Choose The Right Prescalers For PLL Synthesizers

Prescalers act as Frequency dividers in radio- and microwave-frequency translation and signal generation. Often, they are employed in phase-locked loops (PLLs) and frequency synthesizers to match the frequency of a high-frequency source to the frequency of a reference oscillator. With knowledge of how to bias and operate prescalers, engineers can design them into a wide range of high-frequency applications. In the six-page product application note titled, "Selecting Prescalers for PLL Synthesizers," Hittite Microwave Corp. uses its own monolithic-microwave-integrated-circuit (MMIC) prescalers as a basis for educating engineers about prescalers and their implementation.

The note begins by discussing the modulus or divide ratio, which is a fundamental prescaler parameter. With a modulus that is equal to an integer power of 2, fixed modulus prescalers allow higher input-frequency handling than other integer division ratios. For example, Hittite's MMIC prescalers feature high input-frequency capabilities to 13 GHz. In terms of capacitors, the +5-VDC prescalers include an internal, monolithic 15-pF power-supply decoupling capacitor. Yet a pair of external decoupling capacitors connected between the Vcc terminal and ground is highly recommended.

To bypass lower frequencies, one capacitor should be relatively high in capacitance. The company recommends a 1-to-10-F tantalum chip or multilayer ceramic capacitor. To bypass high-frequency components and minimize prescaler input-to-output coupling, the second capacitor should be relatively low in capacitance (for example, 300 pF). The note also describes the optimal placement of the capacitors and how to attach them.

Several standard guidelines are provided for the capacitance and style of the DC blocking capacitors so that they do not cause significant attenuation of the input signal. For the lowest value at which the prescaler will be operated, the engineer should select a capacitance value that presents a value of reactance that is small relative to 50 Osay 3 O or less. In addition, the engineer should check the vendor's capacitor data to make sure that the selfresonant frequency is higher than the highest input frequency at which the prescaler will be operated. The note goes on to examine input power and its effect on the prescaler's output phase noise, how to handle conditions of no or inadequate RF input signal levels, and more. By providing a variety of problem scenarios and general rules of thumb, it offers a valuable guide to prescaler selection.

Hittite Microwave corp., 20 alpha rd., chelmsford, Ma 01824; (978) 250-3343, FaX: (978) 250-3373, internet:

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