IEC 61260-1 Filter Design: Bandwidth, Rolloff, and Attenuation Limits
TL;DR
IEC 61260-1 specifies the filter response shape through a tolerance mask that defines acceptable magnitude response at any frequency. The passband must be flat within class-dependent limits: Class 1 allows ±0.3 dB at centre frequency, rising to ±0.5 dB at band edges. The stopband attenuation must reach specified minimums: at least -65 dB for 1/3 octave Class 1 filters at frequencies more than 4 octaves from centre. The rolloff between passband and stopband follows a defined slope. Filter order (typically 4th to 8th order Butterworth or similar) is chosen to meet these masks.
Tolerance Mask
IEC 61260-1 defines a tolerance mask as a set of attenuation limits at specified frequencies relative to the centre frequency. The mask specifies both maximum and minimum attenuation at each point, creating a "tunnel" through which the filter response must pass.
Passband Requirements
At the centre frequency, the filter gain must equal 0 dB within tight limits:
| Class | At centre freq | At band edges |
|---|---|---|
| 0 | ±0.15 dB | ±0.3 dB |
| 1 | ±0.3 dB | ±0.5 dB |
| 2 | ±0.5 dB | ±0.8 dB |
The band edges are the -3 dB points (half-power frequencies) defined by the bandwidth formula. The filter must be at -3 dB ± the class tolerance at these frequencies.
Stopband Attenuation
The minimum required attenuation increases with distance from the centre frequency:
| Offset from centre | 1/3 octave min atten (Class 1) | 1/1 octave min atten (Class 1) |
|---|---|---|
| ±1 octave | -42 dB | -18 dB |
| ±2 octaves | -62 dB | -42 dB |
| ±3 octaves | -65 dB | -62 dB |
| ±4+ octaves | -65 dB | -65 dB |
The -65 dB floor ensures that energy from distant frequency bands does not leak into the measurement. 1/3 octave filters require steeper rolloff than 1/1 octave because the bands are narrower.
Rolloff Shape
The transition between passband and stopband follows a prescribed slope. For 1/3 octave Class 1 filters, the rolloff must be approximately 60 dB/octave. Butterworth filters of order 6-8 typically meet this requirement. Higher-order filters provide steeper rolloff but may introduce phase distortion.
Filter Families
Common filter designs used to meet IEC 61260-1:
- Butterworth: Maximally flat passband, moderate rolloff. Order 6 meets Class 2, order 8 meets Class 1.
- Bessel: Linear phase (good time-domain response), but requires higher order for equivalent attenuation.
- Chebyshev: Steeper rolloff for given order, but passband ripple must stay within tolerance.
Digital Implementation
See digital filter considerations for how these analog prototype specifications translate to IIR or FFT-based real-time implementations. The SonaVyx RTA uses FFT-based power spectrum method with band energy summation, which inherently meets the response requirements when the FFT resolution is sufficient.
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Last updated: March 19, 2026