Measuring receiver bandwidth
The bandwidth of a receiver determines the total power that reaches the detector from a wideband source of noise or interference. The response of receivers is not ideal, and knowledge of the Equivalent Noise Bandwidth is important to measurement of wideband noise and interference.
Mathematically, it can be written as follows.
Avo is the reference audio voltage or the audio voltage at a reference frequency. In some sense it might seem natural to choose Avo to be the maximum of Av(f), but the purposes of this article is to find an equivalent noise bandwidth that allows determination of receiver Noise Figure from a standard sensitivity measurement or specification that establishes the S/N, (S+N)/N or SINAD ratio at 1kHz, AVo is taken to mean the audio voltage at 1kHz.
This article describes how to measure the bandwidth of an SSB receiver using a PC based audio spectrum analyser.
The IF filter is the dominant determinant of the end to end frequency response of an SSB receiver. Audio shaping commonly employed can modify the response, typically applying a slope across the filter passband.
Figure 1 shows the display of the receiver audio response. Not the substantial difference between the level within the pass-band and above the pass-band. Lack of audio level will result in too small a difference, and too much audio (clipping) will also result in too small a difference. Adjust audio level for maximum difference between the in-band and out-of-band level.
Figure 2 shows the receiver frequency response plotted on a linear power axis, and the response of an equivalent ideal filter with the same gain as the actual filter at 1kHz (where sensitivity measurements are usually made). The equivalent filter has the same total noise power admitted (the area under the red line) as the total noise power admitted (the area under the blue line) by the actual filter.
Perl and Python3 utility to read Spectrogram spectrum log to calculate Equivalent Noise Bandwidth
Example spectrum log from Spectrogram 11 for IC7400 with 2.4kHz SSB filter setting
Excel spreadsheet to perform ENB calcs from data extracted from Spectrogram 11 log.
Audio spectrum analysis
Windows is not supplied with a good, high productivity, flexible, rich interpretive scripting language. Perl fills that gap very well.
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