Novel Approach Yields Fast, Clean Synthesizers

Frequency synthesizers are invaluable components in many systems, from commercial communications networks to test and measurement instruments.¹ Because of their wide application, the RF/microwave industry constantly feels the pressure to deliver higher-performance, higher-functionality, smaller-size, and lower-cost synthesizer designs. In addition, as dictated by the ever-increasing data rates of modern communications systems, a major challenge in synthesizer design is to achieve fast switching speeds without sacrificing performance, size, or cost. In a data communications system, for example, the time spent by the synthesizer transitioning between the frequencies becomes increasingly valuable since it cannot be used for data processing. While many systems still work adequately based on frequency synthesizers with millisecond switching speed, newer systems require microsecond operation together with comparable spectrum purity (i.e., phase noise and spurious) of the lower-speed designs.^2,3

Direct-analog frequency synthesizers offer excellent switching speed and spectral purity characteristics. Unfortunately, today's direct-analog frequency synthesizer designs are hardware extensive and limited to applications that can tolerate fairly high costs. In contrast, indirect phase-lock-loop (PLL) frequency-synthesis architectures bring smaller-size and lower-cost benefits but suffer from serious design tradeoffs. Historically, high-performance microwave PLL frequency synthesizers have relied on YIG-tuned oscillators for broadband operation with excellent phase noise characteristics. However, the high power consumption, relatively large size, high cost, and slow tuning speed inherent to the YIG oscillators encourage the use of frequency synthesizer architectures based on voltage-controlled oscillators (VCOs), which primarily rely on the low-noise characteristics of a low-frequency reference oscillator. Today's commercial oven-controlled crystal oscillators (OCXOs) are capable of outstanding phase-noise performance of -160 to -170 dBc/Hz at a 10 kHz offset from a 100-MHz carrier frequency. Such phase-noise performance can be potentially translated to -120 to -136 dBc/Hz at a 10 kHz offset from a 10-GHz carrier. This theoretical performance corresponds toor even exceedsthe performance of the best YIG oscillators at the same frequency settings. Nevertheless, it is very difficult to provide such an ideal frequency translation since some noise degradation always occurs. Thus, achieving YIG-like noise characteristics for a VCO-based design is not a trivial task and calls for advanced frequency-synthesizer solutions.

To address today's market requirements, Phase Matrix has developed the QuickSyn series of frequency synthesizers, a new generation of microwave frequency synthesizers based on a revolutionary (patent-pending) phase-refining technology that provides a unique combination of fast-switching speed, very low phase noise, low spurious content, and low cost characteristics.⁴ In contrast to traditional approaches (which tend to minimize the PLL loop division ratio), this new technology makes a radical step by completely removing the divider from the PLL feedback path. Moreover, it inverts the PLL division ratio by applying a multiplication within the PLL that drastically improves both phase noise and spurious characteristics. The loop filter bandwidth is significantly extended in comparison to conventional designs (targeting the VCO noise floor region where it becomes competitive with its YIG counterparts), which results in faster switching speed and reduced microphonic effects. This technique combined with the use of a high-frequency, ultra low-noise reference and a custom-built, low-noise locking engine allows us to achieve simultaneously fast-switching speed and instrument-grade spectral purity without the use of expensive and bulky parts. This results in a compact, elegant design, which demonstrates excellent performance and extended functionality (Fig. 1).

The core design of the QuickSyn covers the 2-to-10-GHz frequency range, utilizing a fundamental-frequency, solid- state, voltage-controlled oscillator (VCO) to achieve the desired output frequency. In contrast to widely used frequency multiplication schemes, this approach eliminates possible spectrum contamination by subharmonic products. The use of an advanced direct-digital- synthesis (DDS) approach enables a very fine frequency resolution of 0.001 Hz without the common penalty of slower tuning speed. Since DDS-based designs are normally prone to increased spurious content, both hardware and software techniques5 are used extensively to suppress DDS spurious content to negligible levels, which are managed down to less than -70 dBc (Fig. 2). A distributed output-power amplification scheme results in reduced harmonics, which typically do not exceed -40 dBc (Fig. 3).

The VCO phase noise is controlled by utilizing an ultra-low-noise reference OCXO as well as very wide (a few MHz) loop bandwidth. Thus, the synthesizer phase noise within its PLL filter bandwidth mainly depends on the multiplied reference noise as well as residual noise characteristics of the locking mechanism. Typical phase noise measured at a 10 GHz output and a 10 kHz offset is -120 dBc/Hz. The phase noise drops down to -131 dBc/Hz at a 10 kHz offset from a 2-GHz output signal (Fig. 4), which exceeds the performance of traditional YIG-based synthesizers at the same frequency settings. Phase noise remains flat to a few MHz offset and then rolls down sharply showing a noise floor of about -155 dBc/Hz. Phase hits usually inherent to YIG-based designs are also greatly reduced due to the use of a low-mass VCO and very wide loop filter bandwidth.

