Measuring receiver bandwidth

Overview

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.

Procedure

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.

• Step 1: standardise the receiver filter settings (eg IF shift "normal", notch filters disabled, audio filters disabled, noise reduction / noise blankers disabled).
• Step2:  connect the receiver audio output to the PC sound card and set the level for substantial audio without risk of clipping. It is very important that the audio level is high, but that there is NO clipping. Clipping will create distortion that results in an increase in levels outside the filter passband.
• Step 3: with wideband noise input to the receiver, the audio output is connected to a PC sound card and a recording made of 30s duration at 11.025kHz sample rate, mono.
• Step 4: the recording is analysed using sox (sox $1.wav -n stat -freq 2>$1.dat) to produce a spectrum statistics file.
• Step 5: the sox output file is analysed by the Python script sox2enb.py to produce the ENB statistics and GNU plot file from which a frequency response file can be created.

set SOX=c:\Program Files (x86)\sox-14-4-2\sox.exe

"%SOX%" -r 11025 -c 1 -t waveaudio "Stereo Mix" new.wav trim 0 60

Fig 1 shows the Windows commands to record a 60s file of noise output from the receiver.

sox $1.wav -n trim 0 60 stat -freq 2>$1.dat

Fig 2 shows the command to create spectrum statistics with sox.

$ ./sox2enb.py -l 120 demo.dat

sox2enb.py
Locut 120Hz.
sox: bin_width_hz=2.692Hz
Filter -6dB response: 266-2708Hz=2441Hz.
ENB=2678Hz with respect to gain at 1483Hz (passband centre frequency).
ENB=1493Hz with respect to gain at 1906Hz (max gain frequency).
ENB=2579Hz with respect to gain at 1000Hz.

Fig 3 shows the response to the enb analysis of the sox stats file.

Figure 4: Receiver frequency response

Fig 4 shows the receiver response.

Resources

Python3 utility to read sox spectrum file to calculate Equivalent Noise Bandwidth

Receiver test results.

Links

Changes