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 (ENB) 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 configuration of the SpectrumLab long-term average.
Fig 2, go to the Freq-Resp tab and click the button to make the long-term average the reference.
With the receiver connected and levels setup, go back the the Spectrum (1) tab and press the clr button beside the long-term average button.
Figure 3 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.
Wait a minute and go back to the Freq-Resp tab and click the button to save the reference file.
The data in the saved reference file can be used to calculate the frequency response and ENB.
Fig 4 shows the response to the analysis of the SpectrumLab reference file. In this case, the ENB wrt gain at 600Hz (the nominal centre frequency) has been requested to use in convertion of sensitivity measurements at 600Hz baseband to noise figure.
Fig 5 shows the receiver response from the reference file.
Audio spectrum analysis
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