The switching speed of the QuickSyn frequency synthesizer is also significantly faster in comparison to traditional YIGbased synthesizers. The PLL hardware itself needs just a few microseconds (subject to frequency accuracy definition) to bring the output frequency to a desired value, while the output is completely locked and refined within less than a hundred microseconds. The digital signal processing adds extra delays required to receive a tuning command, perform all necessary calculations, and program individual devices. Most of these delays, however, can be reduced or completely eliminated in the list mode, which is the precalculation and memorization of all necessary parameters required to control the individual components of the synthesizer for a preset list of frequencies. The actual throughput numbers heavily depend on a particular operating scenario and are normally specified between a few tens and few hundreds of microseconds.

The synthesizer provides +15 dBm maximum RF output power, which can be leveled and digitally controlled with a built-in attenuator and digital-to-analog converter (DAC) as depicted in Fig. 5. The synthesizer also includes a temperature sensor to provide all necessary information for output power calibration and further correction if required. Employing a sophisticated frequency and temperature interpolation routine, the QuickSyn frequency synthesizer provides the flat and repeatable output power characteristics across operating frequency and temperature ranges. In addition to the factory preset flat response, a user can easily set a desirable power-to-frequency slope to compensate connecting cables as well as other devices external to the synthesizer. Virtually any power-tofrequency response can be created with a built-in programmable equalizer (Fig. 6). An additional analog power control input provides a closed-loop automaticlevel- control (ALC) capability. By adding an external coupler and RF detector, the signal from the detector can be fed back to the analog power control input to close the loop. This configuration ensures precise, instrument-grade output power characteristics regardless of the signal match.

A built-in 32-b, 200-MHz reducedinstruction- set-computing (RISC) central processing unit (CPU) brings the required horsepower to support all necessary frequency tuning calculations as well as a number of features such as output power calibration and control, independent frequency and power sweep, and list mode. Furthermore, the QuickSyn frequency synthesizer has built-in amplitude modulation (AM), frequency modulation (FM), phase modulation (PM), and pulse modulation capabilities, making it one of the more versatile frequency synthesizers currently available. It supports a variety of interfaces including SPI, which offers full-duplex communication with high throughput and flexibility. The complimentary Universal Serial Bus (USB) connection and customer-friendly interface enable instant deployment or just evaluation of the synthesizer using a laptop or desktop personal computer (Fig. 7).

QuickSyn frequency synthesizer's circuits are shielded in a metal box measuring only 5 x 7 x 1 in. The microwave synthesizer is biased from a single +12-VDC supply and includes custombuilt active filters to prevent possible signal contamination. The power consumption for a basic configuration does not exceed 20 W. It is also worth mentioning that the QuickSyn synthesizer architecture is extremely flexible and can be easily reconfigured for specific customer requirements.

In short, the high-performance QuickSyn microwave frequency synthesizers leverage radically new phaserefining technology to support current market demands for high performance in smaller, low-cost frequency sources. The technology offers faster tuning speed and lower phase noise characteristics in comparison with traditional PLL techniques. The improved performance, extended functionality, and small footprint make QuickSyn frequency synthesizers ideal building blocks for a wide range of commercial, industrial systems and measurement instruments.

In addition to these compact frequency synthesizers, the company is perhaps best known for its tunable oscillator technology, including its broadband voltage-controlled oscillators (VCOs) such as the model VTO-6000-67M/T oscillators. These are low-noise VCOs designed for use from 4 to 8 GHz with typically +10 dBm output power. The oscillators are based on a low-noise silicon bipolar transistor and a hyperabrupt varactor tuning diode that works with tuning voltages of 0 to 20 V. The oscillators tune with minimum tuning sensitivity of 200 MHz/V and maximum tuning sensitivity of 600 MHz/V.

The sources achieve their generous output power through the use of an internal GaAs MMIC buffer amplifier. The phase noise for these VCOs is less than -95 dBc/Hz offset 10 kHz from the carrier. The oscillators exhibit frequency drift over temperature of 80 MHz maximum for operating temperatures from 0 to +70oC. They draw 65 mA from a +5-VDC suppy with typical pulling of 10 MHz into a 12-dB return-loss load.

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The VCOs, which offer a minimum modulation bandwidth of 20 MHz, suffer second harmonics of typically -20 dBc and third harmonics of typically -20 dBc. Spurious levels are -60 dBc or better. The VCOs are ideal for applications in communications systems, such as a clock source in high-speed systems, as well as in test and measurement equipment. They can be supplied in a TO-8 package or in a housing measuring 1.18 x 0.95 x 0.5 in. with SMA connectors. Phase Matrix, Inc., 109 Bonaventura Dr., San Jose, CA 95134; (408) 428-1000, Fax: (408) 428-1500, Internet: www.phasematrix.com.

REFERENCES

1. J. Browne, "Frequency Synthesizers Tune Communications Systems," Microwaves & RF, March 2006.

2. J. Regazzi and R. Gill "Signal Generator Melds Speed with Low Phase Noise," Microwaves & RF, October 2006.

3. A. Chenakin, "Frequency Synthesis: Current Solutions and New Trends," Microwave Journal, May 2007.

4. A. Chenakin, "Low Phase Noise PLL Synthesizer," patent pending.

5. A. Chenakin, "Building a Microwave Frequency Synthesizer - Part 4: Improving the Performance," High Frequency Electronics, August 2008.