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 the audio analysed using a PC spectrum analyser package, in this case Spectrublab. I have used averaging for a smoother line on which to make the measurements. I have exported the reference file and analysed it using the slfr.py Python script to calculate the equivalent noise bandwidth (547Hz) wrt to the gain at 602Hz (passband centre frequency).

Figure 1: SpectumLab long-term average configuration

Figure 1 shows configuration of the SpectrumLab long-term average.

Figure 2: Select reference

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: Spectrogram display of receiver audio output (IC706IIG SSB).

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.

slfr2enb.py -l 120 -c 600 Example500Hz.ref

slfr2enb.py
Locut 120Hz.
SpectrumLab: bin_width_hz=2.692Hz offset_hz=0.000Hz.
Filter -6dB response: 331-877Hz=546Hz.
ENB=547Hz with respect to gain at 602Hz (passband centre frequency).
ENB=490Hz with respect to gain at 767Hz (max gain frequency).
ENB=548Hz with respect to gain at 600Hz.

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.

Figure 5: Receiver frequency response

Fig 5 shows the receiver response from the reference file.

Resources

SpectrumLab settings file ENB.usr

Python3 utility to read SpectrumLab reference file to calculate Equivalent Noise Bandwidth

Receiver test results.

Links

Audio spectrum analysis

• SpectrumLab

Changes