GuidoOverview
Now that the whole project works pretty well as a receiver, including FT8 decoding, it's time to transmit anything.
To understand the FT8 protocol, see FT4_FT8
MYJTDX software (forked from JTDX forked from WSJT-X) decodes well and it is ready to transmit and manage QSO's. Its way to transmit is to send an 8FSK modulated tone at 1500Hz out to the audio out card (to the PL of Zynq7000). This tone would SSB modulate an HF transmitter.
E.g. 8FSK 1500Hz modulates in USB a 7074000 Hz transmitter, obtaining a 7075500 Hz 8FSK modulated carrier.
Now I see two options
1) Follow the JTDX way - external DAC, sending the 1500Hz 8FSK modulated tone to a USB Weaver modulator.
1500 Hz 8 FSK audio samples from PS to PL -> Digital Weaver USB modulator -> hardware DAC -> hardware HF amplifier -> Low Pass Filter -> Antenna
2) Use an external DDS, programming its output frequency to produce 8FSK and amplitude to generate the ramps (raised cosine initial amplitude increase and final amplitude decrease) of FT8.
174 bits encoded sequence from PS to PL -> external AD9851 DDS -> hardware HF amplifier -> Low Pass Filter -> Antenna
For AD9851 Amplitude Modulation see this
PRO'S and CON'S
| Follow the JTDX way | Use an external DDS | |
| Audio stream / Amount of data | +++ The audio stream is produced by JTDX! --- The audio stream must be transferred from PS to PL via DMA or similar --- About 300 KBytes must be transferred from PS to PL every 15 seconds | There's no audio stream +++ Only 174 bits (produced by JTDX) must be transferred from PS to PL every 15 seconds |
| Available HW boards | --- I didn't find any high speed DAC board | +++ I found AD9851 (DDS + DAC) board for around 30 Euros or AD9850 for 20 Euros |
| JTDX addition | +++ None | --- 3 bits FT8 symbol to 40 bits AD9851 programming word |
| Vivado design | --- Weaver modulator (2 x DDS compiler + 2 audio FIR filters + 4 multipliers) | +++ 40 bits serial output with clock, word write and reset --- PWM amplitude modulator for FT8 ramp up and down. |
| Pins Usage | --- Zynq7010 12 pins output (10 bits data + clock + strobe) | +++ Zynq7010 4 pins (serial data, clock, word write, PWM amplitude) |
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