HARDWARE

TX

• Raspberry Pi Zero 2 W sends out the AFSK modulated tones (Bell-202 1200 & 2200 Hz) to its I2S interface, connected to the DAC MAX98357.

• The audio out of the DAC is low passed, regulated by a variable resistor and sent to the SA868S modulation input. 

• SA868S outputs a power of around 2W, narrow band FM modulated.  

RX

• It uses an RTL-SDR USB dongle, with EMI filters on both RX and USB paths to reduce noise and improve RF performance.
• The received packets are decoded and shown on the web interface.

RX-TX switch

• It is built using a micro relay shielded inside a tin plated box.
• It is driven by the PTT (Push to Talk) output.

SOFTWARE

The main features are:

• the excellent APRS packet decoder/encoder of Direwolf
• (new) a good demodulator 
• (new) an accurate RSSI and SNR measurement, before the FM demodulator
• (new) a WEB interface to monitor RX performances in a easy way 

To obtain these (new) features you need to build Direwolf-IQ from source (branch SoapySDRToDirewolf)  

For a complete Direwolf-IQ description, see my Github repository soapysdr_to_direwolf