Practical case: RC filter to smooth noise

Materials

• 1× Breadboard and jumper wires
• 1× Resistor R1 = 1 kΩ (±5% or better)
• 1× Capacitor C1 = 100 nF (film or ceramic, 50 V rating or higher)
• 1× Function generator (noise or sine + DC offset capability)
• 1× Oscilloscope (2 channels) and probes
• 1× Digital multimeter (optional, AC mode for ripple)

Wiring guide

• Place R1 in series between the input node (Vin) and the output node (Vout).
• Connect C1 from Vout to the common ground (GND).
• Connect the function generator output to Vin and its ground to GND.
• Oscilloscope:
• CH1 on Vin at the ●CH1 dot; CH1 ground clip to ●REF (GND).
• CH2 on Vout at the ●CH2 dot; CH2 ground clip to ●REF (GND).
• Abbreviations used:
• Vin: Noisy DC input node from the generator.
• Vout: Filtered output node after R1.
• CH1, CH2: Oscilloscope channels 1 and 2.
• REF: Ground reference point (GND).

Schematic

        Generador de señales (seno + ruido)
            (1 Vpp, 0 V offset)

Vin ──────●CH1───────┬
                       │
                      ┌┴┐
                      │ │   R1 = 1 kΩ (serie)
                      │ │
                      └┬┘
                       │
                     Vout ──●CH2
                       │
                      ┌┴┐
                      │ │   C1 = 100 nF a GND
                      │ │
                      └┬┘
                       │
                       ├──────────── Tierra del generador
                       │
───────────────────────┴─────────────────────────── GND

Measurements and tests

• Initial setup:

  • Set the function generator to a sine wave at 20 kHz, 0.2 Vpp, with a DC offset of +2.5 V (this creates “DC + noise” at Vin).
  • Confirm both oscilloscope probes have their ground clips on ●REF.

• Observe waveforms:

  • On CH1 (Vin), you should see a 2.5 V DC level with a 20 kHz ripple of about 0.2 Vpp.
  • On CH2 (Vout), the 20 kHz ripple should be noticeably smaller, while the DC level remains close to 2.5 V.

• Quantify attenuation:

  • Measure ripple amplitude on CH1 and CH2 by using the oscilloscope’s Vpp measurement.
  • Expected: For R1 = 1 kΩ and C1 = 100 nF, fc ≈ 1/(2πRC) ≈ 1.6 kHz. At 20 kHz, attenuation is roughly 1/√(1+(20k/1.6k)²) ≈ 1/12.6 (≈ −22 dB). So 0.2 Vpp in might drop to ≈ 16 mVpp out.

• Sweep frequency:

  • Sweep the generator frequency from 100 Hz to 200 kHz (keep 0.2 Vpp, 2.5 V offset).
  • Note the frequency where CH2 ripple reaches 0.707 of CH1 ripple (−3 dB point). This should be near fc ≈ 1.6 kHz.

• Optional DMM check (ripple estimate):

  • Put the DMM in AC mode between Vout and GND (●CH2 to ●REF).
  • The reading should be much lower than the AC reading at Vin, confirming ripple reduction.

Understanding the behavior

• The RC low-pass passes DC (desired) while attenuating high-frequency components (noise).
• Increasing C1 or R1 lowers the cutoff frequency, improving high-frequency noise suppression but slowing response to changes in the DC level.

Common mistakes

• Using a polarized electrolytic for C1 with incorrect polarity to ground; for small values like 100 nF, use ceramic/film (non-polarized).
• Forgetting to connect the generator ground to the circuit GND (●REF), causing erratic measurements.
• Probing without a common reference: always clip probe grounds to ●REF.

Safety and good practices

• Keep leads short to minimize pickup of additional noise.
• Start with small signal amplitudes; avoid overdriving the oscilloscope input.
• If you swap to larger capacitors (e.g., 10 µF), ensure their voltage rating safely exceeds the DC level.

Improvements and variations

• Change C1 to 10 µF to better suppress low-frequency ripple (fc ≈ 16 Hz with R1 = 1 kΩ).
• Cascade two RC stages (R1–C1 followed by R2–C2) for greater attenuation.
• Replace R1 with a ferrite bead for better high-frequency suppression in power supply decoupling scenarios.

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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