[old_tv_nut]

In general, the waveform should be flat if your test gear has flat frequency response, but you need to give more details of your lashup including the cable and the demodulator circuit before concluding something is broken.

If your lashup is not loading the generator properly, it could cause problems.

You may need a matching pad to prevent an uneven frequency response due to the loading by your demodulator.

Another thing to note is that your scope should be set to DC coupling, otherwise the slow rate of the waveform could cause distortion. It looks to me like it is set to DC (the baseline is flat), but you didn't say.