Probably the most interesting facets of amateur radio in 2021 lie in the realm of digital modes. Using the limitless possibilities of software defined radios has freed digital radio communication from the limits of what could be done with analogue electronics alone, and as a result this is a rare field in which radio amateurs can still be ahead of the technological curve. On of these newer digital modes is FT8 created by the prolific [Joe Taylor K1JT].
And it’s for this mode that [Charles Hill] has created an easy-to-build transceiver. Its brains are aTeensy 3.6, while the receive side is a Si4735 receiver chip and the transmitter is a Si5351 programmable clock chip driving a Mini-Circuits GVA84 power amplifier with an appropriate filter. The interface is via a touchscreen display. It relies on existing work that applies a patch on-the-fly to the Si4735 receiver chip for SSB reception, and another project for the FT8 software.
The charm of this transceiver is that it can be assembled almost in its entirety from modules. Some radio amateurs might complain that homebrew radios should only use the most basic of components assembled from first principles, but the obvious answer to that should be that anything which makes radio construction easier is to be welcomed. If the 100 mW output power seems a bit low it’s worth remembering that FT8 is a weak signal mode, and given the right propagation conditions the world should be able to hear it despite the meagre output.
The Silicon Labs Si4735 is a single-chip solution for receiving AM, FM, and shortwave radio. With a bit of hacking, it even supports single sideband (SSB). All you’ve got to do is provide it with a suitable control interface, which [Ricardo Lima Caratti] has done with his recent project.
Using an Arduino Pro Mini, a handful of buttons, and a standard TFT display, [Ricardo] has put together a serviceable little receiver with a fairly impressive user interface. We especially like the horizontal bars indicating the signal to noise ratio and received signal strength. The next evolution would be to put this whole rig into some kind of enclosure, but for now he seems content to control the action with a handful of unlabeled buttons on a piece of perfboard.
Of course, the presentation of this receiver isn’t really the point; it’s more of a proof of concept. You see, [Ricardo] is the person who’s actually developed the library that allows you to control the Si4735 from your microcontroller of choice over I2C. He’s currently tested it with several members of the official (and not so official) Arduino family, as well as the ESP32.
The documentation [Ricardo] has put together for his MIT licensed Arduino Si4735 library is nothing short of phenomenal. Seriously, if all open source projects were documented even half as well as this one is, we’d all be a few notches closer to world peace. Even if you aren’t terribly interested in adding shortwave radio reception to your next project, you’ve got to browse his documentation just to see where the high water mark is.
It is getting harder and harder to tell homemade projects from commercial ones. A good case in point is [Mirko’s] all band radio which you can see in the video below the break. On the outside, it has a good looking case. On the inside, it uses a Si4730 radio which has excellent performance that would be hard to get with discrete components.
The chip contains two RF strips with AGC, built-in converters to go from analog to digital and back and also has a DSP onboard. The chip will do FM 64 to 108 MHz and can demodulate AM signals ranging from 153 kHz to 279 kHz, 520 kHz to 1.71 MHz, and 2.3 MHz to 26.1 MHz. It can even read RDS and RBDS for station information. The output can be digital (in several formats) or analog.
The Si47xx series of integrated circuits from Silicon Labs is a fascinating series of consumer broadcast radio products, chips that apply SDR technologies to deliver a range of functions that were once significantly more complex, with minimal external components and RF design trickery. [Kodera2t] was attracted to one of them, the Si4720, which boasts the unusual function of containing both a receiver and a transmitter for the FM broadcast band and is aimed at mobile phones and similar devices that send audio to an FM car radio. The result is a PCB with a complete transceiver controlled by an ATmega328 and sporting an OLED display, and an interesting introduction to these devices.
A look at the block diagram from the Si4720 reveals why it and its siblings are such intriguing devices. On-chip is an SDR complete in all respects including an antenna, which might set the radio enthusiasts among the Hackaday readership salivating were it not that the onboard DSP is not reprogrammable for any other purpose than the mode for which the chip is designed. The local oscillator also holds a disappointment, being limited only to the worldwide FM broadcast bands and not some of the more useful or interesting frequencies. There are however a host of other similar Silicon Labs receiver chips covering every conceivable broadcast band, so the experimenter at least has a good choice of receivers to work with.
If you need a small FM transmitter and have a cavalier attitude to spectral purity then it’s easy enough to use a Raspberry Pi or just build an FM bug. But this project opens up another option and gives a chance to experiment with a fascinating chip.