OpenRTX MD-380 M17 modulation test

Описание: Paul KB5MU demonstrates the M17 transmit modulation from a TYT MD-380 handheld transceiver modified with open source OpenRTX firmware. Versions from April 30 and May 5, 2022, are compared.

Both versions display an anomaly during the first few seconds of the first transmission after powerup. This anomaly is clearly seen on a Rigol RSA5065A spectrum analyzer.

The Rigol instrument also has a Vector Signal Analyzer mode, which displays a one-dimensional constellation diagram for M17's 4FSK modulation. This analysis clearly shows that the April 30 firmware version produces inadequate frequency deviation, about one-third of nominal. The May 5 firmware version corrects this problem.

The VSA analysis also shows that the signal from the May 5 version is not as clean as it might be. A signal from a reference GNU Radio flow graph is shown for comparison. The reference signal has much more precise modulation of the four nominal tones. This may point to a further issue with the OpenRTX firmware, or a limitation of the MD-380 hardware, or perhaps a weakness in the test procedure due to the much stronger signal from the MD-380 as compared to the ADALM Pluto SDR used to transmit the reference signal.

OpenRTX: https://openrtx.org

OpenRTX firmware for M17 on MD-380: https://github.com/OpenRTX/OpenRTX/

Hardware modifications required for OpenRTX on MD-380: https://openrtx.org/#/md380_mods

M17 Project: https://m17project.org

Open Research Institute: https://m17project.org