Development of a RF signal generator for ionosonde transmitter using low-cost SDR

Instituto Geofísico del Perú (IGP), through its scientific facility the Jicamarca Radio Observatory (JRO), monitors the upper atmosphere with different instruments: radars, GNSS receivers, magnetometers, and others. An ionosonde is a type of HF radar system that uses multiple frequencies to examine the ionosphere and provides estimates of electron density. In this work, we present the development of a low-cost radio frequency signal generator to the ionosonde transmitter. This development is based on the SDR Red Pitaya Signal Lab 250-12, and was tested with the VIPIR system operating at IGP-JRO to monitor the ionosphere above Peru through vertical and oblique ionospheric soundings.

The generator consists of an SoC FPGA Zynq-7020 with a sampling frequency of 250 Msps. This device can transmit modulated signals with a frequency sweep ranging from 1 MHz to 60 MHz. The system is flexible enough to be operated with other CW radars. In the design process, we used the Vivado development environment from Xilinx-AMD. The hardware synthesis was based on the VHDL hardware description language, using a behavioral description style for the modules, such as the SPI controller, register map, numerically controlled oscillator (NCO), OOK modulator, multiplexer, synchronization module with a 10 MHz GPS clock input, and trigger for signal transmission initiation. Subsequently, we integrated all the mentioned modules and IP Core Clocking Wizard using the structural description style in the Vivado software. An embedded system was also used for register writing through the SPI protocol.

We tested the RF signal generator developed using the Vertical Incidence Pulsed Ionospheric Radar (VIPIR) located at Jicamarca. Pulsed signals were transmitted successfully, and vertical-sounding ionograms were measured using the VIPIR receiver. We also obtained oblique-sounding ionograms with USRP-based HF radio receivers developed at IGP-JRO. We deployed these receivers in four Peruvian cities (Ayacucho, Piura, Arequipa, and Cusco).