Unfortunately, the AD9833 doesn’t have a way to control amplitude, something which is pretty important in a function generator. So [Faransky] uses a X9C104P 100KOhm 8-bit digital potentiometer ...

It’d probably be unrealistic to expect a handheld computer powered by an Arduino Nano to do much more than that. It’s a tiny board with a 16 MHz ATmega328 microcontroller 32KB of flash memory ...