What type of antialiasing filter is used in a digital scope?

[KJDS]

Thanks KJDS. Looks like I've got my work cut out but it'll pass some cold wintery nights! 

I assume you choose 50MHz because that guarantees a flat response to 30MHz?

And is there any standard way to increase the values of the smallest capacitors - I'd rather avoid having most of the capacitance made up from board parasitics.

I chose 50MHz as that's the 3dB point of a Bessel. That frequency will be squeezed a little lower by the transmission poles. I wouldn't worry too much about parasitic capacitance, the self capacitance of the inductors is likely to dominate. You'll almost certainly need to tweak things a bit when it's on the board so I'd invest in a capacitor kit too. For a one off, I'd hand wind the coils as they'll be easier to adjust a little.