LinHT Rev B status: what works, what broke, and why Rev C is next – M17 Project

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LinHT Rev B status: what works, what broke, and why Rev C is next

Written by

Vlastimil Slinták

in

Hardware, LinHT

Before Friedrichshafen, we want to put the current LinHT development status in one place. We have published many short updates over the last months, and the project moved through several phases: Rev A proof-of-concept testing, Rev B design, manufacturing, first boot, RF tests, audio bring-up, GNSS debugging, and now Rev C planning.

This post summarizes that progress. It starts with what Rev A proved, then goes through what we changed in Rev B, how we built the first boards, what worked during testing, what failed, and what we need to fix before Rev C.

If you want the longer background, start with my older text about the first LinHT prototype: LinHT – Open SDR Handheld for Radio Amateurs. That article describes the Rev A board, the Retevis C62 donor-radio approach, and the basic idea behind LinHT: a Linux-based SDR handheld transceiver, open enough that the radio can be changed in software, not only programmed from a vendor tool.

Rev A was our proof of concept. It showed that the architecture made sense and that the device could work the way we imagined. An i.MX93 system-on-module could boot Linux inside a handheld radio body, run a Yocto image, expose USB networking, talk to the display, run GNU Radio flowgraphs, and use the SX1255 IQ RF front end. We decoded M17, tested TETRA receive, displayed spectrum data on the small screen, and ran enough hardware and software tests to be sure that the concept was valid.

Rev B is the revision where we tried to turn that proof of concept into a much more complete handheld.

What Rev B added

Rev B is still an experimental board, but it is a large step forward from Rev A.

The hardware changed a lot. Rev B has a redesigned power system, an ATtiny-based power-management controller, USB-C battery charging through a BQ25792 charger, an integrated GRF5604 RF power amplifier, RF TX/RX switching, two PE4312 programmable RF attenuators in the receive path, a GNSS module, and a cleaner layout with improved grounding. The board is still designed to fit inside the Retevis C62 enclosure and reuse selected donor parts from the original radio.

The first Rev B boards. Fully assembled in the Czech Republic, EU.

We also registered linux-radio.eu and pointed it at the current LinHT hardware documentation. The page contains the hardware overview, generated manufacturing data, schematics, firmware links, and other project files. The hardware repository remains here: M17-Project/LinHT-hw.

Some parts of Rev B were tested before they were integrated into the main board. For the GRF5604 PA, we designed a separate development board and measured it first. The earlier PA tests are described in the first amplifier tests and part II. On the software side, gr-m17, Codec 2 blocks, GUI work, lower-CPU receive flowgraphs, and updated Yocto images continued while we were waiting for PCBs and assembly. Some of that work was described in the Codec 2 block update, the LinHT internal structure article, and the Yocto image update.

Rev B board became much more complex, with many new features and components, and it is assembled on both sides. It brought together the parts needed for a practical radio: power sequencing, charging, RF output power, receive-path control, GNSS, Linux integration, and a manufacturing process we can repeat.

LinHT Rev B is a replacement mainboard for the Retevis C62 handheld. Some parts still come from the donor radio: the display, side button PCB, SMA connector, jacks, encoder, springs, and mechanical hardware.

Building the first Rev B boards

We originally expected to build the Rev B prototypes in China, as we did with Rev A. The prices were high, customs and import handling were cumbersome, and importing the Compulab’s SoM into China and getting the boards assembled there was quoted at a price that did not make sense for a small prototype run.

Since we already wanted future LinHT revisions, especially larger runs, to be manufactured and assembled in Europe, Rev B simply moved that plan forward. The first Rev B prototypes were assembled locally in the Czech Republic, and we plan to use the same assembly path for Rev C.

The production work itself was normal hardware development: PCB manufacturing, stencils, component sourcing, substitutions, and checks before assembly. The BOM is large enough that sourcing is not trivial. Some parts were only available from specific suppliers, some had to come from Mouser and others from LCSC, and two parts escaped our attention until late because they were already obsolete or difficult to source. Rev C has to replace them.

We ordered 10 PCBs and stencils from JLCPCB in late January. The assembled boards were ready near the end of February, and the first public post went out as LinHT boards have arrived!.

Close-up of one...