For a lot of experiments, knowledge of the Effective 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, 1kHz is usually specified for SSB telephony receiver sensitivity measurement.
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 2400Hz bandwidth soft 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, lacking the usual tapering off above 1kHz (a technique to cheat on sensitivity specs).
The spectrum plot was analysed with sl2enb to produce the following summary.
Locut 120Hz Filter -6dB response: 369-2624Hz=2256Hz. ENB=1777Hz with respect to gain at 1488Hz (passband centre frequency). ENB=1666Hz with respect to gain at 1273Hz (max gain frequency). ENB=2093Hz with respect to gain at 1000Hz.
The important figure is ENB=2093Hz wrt 1000Hz reference frequency. This means this admits the same amount of noise power as a rectangular response of 2088Hz width and gain the same as the actual filter at 1kHz.
For noise power calculations, the ENB of 2093Hz is 0.6dB lower than the nominal filter bandwidth of 2400Hz.