SBHT is based on the Analog Devices ADF7021-N Transciever and controlled by a STM32F4. Speaker and MIC circuits are on board to allow this data modem to also digitally transmit audio with various codecs either placed on the STM32F4 or off board AMBE codec DSP board. This was initially built and tested as modules (PA, SWITCH, LNA, FILTERS, TXRX, MCU, POWER, etc).
The current filter configuration is designed to work well in the 144Mhz - 148Mhz Amateur Radio Band. With filter modifications the design will work in other bands.
The PA module output is controlled by the STM32F4.
This design is an attempt to compromise size / performance. It is small and battery powered. The ADF7021-N is designed for low power operation but we'll see how well optimized the output can be and the PA is adjustable.
NOTE: The ADF7021-N is designed as a low power device. It not really well suited for high power amplification. If you build one of these be sure to check the output for spurs.