Development of a Low Resource, Inexpensive, High Frequency Chirpsounder

Ionospheric chirpsounders produce a linear frequency modulated sweep that typically span the full high frequency radio frequency band (3-30 MHz). These sounders are able to produce oblique ionograms many thousands of kilometers from the transmitter. The chirpsounder had its genesis in the development of frequency synthesizers that must be coupled with a similar and stable time base. More recently, software defined radios, such as the Ettus N2XX series, have been used passively to collect chirpsounder observations and produce ionograms. While it is conceivable that the Ettus N2XX or other series could be used to generate chirp signals spanning the full HF band, there have been few articles that have demonstrated this capability.

Recently, there has been a proliferation of single chip systems that have been used for frequency synthesis, most notable the Analog Devices AD9850 and Silicon Labs Si5351 chips. A more capable chip is the AD9910, which is a direct digital synthesis chip that is able to produce frequency sweeps, along with other waveforms at a suitable sampling rate in the HF frequency band. A commercial-off-the-shelf version of this synthesizer combined with an Arduino is available.

The purpose of this investigation is to demonstrate a simple chirpsounder using the AD9910 as the frequency sythsizer, along with the associated RF hardware. We show results of our implementation in the laboratory and also results from a brief field tests where we further perform radar signal processing to determine the associated range resolution from this set up. Our long term objective is to make it possible to build very low power and low cost HF chirpsounder.