FPGA System A modulator

[ppppenguin]

I'd made an elementary error. Forgot to invert the MSB before sending the signal to the DAC. It's 2's complement coming out of the sine table which puts zero at the mid point of the DAC. Of course it needs to be at the bottom end of the DAC's range.

I tried 1kHz, 41.5MHz and 45MHz outputs. The spectrum looks a bit better, so does the waveform at carrier frequencies though still fairly horrible. The 1kHz output looks perfect so the DAC is happy at 148.5MHz. Using 1kHz as reference it's about 20dB down at 41.5MHz and a further 2dB down at 45MHz. Most of this must be due to the filter. Not wonderful but it will give pictures and sound of some kind once it's modulated. Even at 20dB down the output is a huge 200mV or so p-p. A lot more than an Aurora. Definitely not to be applied to a TV without an attenuator. About 20 to 25 dB should be about right as the modulation plus adding sound and vision will reduce the amplitude.

PS: Just thought about the filter. For its original application it needed to be some way down by 27MHz so not surprising that it's 20dB or so down at 45MHz.