Tunable Bandpass Filters
Tunable bandpass filters are typically characterized by center frequency, bandwidth, and shape factor for a specified level of attenuation. As a rule, only two of the three parameters can be independently specified for a given filter topology (implying a specific filter order) .
A tunable filter bank covering 10 MHz contiguously through 1344 MHz is part of the AM1-S100 frequency synthesizer. A number of the lumped-element tunable bandpass filters contained in that product were prototyped using the hardware shown here. The design details and supporting analysis for these filters is provided in a 3-part series:
Part I: General overview of lumped-element tunable bandpass filter design; detailed design formula for 1st order solutions.
Part II: Continuation of Part I, focusing almost entirely on topologies and detailed design formula for 2nd order solutions.
Part III: Final part on the design of lumped-element tunable bandpass filters. Actual test results are provided for several of the bandpass filters which make up this prototype filter board.
Schematics: 10 – 40 MHz Filters, 40 – 160 MHz Filters, Tune Voltage Creation
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Tunable bandpass filter prototype board testing. Arduino Uno shown in the forefront with LCD-bases slider controls for creating the 4 required varactor tune voltages.
Close-up of the lumped element bandpass filter prototype board with 3 of the 4 filters assembled. Op-amps for tune voltage creation are shown along the bottom.
Filter center frequency tuned to 154 MHz.
Filter center frequency tuned to 80 MHz
Wide span exhibiting good stopband attenuation performance.