HC12 Si4463 based radio with STM8S003F3 processor. This low cost radio module(£3) is available preprogrammed for remote control and monitoring applications. Though normally used with the preprogrammed code for a serial interface they can be reprogrammed with different code for our tracker. The STM8S0003F3 is fairly powerful, but has limited flash memory(8K) and RAM(1K) so fitting the code is challenge. Mass is 1g.
The HC12 module appears to have originally designed an produced by HC01 company www.HC01.com. Subsequently it appears the HC12 has been cloned by a number of other companies with some changes along the way. The primary changes appear to be the use of the 20 pin QFN processor package instead of the 20 pin TSSOP package, and the use of the Si4438 radio instead of the Si4463. The Si4438 is pin and firmware compatible with the Si4463 but has a frequency range limited to the 433MHz band which is not a problem for us. The clone HC12 modules have come under a lot of criticism for poor performance and devices from different suppliers not interworking. The Processor and Radio ICs appear to be genuine manufacturers parts and the issues seem to be related to the poor quality of the crystal used for the radio IC. Crystal Fail!
The Silabs SI4463 Radio IC has been successfully used in a number of existing HAB tracker designs including those from Bristol SEDS and PecanPico. The Si4463 IC is capable of operation over wide frequency range from 119MHz to 1050MHz, but implementations usually limit operation to a single band. In our case we use the 433MHz band but other implementations with different RF configurations and component values can operate in the 144MHz and 868MHz bands. The Si4438 IC is limited to frequencies between 425 and 525 Mhz, and has fewer modulation options. The Si4463 is a transceiver but at this time we are only using the transmit functions.
As quoted above many users have had problems with the crystals used on the clone HC12 module which result in the module being off frequency. We discovered that the crystal problem also manifested itself in wide frequency drift during transmission. Replacement of the crystal on the HC12 module resolved this issue.