EMC Pre-Compliance Testing with PicoScope: Conducted Emissions Measurement in Your Lab

The Cost of EMC Failure

Formal EMC compliance testing at an accredited lab in India typically costs Rs 2–5 lakh per test campaign. When a product fails, the costs multiply: re-design, re-prototype, re-test. Each iteration adds weeks and lakhs to the project budget.

Pre-compliance testing, measuring your product’s electromagnetic emissions in your own lab before sending it for formal testing, catches problems early when they are inexpensive to fix. A PicoScope oscilloscope with its built-in FFT spectrum analyzer provides a practical pre-compliance tool for conducted emissions measurement.

What Are Conducted Emissions?

Conducted emissions are electromagnetic noise generated by your product that travels back through the power cord into the AC mains. Every switching power supply, motor driver, LED driver, and digital circuit generates some conducted noise. Standards like CISPR 32 (multimedia equipment), CISPR 25 (automotive), and IEC 61000-6-3 (generic) set limits on how much conducted noise is allowed in specific frequency ranges, typically 150 kHz to 30 MHz.

Measurement Setup

Equipment Required

1. PicoScope oscilloscope: models with at least 50 MHz bandwidth are suitable. The PicoScope 5000D or PicoScope 6000E provide the bandwidth and dynamic range needed for EMC measurement.

2. LISN (Line Impedance Stabilization Network): a standard 50 ohm / 50 uH LISN per CISPR 16. The LISN provides a defined impedance at the measurement point and isolates the measurement from ambient mains noise. LISNs are available from multiple suppliers; GSAS can recommend options compatible with your power level and frequency range.

3. Coaxial cable: 50 ohm BNC cable from LISN output to PicoScope input. Keep it short (under 1 metre) to minimize cable losses at higher frequencies.

4. Ground plane: a conductive metal sheet (copper or aluminium) under the DUT and LISN, bonded to the LISN ground terminal. This provides the ground reference required by the measurement standard.

Physical Setup

Place the LISN between the AC mains and the device under test (DUT). The LISN’s measurement output connects to the PicoScope via coaxial cable. The DUT, LISN, and PicoScope all sit on or connect to the ground reference plane.

Position the DUT at 40 cm from the ground plane edge and 80 cm from any walls or other equipment (per CISPR 16 guidelines for pre-compliance setups).

Measurement with PicoScope 7

Spectrum Mode Configuration

PicoScope 7’s spectrum mode converts time-domain captures to frequency-domain displays using FFT. Configure as follows:

• Frequency range: 150 kHz to 30 MHz (the standard conducted emissions band)
• FFT size: Maximum available (up to 1M points on PicoScope 6000E), larger FFT size provides finer frequency resolution
• Window function: Hanning or Blackman-Harris, these provide good frequency resolution with manageable spectral leakage
• Averaging: Enable spectrum averaging (16–64 averages) to reduce measurement noise and reveal the stable emission peaks

Taking the Measurement

1. Power on the DUT and let it reach steady-state operation
2. If the DUT has multiple operating modes, test each mode, emissions often vary with load and operating state
3. Capture the spectrum with averaging enabled
4. Record peak frequencies and amplitudes

Interpreting Results

Compare measured emission levels against the applicable CISPR limit line. PicoScope 7 does not include built-in CISPR limit lines, but you can:

• Export the spectrum data to CSV and plot against limit lines in a spreadsheet
• Use the cursor measurements in PicoScope 7 to read peak amplitudes at specific frequencies and compare manually

Key frequencies to watch:

• Switching frequency and harmonics: The fundamental switching frequency of the power supply and its harmonics (2x, 3x, 4x, etc.) are typically the strongest emissions
• 150 kHz – 500 kHz: Where CISPR limits are most stringent for quasi-peak detection
• FM broadcast band (88 – 108 MHz): Not within conducted emissions range, but radiated emissions in this band can cause consumer complaints

Margin Assessment

Pre-compliance measurements in a non-shielded lab environment have inherent uncertainty, typically 6–10 dB. To account for this:

• If your measurement shows emissions more than 10 dB below the limit, you are likely safe
• If emissions are within 6 dB of the limit, you are at risk, consider design changes
• If emissions exceed the limit, you will almost certainly fail formal testing

Common Fix Strategies

When pre-compliance testing reveals excessive conducted emissions:

• Input filter design: Add or modify the common-mode and differential-mode filters on the power input. Typical components: common-mode chokes, X-capacitors, Y-capacitors.
• Switching frequency adjustment: Moving the switching frequency can shift harmonics away from the most sensitive frequency bands.
• Layout optimization: Minimizing loop areas in the power switching circuit reduces the noise source itself.
• Snubber circuits: RC or RCD snubbers on power switches reduce high-frequency ringing that generates wideband emissions.

Why Buy PicoScope from GSAS

GSAS Micro Systems is an authorized Pico Technology partner. Our engineers support EMC pre-compliance setups, scope selection, probe configuration, LISN recommendations, and measurement technique guidance.

• INR invoicing with GST-compliant documentation
• Demo units at offices in Bengaluru, Hyderabad, Chennai, Pune, Mumbai, and Delhi NCR
• Application engineering for EMC measurement setup
• Near-field probe guidance for radiated emissions pre-compliance

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