sweep generator

I am renovating my house so I have not much time to work on the project.
Nevertheless I am thinking about the design of sweep generator.

The span (MHz/division) must be controlled by the microcontroller. So I think I make a sawtooth wave generator followed by a programmable attenuator. I found some cheap digital potentiometer chips that should do the trick.
They have 64 taps. This is not enough. I need more resolution as I want the following spans:
500 KHz/div
1 Mhz/div
2 Mhz/div
5 MHz/div
10 Mhz/div
20 MHz/div
50 MHz/div
and 500 MHz full screen.

I think it can be done by cascading two of those potentiometers. Then I get 64*64 different spans. Resolution will be 500/(64*64) = 122 KHz. That should be enough.