THE DESIGN BRIEF

Most engineers underestimate how little margin exists in FCC Part 15 Class B limits. Above 960 MHz, you have just 54 dBμV/m at 3 meters—that's only 0.5 mV/m. For perspective, a typical switching regulator clock harmonic at 2.4 GHz can easily generate 45-50 dBμV/m without proper layout techniques.

The critical design rule: keep your highest-energy switching frequencies below 200 MHz when possible. Every octave increase in frequency reduces your shielding effectiveness by approximately 6 dB for typical enclosures. If you must switch above 200 MHz, budget for 20-30 dB additional suppression through ferrite loading and copper pour strategies. Remember that Class B conducted limits (46-54 dBμV from 150 kHz-30 MHz) are measured per ANSI C63.4, so your LISN setup during design validation must match the test lab configuration exactly.

• • MIL-STD-461 CS101 power input susceptibility now requires testing up to 400 Hz for modern DoD platforms with variable-frequency drive systems
• • CISPR 25 automotive limits extend to 1 GHz for radiated emissions, with stripline coupling methods requiring 50Ω ±2Ω impedance control
• • IEC 60601-1-2 professional healthcare immunity levels demand 10 V/m field strength per IEC 61000-4-3, vs. 3 V/m for home use
• • Via stitching effectiveness drops below 40 dB isolation when via spacing exceeds λ/8 at your highest harmonic frequency