rfmeter

A native Linux desktop "cockpit" for the Chinese USB RF Power Meter V3 (USB VID 0483 / PID 5740, enumerates as a STM32 CDC ACM virtual serial port at 921600 baud).

The meter streams a single scalar power reading; this app shows it as a big numeric readout, a rolling 60 s time-series plot, a histogram of recent samples, and exposes the meter's 8 calibration "bands" with per-band frequency + offset.

Build & run

go build -ldflags="-s -w" -o rfmeter ./cmd/rfmeter
./rfmeter

Or via the Makefile: make (Linux binary), make windows (cross-build rfmeter.exe), make all, make run, make test, make clean. On Windows without make, use .\build.ps1 (same targets) — see Building on Windows.

Requires:

• Go 1.22+ (1.25+ pulled in transitively by go.bug.st/serial)
• System libs (Debian/Ubuntu): libxkbcommon-x11-dev libx11-xcb-dev (plus the normal X11 / Wayland / EGL devel packages Gio needs)
• User in the dialout group to read /dev/ttyACM*

On Windows the system libs and dialout group don't apply — see Building on Windows.

Tests

go test ./...

Parser, ring buffer, stats/histogram, and CSV logger have unit coverage. The UI is verified manually against the live meter.

How it works

• Auto-detect: on startup the app enumerates USB serial ports and connects to the first one matching the meter's VID/PID. If you unplug, the status dot turns red and a 1 s ticker reconnects on replug.
• Bands (F3–F10): the meter has 8 calibration entries, each with a frequency (MHz) and offset (dB). Pre-populated with the factory defaults (100, 200, 300, 400, 1000, 2000, 5000, 6000 MHz) on first launch; refreshed from the meter on connect. Clicking a band button (or pressing its F-key) re-sends the stored config to the meter, which switches its active calibration to that band.
• Edit band: type a new freq / offset and click Apply to update the currently selected band on the meter.
• Sample rate (S0/S1/S2): slow / medium / fast — sent to the meter as a serial command.
• CSV log (F11): writes rfmeter_YYYYMMDD_HHMMSS.csv to CWD with time_iso,dbm,linear_w,unit,page columns. Toggle to stop.
• PNG snapshot (F12): saves a 1920×1080 image of the current cockpit (readout + plot + histogram + stats).
• Attenuator helper (F2): given expected signal power and the meter's max safe input (default 0 dBm), computes the attenuation you need to add. Apply writes that as the current band's offset.

Caution

Use attenuators. Use attenuators. Use attenuators. The meter's safe input ceiling is 0 dBm (1 mW). Most real transmitters — handhelds, LoRa nodes, anything with a PA — put out +20 to +30 dBm (0.1–1 W) and will destroy the meter the instant you connect them. Always insert an attenuator (rated for the full source power) inline first. See Testing Meshtastic devices for sizing examples.

Guides

• Testing Meshtastic devices — measuring LoRa node TX power safely: W↔dBm table and attenuator examples.
• Building on Windows — cross-platform build, COM-port notes, and cross-compiling.

Keyboard

Key	Action
F1	Help modal
F2	Attenuator helper
F3..F10	Select band
F11	Toggle CSV log
F12	Save PNG snapshot
Space	Pause / resume the time-series plot
Esc	Close any open modal

Project layout

cmd/rfmeter/      main, wires Controller + Gio window
internal/meter/   serial I/O, frame parser, config parser
internal/state/   ring buffer of frames, rolling stats, CSV log
internal/ui/      Gio cockpit, widgets, modals, snapshot rendering
docs/             spec, plan, device photo, vendor docs

Protocol notes

• Streaming frame is 12 ASCII bytes: a±DDDLLLLL[uwm]A
  • ±DDD = dBm × 10 (e.g. -392 = -39.2 dBm)
  • LLLLL = linear power × 100
  • unit byte: u µW, m mW, w W
  • example: a-39200011uA = -39.2 dBm = 0.11 µW
• Commands end with \r\n:
  • S0/S1/S2 — sample rate
  • <A-H><freq><±off.o> — set/activate band (e.g. A2400+10.0)
  • Read — dump current 8-band config as R<freq><±off.o> × 8
• The firmware echoes recent host writes back into the stream; the parser is garbage-tolerant.

Vendor's original docs are in docs/rfmeter_doc.txt (plain text extract) and rf_power_meter_100khz_to10Ghz/Dock/USB-RF-Power-Meter.docx (extracted from the vendor zip).