I think you have an extra factor of two there. If you need 20 MHz without aliasing, you either need real sampling at 40 MS/s or IQ sampling at 20 MS/s, but IQ s

I think you have an extra factor of two there. If you need 20 MHz without aliasing, you either need real sampling at 40 MS/s or IQ sampling at 20 MS/s, but IQ sampling at 40 MS/s is overkill. Nyquist works differently if your samples are complex.