Know the Nuances Of SSB Phase Noise

Feb. 24, 2009
Understanding phase noise in oscillators and synthesizers makes it easier when comparing the performance of different sources at different frequencies.

Phase noise is one of the first performance parameters that oscillator specifiers review on a data sheet. Whether it is a low-frequency crystal reference oscillator or a higher-frequency tunable oscillator, such as a voltage-controlled oscillator (VCO), all signal sources contribute phase noise to a systema key in designing or specifying an oscillator is to keep the phase noise as low as possible and practical. Understanding what it is and how it affects performance at the system level can aid in the appreciation of how difficult the task is of achieving truly low phase noise.

In the frequency domain, phase noise is a measure of the random, short-term fluctuations of a signal's phase; in the time domain, it is known as jitter. An additional time-domain representation of this noise is as the Allan variance, which describes phase noise in terms of the amount of frequency drift in parts per million over a period of time. Because the frequency of an oscillator or synthesizer can not be perfectly controlled, minute fluctuations in the output frequency occur over time and are measured as phase noise or jitter. Although phase noise may not be critical for some systems, it can impact the performance of radar systems that rely on reading signal phase and communications systems employing digital modulation formats.

In a perfect output signal, all of the energy would be focused in a single frequency. But real signals cannot maintain a single frequency with zero fluctuations in frequency and phase, and the result is a spreading of the signal energy outward from the desired frequency. That signal spreading is due to a variety of noise sources, including shot and flicker noise from the transistor used in an oscillator as well as noise carried on the power line. The shape of that spreading can tell a designer about the sources of noise and, by minimizing those sources, how to improve the phase noise of the oscillator.

On its web site, Vectron International offers an excellent graphical depiction of a SSB phase-noise plot (see figure). It shows the three basic regions of noise for a given signal: close-in or flicker frequency-modulation (FM) noise, 1/f noise generated by an oscillator's active device, and broadband noise further from the carrier. The site's literature also provides information on setting up several different measurement systems for evaluating phase noise. Additional details on measuring phase noise can be found on the web site of long-time low-noise crystal oscillator supplier Wenzel Associates. Furthermore, an application note from Mini-Circuits, "VCO test methods," offers block diagrams with various approaches to measuring VCO phase noise. To minimize power-line noise, batteries are used in these test setups to power the source under test and reference source.

Phase noise can be measured in various ways, from simple to elegant. Some of the simplest approaches involve a double-balanced mixer fed by the oscillator or synthesizer under investigation and a known reference oscillator, such as a high-performance oven-controlled crystal oscillator (OCXO). With the oscillator under test and reference serving as the RF and local oscillator (LO) signals to the mixer, an intermediate frequency (IF) output results that can be filtered to remove unwanted frequencies and evaluated with a spectrum analyzer. Oscillators can be specified in terms of single-sideband (SSB) or double-sideband (DSB) phase noise, although the former convention is more widely accepted. In SSB measurements, the amount of phase noise is normalized to a 1-Hz measurement bandwidth, given in terms of decibels from the carrier (dBc), and referenced to a particular offset from the carrier frequency. For example, a VCO might have phase noise of -90 dBc/Hz at an offset of 10 kHz from the carrier. Because of the shape of a typical phase-noise plot, phase noise drops rapidly with distance from the carrier and as it approaches the noise floor of the system.

A more elegant (and more expensive) means of measuring phase noise involves a dedicated automatic test system, such as the E5505A phase-noise test system from Agilent Technologies. It is designed to measure the phase noise of carriers from 50 kHz to 110 GHz at offsets from 0.01 Hz to 100 MHz. The system, which can operate automatically under software control, features a noise floor of -180 dBc/Hz. In addition to SSB and DSB measurements, it can also provide integrated phase noise results, where the phase noise is expressed as a value integrated over a range of offsets.

The choice of oscillator type can limit phase-noise performance. While a VCO can provide good phase-noise performance, component tolerances and tuning capability tend to force compromises in the lower limits of phase noise. Crystek, which supplies a wide range of typically narrowband VCOs, recently announced its model CVCO55CC-2300-2450 VCO with a tuning range of 2300 to 2450 MHz. Its phase noise is -106 dBc/Hz offset 10 kHz from the carrier with +4 dBm output power over the frequency range. Over a wider tuning range, model DCRO196265-10 from Synergy Microwave Corp. tunes from 1960 to 2650 MHz with +2 dBm output power and phase noise of -106 dBc/Hz offset 10 kHz from the carrier. It features planar resonator construction to fit into a lead-free, surface-mount package measuring just 0.5 x 0.5 x 0.17 in.

For voltage tuning over an extremely narrow range, the VS-705 series of voltage-controlled surface-acoustic-wave (SAW) oscillators (VCSOs) from Vectron International operate at fixed frequencies from 122.88 to 985 MHz in a package measuring just 5.0 x 7.5 x 2.5 mm. The phase noise at 155.52 MHz, for example, is -44.8 dBc/Hz offset 10 Hz from the carrier, and -120 dBc/Hz offset 10 kHz from the carrier. In contrast, the company's DC-170 line of OCXOs operate at standard outputs of 5 and 10 MHz with phase noise of -90 dBc/Hz offset 1 Hz from the carrier and -140 dBc/Hz offset 1 kHz from the carrier.

Different resonator types yield different results in phase noise and oscillator performance. For extremely wide tuning range, few designs can match the tuning capability of a microwave YIG oscillator, such as the MLXB-0218 from Micro Lambda Wireless, which tunes from 2 to 18 GHz with +13 dBm output power. In place of good close-in phase noise, the source provides the wide tuning range, but does achieve phase noise of -120 dBc/Hz offset 100 kHz from carriers of 2 to 12 GHz and -112 dBc/Hz offset 100 kHz from carriers of 12 to 18 GHz.

Dielectric resonator oscillators (DROs) are sold as free-running and lower-noise phase-locked versions. The model PLDRO40000 from MITEQ, for example, is a phase-locked DRO-based oscillator series available with output frequencies from 26.8 to 40.0 GHz. The phase noise is typically -96 dBc/Hz offset 1 kHz from the carrier and -129 dBc/Hz offset 1 MHz from the carrier. The DRO series of free-running DROs from Herley CTI cover outputs from 3 to 18 GHz with -125 dBc/Hz phase noise offset 100 kHz from a 5-GHz carrier.

Reference-quality phase noise is still largely the domain of an OCXO. Wenzel Associates offers a novel approach to controlling the phase noise of higher-frequency OCXOs by packing two stress-compensated (SC) crystal resonators and oscillator circuits in a single SC Sorcerer II package measuring 4.7 x 3.5 x 1 in. The package houses 5- and 100-MHz OCXOs with the lower-frequency source phase-locked to the 100-MHz oscillator. Both deliver +10 dBm output power. For the 10-MHz oscillator, the SSB phase noise is -150 dBc/Hz offset 10 Hz, -172 dBc/Hz offset 1 kHz, and -175 dBc/Hz offset 10 kHz from the carrier. For the 100-MHz OCXO, the phase noise is -120 dBc/Hz offset 10 Hz, -155 dBc/Hz offset 1 kHz, and -174 dBc/Hz offset 10 kHz from the carrier.

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