Practical case: Inductor in low-pass filter for audio

Materials

• 1x L1: 10 mH inductor (low DCR, ≥100 mA current rating)
• 1x R1: 100 Ω, 0.25 W resistor (acts as load/simulated speaker)
• 1x Audio signal generator (sine, 20 Hz–20 kHz, 1 Vpp, 0 V offset)
• 1x Oscilloscope with 2 channels (CH1, CH2) or 2 multimeters in AC mode
• 1x Breadboard or terminal block
• 6x Jumper wires (or clip leads)
• Optional: 1x BNC-to-clip lead cable for the generator

Wiring guide

• Place R1 (100 Ω) so one end will connect to the output node and the other to GND.
• Place L1 (10 mH) so it will be in series between the generator output and the output node.
• Connect the signal generator output (hot) to the left node that feeds L1 (this is VIN).
• Connect the far end of L1 to the output node (this is VOUT).
• Connect the top of R1 to the same output node (VOUT).
• Connect the bottom of R1 to the generator ground, then to GND.
• Connect oscilloscope:
• CH1 probe tip to VIN; CH1 reference to GND.
• CH2 probe tip to VOUT; CH2 reference to GND.
• Abbreviations used:
• VIN: input voltage node from the generator referenced to GND (measure at the ●VIN dot).
• VOUT: output node after the inductor, referenced to GND (measure at the ●VOUT dot).

Schematic

              GEN (seno 100 Hz–10 kHz)         L1 1.2 mH                SPK1 8 Ω
            ┌─────────┐                      ┌─────────┐              ┌─────────┐
            │         │──────────●Vin────────│         │─────●Vout────│         │
            │         │                      │         │              │         │
            │         │                      │         │              │         │
            └─────────┘                      └─────────┘              └─────────┘
                 │                                 │                      │
                 │                                 │                      │
─────────────────┴─────────────────────────────────┴──────────────────────┴──────
                                                                           GND

Measurements and tests

• Expected cutoff:
  • For RL low-pass, fc ≈ R1 / (2πL1) ≈ 100 / (2π·0.01) ≈ 1.59 kHz.
• Initial check:
  • Set generator to 1 kHz, sine, 1 Vpp, 0 V offset.
  • Verify VIN ≈ 1 Vpp and VOUT is close to VIN (slight drop is normal due to L1’s DCR).
• Frequency sweep (magnitude):
  • Sweep 100 Hz → 10 kHz.
  • Below ~500 Hz: observe VOUT ≈ VIN (passband).
  • Near 1.6 kHz: VOUT ≈ 0.707·VIN (−3 dB point).
  • Above ~5 kHz: VOUT should fall ~20 dB/decade (first-order roll-off).
• Phase observation (oscilloscope):
  • Compare CH1 (VIN) and CH2 (VOUT).
  • Near fc, VOUT lags VIN by ~45°; above fc, increasing lag tends toward 90°.
• Stability and loading:
  • Try replacing R1 with an 8 Ω speaker if available; recalculate fc ≈ 8/(2π·0.01) ≈ 127 Hz.
  • Note: lower R shifts fc downward and increases current; reduce VIN if needed.

Common mistakes and tips

• Using too small an inductor value: fc shifts too high; use ~10 mH with 100 Ω load to keep fc in the audio band.
• High inductor DCR: causes extra attenuation even at low frequency; prefer low-DCR inductors.
• Loose or long leads: add parasitic resistance/inductance; keep wiring short and neat.
• Ground reference errors: ensure both scope channels and generator share the same GND point.
• Overdriving loads: if using a real speaker (low Ω), lower the generator amplitude to avoid overheating the inductor or generator output.

Improvements

• Add a small series resistor (e.g., 1–2 Ω) with L1 to limit current spikes; update fc accordingly.
• Use a power inductor rated well above expected RMS current to minimize saturation and distortion.
• Implement a 2nd-order filter by adding a shunt capacitor across R1 (forming an L–C low-pass) for steeper attenuation.

Validation: The schematic includes +V at the top and GND at the bottom, all components are labeled and fully connected, and measurement dots (●VIN, ●VOUT) are placed on the circuit with abbreviations explained in the Wiring guide.

More Practical Cases on Prometeo.blog

Carlos Núñez Zorrilla

Electronics & Computer Engineer

Telecommunications Electronics Engineer and Computer Engineer (official degrees in Spain).

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