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Attenuating RF signals is commonly done in RF test instrumentation and receiver front ends to protect downstream circuitry
and to increase dynamic range. Using discrete attenuators and
switches maximizes design flexibility and routing options. In the Figure 1 circuit, two ADGM1304 single-pole, four-throw (SP4T) RF MEMS switches in a back to back configuration yield
four independently switchable paths between input and output. Two of the paths are straight through transmission lines, the third path contains a 6 dB attenuator, and the forth path contains a 9 dB attenuator. Key to realizing this application is the use of ultralow insertion loss and highly linear switches to multiplex between the different path options.
The switches must be as transparent as possible to the RF signal
and add as little insertion loss and distortion as possible. The
ADGM1304 switches offer best in class insertion loss of 0.26 dB typical at 2.5 GHz, and a third-order intercept (IP3) performance
of 69 dBm typical. In addition to insertion loss and distortion,
another key benefit that the MEMS switch brings to this
application is its ability to operate down to true dc. This means
the switches do not limit lower frequency operation in a typical
RF instrumentation attenuator switching application, and enables the instrument to pass dc bias voltages when required.
The physical size of the ADGM1304 device at 4 mm × 5 mm ×
0.95 mm yields a significant reduction in printed circuit board
(PCB) area compared to traditional electromechanical relays switching solutions. In addition, the actuation speed of the ADGM1304 switch is 30 μs, a significant improvement over
electromechanical relays, which are in the order of milliseconds and introduce significant time lag in measurement systems.
The actuation lifetime of the ADGM1304 device is guaranteed for one billion cycles, which is a major improvement over
electromechanical relays and significantly increases overall system reliability.