
for Synchronous Optical Network (SONET) systems call out for
low phase noise close to the carrier (1 kHz), these SAW sources
are well-suited for SONET and synchronous-digital-hierarchy (SDH)
applications. The 622-MHz oscillators, for example, exhibit close-in
phase noise of -80 dBc/Hz offset 100 Hz from the carrier **(Fig.
3)**. The phase noise drops to -110 dBc/Hz offset 1 kHz from
the carrier, -135 dBc/Hz offset 10 kHz from the carrier, and
- 155 dBc/dHz offset 100 kHz from the carrier. At 300 kHz from
the carrier, the phase noise rolls down toward the noise floor
of - 65 dBc/Hz. It should be noted that these phase-noise specifications
translate into very-low-phase jitter performance (on the order
of picoseconds) in terms of time-domain system performance.

The voltage-tuned SAW oscillators are inherently resistant to microphonics, which are essentially frequency-modulated (FM) signals that result as a function of vibration. When an oscillator is subjected to single-tone-type vibration, it will produce sideband energy at a carrier frequency offset equal to the vibration frequency. When the oscillator is subjected to vibration of a more random nature, the resulting noise is similar to phase noise, due to the mechanical stressing of the SAW resonator. The immunity of a SAW oscillator to vibration can be expressed in terms of gravitational-force (G) sensitivity or acceleration sensitivity, which is the ratio of the power in the vibration sidebands to the carrier power per G unit of vibration. The typical immunity to vibration sidebands and microphonics for the 622-MHz oscillator is 5 X 10-10/G, with similar performance for the 2.488-GHz unit.

The voltage-tuned SAW oscillators are suitable for local-multipoint-distribution-system (LMDS) applications, for clock cleanup/recovery applications in digital- and optical-communications systems, and for digital microwave radios employing higher-order modulation schemes, such as 16QAM and 64QAM. The first series of these stable sources operates at OC-12 (622 MHz) and OC-48 (2.488 GHz) frequencies and are well-suited for surface-mount assembly.

SAWs Stabilize Low-Phase-Noise Voltage-Tuned SourcesThese SAW oscillators provide high fundamental frequency outputs at 622 and 2488 MHz with low phase noise and high immunity to vibration.