analog output signal

create analog output signals by 2 or 3 digital out and 3 resistors and 1 capacitor. pls find my BLOG

as i use there also a arduino or a max32 board as scope,
who has a idea for a check on the sample rate…?
where to find a accurate signal i can measure with that poor man scope equipment
( without spending money… ) for calibration?
a watch … if accessable measuring points, might have a clock frequency what is too high for me.