Series LC Band-Pass Filter Calculator
Design a series RLC band-pass filter using your target center frequency, bandwidth, and circuit resistance.
What this LC bandpass filter calculator does
This calculator helps you size the inductor (L) and capacitor (C) for a practical series RLC band-pass filter. You enter your desired center frequency and bandwidth, plus the resistance in the circuit, and it computes the required component values instantly.
It also reports useful design metrics such as quality factor (Q), lower cutoff frequency (f₁), upper cutoff frequency (f₂), and reactance at resonance. This makes it easier to move from concept to prototype.
LC band-pass filter fundamentals
A band-pass filter allows a selected frequency band to pass while attenuating frequencies below and above that range. In a classic series RLC implementation:
- The circuit resonates at the center frequency, where inductive and capacitive reactance are equal in magnitude.
- At resonance, impedance is minimized (equal to R), so current is highest.
- Around resonance, the sharpness of the response is controlled by Q.
Core relationships used by the calculator
These equations are standard for a damped series resonant network and are widely used in RF, audio, instrumentation, and sensor-interface design.
How to use the calculator effectively
1) Pick your target center frequency
This is the frequency you want to pass most strongly. For example, in a communications front-end, this might be your channel frequency.
2) Define the required bandwidth
Bandwidth is the passband width between -3 dB cutoff points. Smaller bandwidth means more selectivity and usually a higher Q.
3) Enter realistic circuit resistance
Resistance strongly affects damping. In real circuits, it can represent source resistance, load resistance, winding resistance, and other loss terms combined into an effective value.
4) Build and tune
Real parts have tolerance and parasitic behavior. Use trimmer capacitors, slug-tuned inductors, or component binning if precise alignment is required.
Worked example
Suppose you want:
- Center frequency: 100 kHz
- Bandwidth: 10 kHz
- Resistance: 50 Ω
The calculator returns approximately:
- Q ≈ 10
- L ≈ 795.8 µH
- C ≈ 3.183 nF
This gives a practical starting point for simulation (SPICE) and bench validation.
Practical design tips
- Component tolerance matters: 5% inductors and capacitors can shift center frequency significantly.
- Inductor Q matters: low-Q inductors increase loss and broaden the response.
- Layout is part of the circuit: parasitic capacitance and lead inductance can detune high-frequency designs.
- Check source/load interaction: loading can lower effective Q and alter cutoff points.
- Simulate first: use AC sweep in LTspice, ngspice, or similar tools before PCB/final assembly.
When this model is ideal vs. when it is not
This calculator assumes a simple linear series RLC model. It works very well for first-pass design and educational use. For precision RF filters, crystal filters, or multi-pole active topologies, additional effects must be modeled:
- Parasitic ESR, ESL, and self-resonance of components
- Temperature drift and bias dependence
- Coupling between nearby inductors/traces
- Impedance matching requirements across stages
Quick FAQ
What happens if bandwidth is very wide?
Q becomes low, selectivity decreases, and the response is less “peaked.” This can still be valid, but it may no longer behave like a narrowband resonator.
Can I use this for MHz-range filters?
Yes, as a starting point. At higher frequencies, parasitics and PCB layout become increasingly important.
Does this calculator include active op-amp band-pass filters?
No. This page is specifically for passive LC (series RLC) band-pass design.
If you want, you can now plug in your target values above and get instant LC results for your next filter build.