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How do you implement a variable capacitor these days? Mechanical caps are out. That leaves varactors, which work fine but require higher voltages that often are not available, and their response is nonlinear. Yet the Peregrine Semiconductor serial peripheral interface (SPI) PE64904 and I2C interface PE64905 DuNE Digitally Tunable Capacitors (DTCs) let you vary the capacitance digitally in 32 steps with a 5-bit serial input word.
Overall capacitance ranges from 0.7 to 4.6 pF in 131-femtofarad (fF) steps. Each device comprises five capacitors in series...