Twin-T notch filter
Two T-shaped networks in parallel: one low-pass (R–2C–R), one high-pass (C–R/2–C). At one frequency their outputs are equal in size and opposite in phase, so they cancel: a deep notch at f₀ = 1/(2πRC).
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How it works
f₀ = 1 ÷ (2π·R·C) = 1.592 kHz, provided the parts are in exact ratio: R/2 and 2C. A 1 % mismatch turns an infinitely deep notch into one about 40 dB deep.
Try it on the bench
- Change R/2 to 4.7 kΩ: the notch becomes shallow and shifts.
- Feed it a square wave at f₀: the fundamental vanishes and the harmonics remain.
Parts on the bench
- AC source
- 4 × Resistor
- 3 × Film capacitor
In the course
Electronics Engineering, module “Filters and frequency response”.