Frequency Response
What is this?
To measure the audio response of your transmitter, you can generate an audio file, transmit it, receive it, record it and compare the spectrograms between the original and the recorded audio. Adjusting filters, equalisers, gain and other transceiver settings will result in the spectrogram changing.
Last edited: 11 February 2022
The software used to generate the images on this page is available from GitHub. It consists of several bash scripts that use SoX to generate and play audio sweeps, as well as generate the spectrograms shown below, and rtl_fm to record it.
The transmit hardware is a Yaesu FT-857d connected to a dummy load using 5 Watts FM.
The receive hardware is a Raspberry Pi connected to an RTL SDR v3 dongle with a telescopic rabbit ear antenna (that came with the dongle).
This project will work just as well using alternative hardware. Although I'm using the specified hardware, the aim is to get an audio file to play on a transmitter and to record it off air from a receiver. If you have digital modes like FT8 or WSPR working, you're most of the way there.
You can even use simpler tools than that. For example, you can play the audio file with your mobile phone into the microphone of your radio and record it off air with your computer, or a handheld recorder.
The examples are available for download.
Transmitted FM
The audio file for this spectrum is called double-sweep.48k.wav.
It represents 0 to 5 kHz audio over five seconds.
Transmitter: Yaesu FT-857d
Received FM
The resulting received audio has many visible artefacts, though they are of very low strength, they come from somewhere. The frequency response is pretty close to the original 5 kHz.
Receiver: RTLSDR
Transmitted FM
The audio file for this spectrum is called fm.double-sweep.48k.wav.
It represents 0 to 15 kHz audio over five seconds.
Transmitter: Yaesu FT-857d
Received FM
The resulting received audio has different artefacts, but notice that the maximum frequency is 9 kHz.
Receiver: RTLSDR
Transmitted FM
The audio file for this spectrum is called fm.double-sweep.48k.wav.
It represents 0 to 15 kHz audio over five seconds.
Transmitter: Yaesu FT-857d
Received FM
The resulting received audio is shown as a stereo signal, since the recorder generated both left and right. The receiver is a Yaesu FT-857d and the frequency response is 15 kHz.
Receiver: Yaesu FT-857d
Recorder: Zoom H5