Well for FM, I believe they are just adjusting a PLL's output frequency by loading new divider values.
The output spectrum contains spurs and harmonics, so it's not a pure RF tone. So if one actually wanted to transmit into the air an appropriate bandpass filter would be needed. But for playing with test equipent or an SDR its no big deal.
How they create AM or SSB, does feel a bit like voodoo at this point. It would seem they have devised a method to go from IQ samples to a GPIO pin state, i'm not entirely sure what that involves, must be magic!
An impressive amount of fun for just soldering a resistor to a GPIO pin!
Well for FM, I believe they are just adjusting a PLL's output frequency by loading new divider values.
The output spectrum contains spurs and harmonics, so it's not a pure RF tone. So if one actually wanted to transmit into the air an appropriate bandpass filter would be needed. But for playing with test equipent or an SDR its no big deal.
How they create AM or SSB, does feel a bit like voodoo at this point. It would seem they have devised a method to go from IQ samples to a GPIO pin state, i'm not entirely sure what that involves, must be magic!
An impressive amount of fun for just soldering a resistor to a GPIO pin!
For AM I believe you change the duty cycle of carrier, but don't quote me on that. I don't understand why they don't use the full FM bandwidth with the RPITX. Other cool stuff that I've seen is that some have transmitted even colored NTSC signals. FPGAs are also pretty good at this because you can get very accurate pin control at very high speeds.
