Four-Channel Digitally Tunable RF-Filter IC Spans 0.5 to 9 GHz

In today’s fast-moving, constantly changing RF world with new standards, devices, and application demands, a fixed-function filter may be too static to meet the system requirements. Instead, a filter with key parameters that can be changed “on the fly” would offer significant advantages in flexibility and response to changing needs.

There are several ways to achieve this goal, and now another choice has been added to the list. Analog Devices’ ADMV8809 is a fully monolithic microwave IC (MMIC) that features digitally selectable operating frequencies and functions as a single-chip replacement for discrete filter banks. It’s also compact, housed in a 52-terminal land-grid-array (LGA) package measuring just 8 × 8 × 1.07 mm (Fig. 1).

The device integrates four independently controlled high-pass filters (HPFs) and low-pass filters (LPFs) with center frequencies spanning 0.5 to 9 GHz, while its flexible architecture allows the 3-dB cutoff frequency of the high-pass and low-pass filters to be controlled independently. The digital logic control on each filter is 3 bits wide (eight states) and controls the on-chip reactive elements to adjust that cutoff. Typical insertion loss is below 6.2 dB, while rejection is up to 40 dB.

This sort of programmability precludes “ready-to-go” operation. Instead, the ADMV8809 is initialized and controlled via an SPI port, with a well-defined flow chart (Fig. 2). In addition, it has a lookup table (LUT) feature that can be used to store multiple filter conditions on LUT registers.

The 52-page datasheet provides the detailed characteristic curves, tables defining performance, and more, as well as details for programming this filter. It also includes design-in suggestions, such as noting that the ADMV8809 can use standard high-quality dielectric materials between the top metallization layer and internal ground layer like the Isola 370HR. Rogers 4003 or Rogers 4350 don’t have to be used. Of course, the characteristic impedance of the transmission lines to the RF1 and RF2 pins must be controlled to 50 Ω to ensure optimal RF performance.

Evaluating an RF component operating into the gigahertz range is never trivial. To facilitate the process, Analog Devices offers the EVAL-ADMV8809 evaluation board (Fig. 3).

It incorporates the ADMV8809 chip, as well as a negative-voltage generator, low-dropout (LDO) regulators, and a connector for the SPI interface to the SDP-S controller board to allow for simple and efficient evaluation. The negative-voltage generator and LDO regulators enable the ADMV8809 to be powered by either the 5-V USB-supply voltage from the PC via the controller board or by using two external power supplies.