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We’ve just got LinHT to work as a complete M17 transceiver – able to transmit and receive RF signals without the need to use SSH to issue any commands. This was not as easy as we expected, due to ALSA-related problems (Linux users probably know what I mean).

The issue was GNU Radio not allowing us to use SX1255 baseband source and sink within a single flowgraph. Just to remind you – the SX1255 is seen by the OS as a sound device. I and Q baseband branches are disguised as Left and Right audio channels 🙂 Andreas OE3ANC proposed a simple workaround – daemonized ZeroMQ publisher/subscriber proxy for both directions. Then, instead of using GNU Radio’s Audio Sink/Source blocks, the baseband is accessed through ZMQ PUB/SUB blocks. PTT signal is handled as GNU Radio’s native “Message PMT”.

Right now, using LinHT for M17 is as simple as turning the device on and a PTT press. The GUI daemon loads all the RF front-end and M17 settings from a YAML file. There is still a lot of work to do, especially to improve the receiver path, as the SX1255 is really sensitive and gets saturated easily. This is why we added a variable RF attenuator in Rev B.

None of the achievements or work mentioned above would be possible without our core contributors: Andreas OE3ANC and Vlastimil OK5VAS. Working on LinHT with you is an amazing journey!

A demo video will be shared later, while LinHT’s Revision B is still being prepared, so stay tuned!

Edit: this is how our M17 transceiver looks like, defined with a GNU Radio flowgraph. Two upper signal paths define the receiver, the last one (bottom) is the transmitter. Big shout out to Jean-Michel Friedt for all the help with gr-m17 development.

This is how the Rev B board looks like (WIP). PCB designed by Vlastimil OK5VAS.

There is still some more work to do, so stay tuned. Big shout out to everyone who donated to our Foundation. Every dollar counts: your donations help us do magnificent things.

After receiving a new 10W 40dB attenuator, I decided to give the GRF5604 a spin again. I connected it to my LinHT prototype (Revision A hardware, for the record) and supplied with 5.1V. I’ve set the SX1255’s DAC gain to 0dB and mixer gain to -13.5dB, to prevent the power amplifier from saturating.

Here’s a spectrum plot showing harmonics with the fundamental at 36dBm (M17 signal at 433.475MHz):

Harmonics are below -60dBc, which I would call a good result (the output filter is a MiniCircuits LFCN-490+). The peak to the left is the PLL reference leakthrough (32MHz).

Stay tuned for more news.