Military communication radios typically use switched filter banks that have inherent disadvantages, such as size, weight and high-power dissipation. To alleviate these issues, a miniaturized high power tunable UHF filter uses micro-electromechanical system (MEMS) switches and lumped element components to tune from 225 to 512 MHz. The filter is configured in seven discrete steps using multiple SP4T switches to adjust the resonant frequency by selecting inductor values. The novel design approach can handle 60 W with insertion loss below 1.5 dB, which is up to 3 dB less than traditional switched filter banks that use solid-state switches. The single-resonator architecture has an ultra-small form factor of 3.4 in. x 1.6 in. x 0.7 in., which is greater than a 90 percent reduction in volume when compared to solid-state solutions.
The miniaturized UHF tunable filter bank uses MEMS switches that have inherent advantages over solid-state technologies. These switches feature lower loss, larger power input handling, greater RF linearity and reliable operation for more than 3 billion switching operations. A critical feature when comparing MEMS-based designs with solid-state technologies is the very low on resistance (Ron), which results in the lowest possible insertion loss, and an ultra-low off capacitance (Coff) that greatly reduces signal leakage. These lower parasitics offer improved “on” and “off” state operation, enabling a high Q tunable resonator with little degradation in RF performance when compared to a fixed resonator. This also offers the advantages of increased RF input power handling and minimal performance variation over the –40°C to +85°C operational temperature range.
The filter’s design topology uses four SP4T MEMS switches with discrete inductors and capacitors configured as a four-pole bandpass filter. The design can be easily scaled to increase performance further by adding more MEMS switches and components for additional filter poles and zeros without drastically changing loss or size.
This filter operates in the UHF band but is not limited to these frequencies as the MEMS switches can operate from DC to 18 GHz, allowing broader bandwidth operation. This article describes the design and performance of a UHF discrete tunable MEMS filter as compared to tradition switched filter banks.
TUNABLE FILTERS VERSUS SWITCHED FILTER BANKS
Switched filter banks (see Figure 1a) are key building blocks of frequency selective RF and microwave portions of many systems both in the receive and transmit paths. The topology of an ‘N channel’ switched filter bank is simply N filters placed between a pair of SPNT switches. This is very effective when the number of channels, N, is small; but as N increases, complexity and size rapidly become unmanageable. Increasing insertion loss due to cascading SPNT switches causes further power dissipation as well, which can create significant thermal challenges for the entire system.
Figure 1 Switched filter bank (a) and tunable filter (b).
Figure 2 Tunable 225 to 512 MHz MEMS filter.
For larger numbers of channels, a tunable filter (see Figure 1b) may provide better performance and a more compact solution than a switched filter bank. However, traditional tunable filters based on varactor diodes have significant limitations in terms of linearity and power handling. The use of MEMS switches in the frequency selective elements offers substantial advantages. The challenges in designing a tunable filter are maintaining high Q factors, maintaining performance with high power handling and temperature variation, and ensuring filter performance is maintained over the full tuning range.
RF MEMS TUNABLE FILTER
The tunable filter shown in Figure 2 uses four RF MEMS SP4T switches from Menlo Microsystems (MM5130) to realize a four-section filter that is tunable from 225 to 512 MHz in discrete steps. The filter measures 2.5 x 1.25 x 0.25 inches without connectors. It tunes over the frequency range in seven discrete steps, but with 4 bit binary switch control a total of 16 steps could be implemented if required.
COMPARISON WITH A TYPICAL SOLID-STATE SWITCHED FILTER BANK
Table 1 shows the performance of the MEMS tunable filter compared with a typical high power switched filter bank, and Figure 3 is a picture comparing size. Key benefits are in the areas of insertion loss and size, weight and power consumption (SWaP).
Figure 3 Tunable MEMS Filter compared with a typical switched filter bank.
