The GG56tv spectrometer is an experiment developed by members of the SDR-BR amateur radio group. The experiment is located in the rural QTH of PY2GN (William) in the city of Pardinho (near the georaphic center of Sao Paulo state in Brazil). The system records a 48 kHz segment of the 40m Amateur Radio band using a quadrature radio receiver, processes the samples using Fourier analysis and Software Defined Radio (SDR) software, and presents the results via a web interface.

System Description

The spectrometer hardware consists of a SDRZero receiver and an inverted-V dipole antenna elevated about 5 meters above ground (to favour NVIS signals). The software used is a combination of generic Linux ALSA utilities, DttSP sdr-core (dttsp-ol version) and some tools and web software developed by myself for the application. A block diagram for the system is shown on Figure 1 below.

Figure 1 - Block Diagram

The system dataflow is as follows:

1. Audio samples are captured by the arecord ALSA utility and are stored in a .wav audio file containing 1 minute of samples recorded at a rate of 48 kilo samples per second using 16 bits per sample.
   
   
2. The .wav file is then processed by a utility writen in C that performs Fourier analysis of the samples and saves the power spectrum representation in a .png file. The results from the FFT are plotted on a spectrogram image where different color tones between black and white, with intermediate values in blue, represents different power levels. The software automatically adjusts the color contrast level to match the highest detected power and the noise floor. The images are created using the GD library.
   
   
3. The .wav file can be used further as input to the SDR processing software (sdr-core) that can, by user request, demodulate a segment of the sampled spectrum. The output of sdr-core is a .wav file that is then converted to MP3 and is presented to the user for download.
   
   

Some suggested uses are:

1. Online Spectrum Analyser for measuring and adjusting transmitters and antenna systems.
   
   
2. Online Radio Analyser for performing qualitative measurements and monitoring of radio transmissions.
   
   
3. Panoramic observation of ionosferic propagation of radio signals.
   
   

If you have any enhancement suggestions, comments, or alternative uses for the system, I would be very interested to know and would be glad to hear from you.

73,

Edson, PU1JTE, N1VTN, JF1AFN
ewpereira {at} gmail {dot} com