SDR Without Complicated RF

Software-defined radio (SDR) is a hot topic right now in the engineering world. An SDR system moves as much of the analog RF circuitry into the digital domain such that it can be easily reconfigured. As a result, the SDR system can handle various systems and protocols, all within the same hardware. The systems can be highly complex, however they offer a great learning platform for topics such as radio frequency and digital signal processing. This is great, but if you are not familiar with either of the topics, it can be hard to learn about a single one when you need to know a little bit about both to operate a system. This is what motivated Damian Bonicatto to create an SDR setup without RF!

The idea for the project came from a need to send wireless data from a solar system to a shop located about 150 feet away. Wi-Fi was the first method to come to mind to accomplish the task, though with dozens of other devices already running on Wi-Fi, the decision was made to find an alternative method to avoid over crowding the Wi-Fi spectrum. A system utilizing ultrasonic frequencies (about 20KHz to 2MHz) offers a relatively clean spectrum, and more importantly eliminates a lot of the difficulties found when working with RF such as impedance matching, component parasitics, and RF PCB layout techniques. In addition, it eliminates the need for RF test equipment which can easily cost tens of thousands of dollars. All the signals in the system can be viewed and measured using an inexpensive low frequency oscilloscope.

The lower frequencies used in the system greatly simplifies the system design. An Arduino Nano is employed for all the signal processing functions as well as for analog to digital conversion. The receive circuitry consists of only a transducer, amplifier, and bandpass filter cascade while the transmit portion consists of a digital to analog converter, amplifier, high pass filter, and transducer. The filters and amplifiers are implemented using op amp circuits such as Sallen-Key filters with gain. Furthermore, a 3D-printable parabolic dish is used in combination with the transducers, which results in about a 9dB improvement in reception.

Using the Arduino as the core processing chip comes with certain constraints such as a small amount of RAM, a large tolerance clock, and lack of floating point. However, with these constraints in mind the firmware can still be efficiently designed. The carrier frequency of the system is 40KHz with a sample rate of 320ksps for the data-converters. This is eight times the carrier frequency with a baud rate of 250. It is a modest data rate, but is sufficient for the end use case of sending small amounts of data between a solar system and shop.

The code available for the Nano contains everything needed for processing the transmit and receive data. This includes the following:

• Retrieving packet to send
• Processing the packet
• Creating modulated symbols
• Breaking the packet up into sub symbols
• Transmitting the packet
• Receiving the packet
• Printing the received information to a serial port

Different types of modulations are included such as on-off keying (OOK), binary phase shift keying (BPSK), no modulation, and a noise option.

The ultrasonic transducer system, also known as the SDU-X can be fully recreated using all the information found on the Thingiverse website. This includes the source code, schematics, PCB design, STL files for the 3D printed parts, as well as notes on the design. More information on the system can also be found through Bonicatto's original article published on EDN. The entire project is very well documented and offers a great low-cost method of beginning to work in the exciting area of SDR.