Miniature Bias Filter Networks with High Isolation from RF to mm-waves

Miniature Bias Filter Networks with High Isolation from RF to mm-waves

Dielectric Laboratories Inc.
Cazenovia, NY

Today's broadband active components and circuits are particularly susceptible to unwanted RF, microwave and mm-wave signals that appear on the bias, control and supply lines that are necessary for proper circuit operation. As a result, broadband filter networks must be constructed and placed in series with the bias lines to provide isolation to these unwanted signals. These filter networks often are discrete components or distributed networks that take up valuable space and achieve less-than-desirable results.

The BFN01 family of bias filter networks are easy-to-use hybrid components designed specifically to filter unwanted signals from bias and control lines over the 10 MHz to 40 GHz frequency range. These new networks provide over 40 dB of isolation from 50 MHz to 40 GHz, as shown in Figure 1 , with a chip size of 0.028" x 0.053". Isolation comparable to stud-mounted feedthru filters is achieved at a fraction of the size and cost. Table 1 lists the filters' performance specifications.

The BFN01 bias filter networks are optimized for high isolation in low DC current applications. The filters are well suited to voltage-controlled circuit elements such as tuning varactor control lines, and GaAs FET and MMIC gate biasing. In these applications, the networks can replace three to five chip components with a single chip that features a much smaller overall size, less assembly and lower cost, and achieves better isolation repeatability. The bias filter networks are designed for wirebond hybrid assembly, as shown in Figure 2 . Large 4 mil wirebond pads allow for ease of assembly.

The networks filter noise and conducted RF effectively from power supply lines and reduce RF feedback significantly through bias supplies. In the case of cascaded high gain modules, the filters enable the successive stages to share a common gate supply input voltage. All of these applications permit the designer to utilize higher levels of functional integration in the design to reduce size and component count, as well as assembly complexity without compromising high isolation performance. Figure 3 shows typical bias filter network applications.

Several innovations were necessary to achieve this performance, including the processing of conductor/resistor thin films combined with high dielectric constant materials, and a new microwave design method. The bias filter networks are produced currently using two proprietary barium titanate ceramics with dielectric constants of 2400 and 4500. These high dielectric constant materials enable the miniaturization and broadband frequency response, and provide 95 and 140 pF versions of the networks.

Another innovation is the application of the thin-film conductor/resistor materials to these high dielectric materials. Tantalum nitride/titanium tungsten/gold, a highly reliable thin-film system used widely in the industry, was adapted along with photo lithographic processes and chemistries for these ceramic materials. Finally, the concept and configuration of this device was designed and modeled specifically to achieve the broadband isolation.

Typical applications for the bias filter networks involve GaAs FET gate bias decoupling and filtering in broadband, high gain amplifier modules, microwave and mm-wave communications modules, oscillators and low noise amplifiers. In these applications, the networks reduce noise, RF feedback and stability problems, and prevent spurious signals from modulating the output.

Other applications involve varactor-control RF decoupling and filtering in VCOs, microwave and mm-wave frequency synthesizers and phase-locked loops. The filters reduce phase noise and spurious output frequencies caused by conducted modulating signals.

The bias filter networks also are used in MMIC and multichip modules used for T/R modules and mm-wave phased arrays where the networks facilitate higher levels of functional integration to reduce size, component count and assembly complexity while maintaining repeatable high isolation performance. In mixed-signal modules, that is, modules that combine microwave or mm-wave analog signals with digital signals, the bias filter networks can filter high clock rate digital signals and their harmonics, and prevent them from corrupting signal fidelity of the microwave analog signals or functions such as in digital signal processors.

For today's wireless applications where reduced size and cost is a prime requisite, these new bias filter networks provide an easy-to-use solution to the designer's desire for effective RF bias isolation. The devices are available currently with a delivery of stock to four weeks at a price of 98¢ each in 1000-piece quantities.

Dielectric Laboratories Inc.,
Cazenovia, NY
(315) 655-8710.