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www.arrl.org QST June 2019 1
Alberto di Bene, I2PHD, devel-oped a fascinating software-defined receiver based on an STM32F429ZIT6 “Discovery Board” manufactured by STMicroelectronics. These boards are designed for experi-menters and even include color LCD displays. Best of all, they are surprisingly inexpensive. At the time of this writing, Digi-Key was selling them for only $29.95 (part no. 497-16140-ND; www.digikey.com).
The board features an ARM microprocessor that sports three 12-bit analog-to-digital converters (ADCs), each capa-ble of a sampling frequency of 2.4 MHz. Considering its capabilities, Alberto realized that the Discovery board could be turned into a software-defined radio receiver for low- and medium-wave frequencies. He needed to add only two outboard cir-cuits, both of which are quite simple.
Anti-Aliasing and DAC “Reconstruction” FiltersYou could connect the Discovery Board’s ADC directly to an antenna, but it wouldn’t work well. For proper operation, the board needs an anti-aliasing filter between the ADC input and the antenna. Alberto designed his filter to attenuate all frequency components greater than 893 kHz, which sets the upper limit of recep-tion. He also included a 10 dB pre-amplifier. The filter consists of less than 20 components.
At the other send of the process, the audio output is intended to be fed to an amplified speaker. Before you can do that, however, the output of the digital-to-analog converter (DAC)
Eclectic TechnologySteve Ford, WB8IMY, [email protected]
must be filtered to remove the sam-pling frequency component. Alberto calls this his “reconstruction filter.” Once again, it is a simple design with less than 15 components.
Alberto has posted a detailed PDF document on his website at www.weaksignals.com, where you’ll find the filter diagrams and much more.
SoftwareOn Alberto’s website, you’ll also find the source code (written in C) that enables the Discovery Board to work its magic. Be sure to read the “Read Me First” file after downloading the code. In there, you will discover that you’ll need to compile the code using a Keil MDK compiler, available at www2.keil.com/mdk5/, and add the legacy support packs. Once the object code is ready, you can load it to the ARM chip’s flash memory using the ST-Link utility found here: www.st.com/en/development-tools /stsw-link004.html#.
When the code is loaded, and power is applied to the radio, you’re rewarded with an SDR receiver that covers from 8 to about 900 kHz, with AM, LSB, USB, and CW demodula-tion modes, narrow and wide band-widths, and fast or slow automatic gain control. As you can see in Figure 1, the LCD touchscreen is nicely designed with pushbutton con-trols, a large frequency display, and an audio volume slider.
On his website, Alberto provides a few audio recordings so that you can hear how well his ARM Radio per-forms. In fact, Alberto entered the project in an ARM microcontroller design contest.
Depending on where you purchase your parts, you could probably build your own ARM Radio for less than $100 — probably much less. It could even become part of a station for 2200 and 630 meters.
Welcome to 21st century homebrewing!
Figure 1 — Alberto di Bene’s ARM Radio, an inexpensive software-defined receiver that covers 8 to 900 kHz. Full details are available at www.weaksignals.com. [Alberto di Bene, I2PHD, photo]
