This paper describes a tunable transient filter (TTF) design for soft error rate reduction in combinational logic circuits. TTFs can be inserted into combinational circuits to suppress propagated single- event upsets (SEUs) before they can be captured in latches/flip- flops. TTFs are tuned by adjusting the maximum width of the propagated SEU that can be suppressed. TTFs require 6-14 transistors, making them an attractive cost-effective option to reduce the soft error rate in combinational circuits. A global optimization approach based on geometric programming that integrates TTF insertion with dual-VoD and gate sizing is described. Simulation results for the 70 nm process technology indicate that a 17-48X reduction in the soft error rate can be achieved with this approach.