For a lot of experiments, knowledge of the Equivalent Noise Bandwidth (ENB) of a receiver is necessary. The ENB is the bandwidth of an ideal rectangular filter with the same gain as some reference frequency.
Though filters are often specified in terms of bandwidth at x dB down, that metric is of relatively little value, the x is often 6dB but not always, the filters depart significantly from ideal or even common response.
In brief, a white noise source is connected to the receiver input, Filter2 (nominal 500Hz bandwidth sharp response) selected and set to standard PBT, and the audio output captured on a PC based audio spectrum analyser, Spectrogram 16 in this case.
Spectrogram is set to integrate over 30s to average the variations due to the noise excitation. The resulting graph and text spectrum log are saved.
The method is explained in detail at Measure IF Bandwidth.
Above is the spectrum plots, as receivers go this is relatively flat.
The spectrum plot was analysed with sl2enb to produce the following summary.
Locut 80Hz Filter -6dB response: 342-853Hz=511Hz. ENB=529Hz with respect to gain at 603Hz (passband centre frequency). ENB=528Hz with respect to gain at 600Hz (specified).
The important figure is ENB=528Hz wrt 600Hz reference frequency. This means this admits the same amount of noise power as a rectangular response of 528Hz width and gain the same as the actual filter at 600Hz.
For noise power calculations, the ENB of 528Hz is 0.24dB higher than the nominal filter bandwidth of 500Hz.