The R-C twin-tee passive circuit supplies band-reject (notch) filtering to portable applications. It has a circuit Q (loaded) of 0.25. Satisfactory rejection can be achieved when the bridge is balanced (close tolerances of adjustable components) and the capacitor unloaded Qs (a function of capacitor ESR) are large compared to the circuit Q. A schematic of a symmetrical twin-tee circuit is shown in the figure. The design equations are as follows:

F_O = 10⁶/(2 πRC) = notch frequency

where R = resistance in ohms = (√2)(Z_O) typical; Z_O = source/load impedance in ohms; and C = capacitance in microfarads.

For convenience, an impedance of 50 Ω was used to test the circuit. The twin-tee circuit was designed for a notch frequency of 60 Hz. It was assembled on vector board (type 169P44C1) in a die-cast aluminum enclosure. The four capacitors are 47-μF, 35-V electrolytics with a ±20% tolerance. The four resistors are 56-Ω, 1/2-W metal-film types with a ±1% tolerance. The measured amplitude response of the 60-Hz twin-tee circuit is shown in the table.

The choice of capacitors is crucial to realizing notch selectivity. The capacitors were obtained from Mouser Electronics per stock number 5985-AFK35V47. The surface-mount components have a maximum ESR of 0.36 Ω at 20°C and 100 kHz. These electrolytic capacitors are polarized, and correct polarities must be maintained throughout the circuit.

The coarse tolerances of the capacitors may adversely affect the reproducibility of the 60-Hz twin-tee circuit during manufacture. This probably could be mitigated by using single-turn, 100-Ω variable resistors in place of the two fixed, grounded shunt resistors.

With the advent of portable electronics equipment, passive circuits can be advantageous. They reduce battery current drain and eliminate some wiring. Passive R-C twin-tee circuits have been around for decades.1 Hopefully, component tolerances and capacitor ESRs will improve. In the era of supercapacitors, surface-mount components, and enhanced pc-board materials, ongoing applications for R-C twin-tee circuits should be considered.

See Associated Table

References:

1. Zverev, A.I., Handbook of Filter Synthesis, John Wiley & Sons, New York, p. 23-24, 1967

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