Frequency synthesizers come in many forms, depending upon the needs of an application.For the price of banks of filters and large bulk, direct analog frequency synthesizers offer unmatched frequency switching speed, although usually with trade-offs in spurious and phase noise. By sacrificing switching speed, few sources can match the low-noise performance of a phase-locked-loop (PLL) synthesizer with narrow loop filters and a high-quality crystal reference oscillator.^1-7 Bridging the gap between low phase noise and fast switching speed has been a goal of many synthesizer designers and a driving force behind the development of the new fast-settling frequency synthesizers (FSFS) series of miniature synthesizers from Synergy Microwave (Paterson, NJ). Among the first models is the FSFS315555-500 with wide tuning range of 3150 to 5550 MHz and switching speed of a mere 50 µs, all in a surface-mount-technology (SMT) package measuring only 1.25 X 1.00 in.

The FSFS315555-500 (Fig. 1) is barely larger than the voltage-controlled oscillator (VCO) upon which it is based. It delivers at least +5 dBm output power over its broad 2400-MHz tuning bandwidth, with a nominal output impedance of 50 W. The synthesizer achieves its fast tuning speed in 5-MHz steps. The switching speed (see table) of 50 Ωs is measured as the time required to settle to within 10 kHz of a new frequency. Measurements taken of the FSFS315555-500's switching speed (Fig. 2) show the specification to be conservative, and included the programming time of the PLL.

The FSFS315555-500 works with external reference input frequencies from 10 to 250 MHz at reference input voltages from 0.8 to 3.3 V peak to peak. It requires bias voltages of 65 mA maximum at +5 VDC for the internal VCO, less than 10 mA at +15 VDC for tuning, and less than 65 mA at +5 VDC for its digital PLL circuitry.^1-3 The synthesizer exhibits typical phase noise of typical phase noise of –80 dBc/Hz offset 10 kHz from the carrier, –80 dBc/Hz offset 100 kHz from the carrier, and –100 dBc/Hz offset 1 MHz from the carrier, anywhere in its 2400-MHz

tuning range (Fig. 3). Spurious content is no greater than –60 dBc and typically –75 dBc. Harmonic levels are –10 dBc or less. The miniature synthesizer is designed to hold its performance levels over an operating temperature range of –20 to +85°C.

These new FSFS synthesizers actually represent a design platform that can work with virtually any 0.5 X 0.5 in. VCO in the company's product lineup, including smaller oscillators in the DCYR and DCYS YIG-replacement product series (see Microwaves & RF, April 2006, p. 80). For example, model DCYS2004005 is one of the aforementioned YIGreplacement oscillators (so named for their small size, low power consumption, and YIG-like low phase noise) with an output frequency range of 2000 to 4000 MHz and +2 dBm minimum output power. Additional examples of VCOs that can be "transformed" into FSFS frequency synthesizers are the model DCMO1857 with +3 dBm minimum output power from 180 to 570 MHz, model DCMO60170-5 with +3 dBm minimum output power from 600 to 1700 MHz, and model DCMO190410-5 with +2 dBm minimum output power from 1900 to 4100 MHz. The company's engineering team currently has designed four different synthesizer circuit boards to accommodate the different mechanical and electrical requirements of the firm's smaller VCO lines in order to quickly meet customers' synthesizer needs.

The new FSFS frequency synthesizers are based on the ADF4106 PLL frequency synthesizer integrated circuit (IC) from Analog Devices (www.analog.com), a single-integer N type device that accepts input frequencies to 6 GHz and maximum reference frequencies to 300 MHz, and exhibits nominal phase noise of –219 dBc/Hz. Because the AD4106 features a separate charge-pump supply with programmable current as well as a programmable dual-modulus prescaler with division ratios of 8/9, 16/17, 32/33, and 64/65, Synergy Microwave designers such as Juergen Schoepf, vice president of engineering, and his group were able to develop an optimization scheme by which the normally conflicting parameters of switching speed and phase noise could be dynamically traded off for the best combination of performance.

Schoepf notes that the optimization flexibility afforded by the AD4106 PLL makes it possible for the FSFS frequency synthesizers to overcome some of the normal trade-offs in synthesizer performance, "Due to the change in tuning sensitivity especially at lower frequencies, the loop filter bandwidth varies with the output frequency." The company's DCMO60170-5 VCO, for example, is specified for typical tuning sensitivity of 30 to 90 MHz/V to achieve its output range of 600 to 1700 MHz. It achieves this range courtesy of 0 to 25-V tuning voltages. Optimization of the loop bandwidth in the FSFS frequencies helps maintain fast and accurate tuning without sacrificing noise performance. The response of the loop filter actually changes with tuning to "equalize" the settling response of the synthesizer's VCO following a frequency jump.⁶

Finding the optimum setting under all operating conditions is sometimes a tedious task for the application engineer. To shorten product-development time, all of Synergy's synthesizers can be equipped with build-in intelligence (IIS) in the form of a microcontroller. The microcontroller takes care of programming the PLL chip with the optimum settings across the frequency band and provides, independent of the PLL chip and frequency range, a standard programming interface to the user. All there is to program is the desired frequency in the form of a serial or parallel protocol.

This allows the engineer in the field to switch models and types of synthesizers (Integer, Fractional, DDS) without the hassle of changing the application software. Additionally, the microcontroller monitors operating parameters to detect malfunction of the synthesizer and provide status information.^1-7