Your Oscilloscope has ExPloDEd ! Make a sine wave
Python 3, 128 \$\cdots\$ 107 103 bytes

Added a byte to fix and error kindly pointed out by blueteeth.
Saved 23 27 bytes thanks to Jonathan Allan!!!
Added a byte to fix a fudge kindly pointed out by Jonathan Allan.   



lambda a,f:[[' .'[~j&lt;=a*sin(f*i*pi/6)+.5&lt;-j]for i in range(100)]for j in range(~a,a)]
from math import*


Try it online!

Inputs an integral amplitude and a float for frequency.
Uses a period of 12 characters for 1Hz.
Returns a list of lists of characters.