LC Filter Design
Design 2nd order LC Low Pass and High Pass filters.
What is an LC Filter?
An LC Filter uses Inductors (L) and Capacitors (C) to block or pass specific frequencies. Unlike simple RC filters, LC filters are 2nd Order (or higher), meaning they have a much steeper "roll-off" (-12 dB/octave).
Because Inductors and Capacitors waste very little energy as heat (unlike Resistors), LC filters are standard for Power Suppliers and Radio Frequency (RF) circuits.
Filter Topologies
We use the "Butterfly" or "Pi" and "T" configurations. Ideally, we design for a specific system impedance (Z0).
Resonant Frequency: f0 = 1 / ( 2π√(LC) )
Butterworth Response
This calculator designs for a "Butterworth" response, which gives the flattest possible passband (no ripples), at the cost of a slightly slower transition to the stopband.
Practical Applications
- Power Supplies: A "Pi Filter" (C-L-C) after a rectifier smooths the ripple voltage much better than a capacitor alone.
- Radio Transmitters: A Low Pass LC filter is placed after the amplifier to remove harmonic frequencies (e.g., transmitting on 100MHz but suppressing 200MHz and 300MHz).
- Audio Crossovers: As mentioned in the Crossover tool, tweeters and woofers use LC networks.
FAQ
Pi Filter vs T Filter?
Mathematically they perform similarly. Pi Filters (Shunt-Series-Shunt) are better if the input impedance is high. T Filters (Series-Shunt-Series) are better if the input impedance is low. In power supplies, Pi is dominant.
What is the Q Factor?
The Quality Factor (Q) describes how "sharp" the filter is depending on the load resistance. If you remove the load (open circuit), an LC filter can ring (oscillate) at its resonant frequency, causing voltage spikes!
Air Core vs Iron Core Inductor?
For Audio (low freq), use Iron/Ferrite Core to get high inductance in a small space. For RF (high freq), use Air Core (empty coil) to avoid core losses and saturation.