The developer built a test bench for a QRP SSB transmitter using the polar modulation method (EER). The hardware is based on an STM32F411 microcontroller, a Si5351 generator, and an RD16HHF1 transistor. The system operates on the 40-meter band with a 12V power supply.
The EER method separates the signal into phase and amplitude components. The phase is handled by the Si5351, while the amplitude is managed by a 100 kHz PWM modulator that adjusts the drain voltage of the output transistor. This approach allows the power amplifier to operate in class C or E with high efficiency.
The firmware is written in C using CMake and CMSIS-DSP. Communication with the PC is performed via a COM port using the Kenwood TS-480 CAT protocol, which is compatible with software like WSJT-X, JTDX, and fldigi. Settings are stored in the microcontroller's flash memory.
Current limitations include an 8-bit amplitude resolution and a 10-bit PWM duty cycle, resulting in a step of approximately 12 mV. Future development plans include designing a dedicated PCB, implementing PWM dithering, and adding envelope pre-correction to improve signal linearity.
