Noise Bandwidth
Understanding Noise Bandwidth
Noise bandwidth is often confused with 3 dB bandwidth, but they are different quantities. NBW accounts for the total noise passed by the filter's skirts beyond the 3 dB points, which contributes to the noise floor.
Noise BW Correction Factors
| Filter Type | NBW / BW_3dB |
|---|---|
| Ideal brick-wall | 1.000 |
| 1st-order RC | 1.571 |
| 2nd-order Butterworth | 1.111 |
| 4th-order Butterworth | 1.026 |
| Gaussian | 1.065 |
| 5-pole Chebyshev (0.1 dB) | 1.010 |
Frequently Asked Questions
What is noise bandwidth?
NBW is the equivalent rectangular bandwidth passing the same noise as the actual filter. NBW >= BW_3dB because filter skirts pass additional noise. Used in sensitivity: MDS = -174 + NF + 10*log(NBW).
Does filter shape matter for sensitivity?
Yes. A 1st-order filter has NBW = 1.57 x BW_3dB, passing 2 dB more noise than a brick-wall filter. Higher-order filters approach NBW = BW_3dB. For accurate sensitivity calculation, use NBW, not BW_3dB.
How is noise bandwidth measured?
Integrate the filter power response over all frequencies and divide by the peak response: NBW = integral(|H(f)|^2 df) / |H(f_peak)|^2. The result is the width of a rectangle with equal area and equal peak height.