Introduction
The uSDX SSB transmitter utilizes digital direct frequency synthesis technology. After processing the input audio sample, it uses the SI5351 to control a frequency of 4800 Hz per second and PWM keying or PWM PA bias voltage adjustment to modulate the amplitude, ultimately reproducing the original signal.
In other words, the audio sampling rate of the uSDX is 4800 Hz, and its effective bandwidth is only 2400 Hz. Frequencies above 2400 Hz will experience Nyquist aliasing (it's best to strictly control the input audio so that it does not exceed 2400 Hz).
Here are some example images for a simple test:
The complete test video can be found at:https://www.bilibili.com/video/BV1iomwYbEQn/
FSK testing:
The transmit quality of both FT8 and Q65 is very good, and the spectrum is clean.
SSTV testing:
Scottie 1 test image
Black and white test images
Original image
Human voice and music testing:
(Refer to video) The spectrum starts to become noisy, and the transmission quality deteriorates, with interference starting to appear.
Summary
uSDX is an experimental radio designed for FSK and other modulation formats that require low sampling rates and small frequency variations. Its primary purpose is for learning, rather than for comparison with commercial radios.
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