July 16, 2018

I thought the LoRa SensorTile (23 mm x 23 mm) was about as small as I could go using the CMWX1ZZABZ LoRa module but the Gnat at 20 mm x 20 mm is 3/4 of the area of the LoRa SensorTile.

The Gnat uses the same Murata CMWX1ZZABZ module with STM32L082 32 MHz Cortex M0+ host and SX1276 LoRa radio, of course, and includes the smaller U-Blox MAX M8Q (sibling of the CAM M8Q on Cricket) as concurrent GNSS engine. There is a BMA280 accelerometer for wake-on-motion/sleep-on-no-motion control of the GNSS engine and four GPIOs exposed at the board edge (2 digital/SWD ports and 2 analog ports) but no battery charger, SPI flash, BME280, VEML6040 or the other ~10 GPIOs of the LoRa Sensor Tile or Cricket devices.

The MAX M8Q requires an active antenna since the internal chip antenna is absent and the ground plane of the Gnat is simply to small to support a chip antenna solution anyway. The CMWX1ZZABZ requires a whip antenna like the Cricket and LoRa SensorTiles do, although I am experimenting with a longer (80 mm x 23 mm) version of the Cricket that has 1) a chip antenna for the CMWX1ZZABZ like the Grasshopper uses and 2) an extended ground plane (71 mm from 46 mm in the Cricket) to increase the sensitivity of the CAM M8Q chip antenna.

I am testing the power usage now to see if I can get as good performance out of the chip antenna as I do with the active patch antenna. If this longer design works well enough, I might have a small production run made. It is very convenient in some applications (animal tracking, for example) to use an asset tracker that requires no additional antennas attached to the board.

So Gnat is a (slightly) reduced-capability mashup of the Cricket and LoRa Sensor Tile devices in a form factor that is approaching the limit of what can be done to shrink the size of any device using the CMWX1ZZABZ module. It is useful for those applications that require accurate location tracking with periodic reporting via LoRa(WAN) but in the smallest weight and form factor (volume, really) possible.