1. I am trying to understand this application note from Microchip about using PWM and a low pass
filter to generate an analog output. I have a few questions regarding to the article:
In the example they use a 20kHz PWM with K = 5 so we have a 4kHz cut-off frequency. Why
choose K = 5? Is it a practical rule? Or is there any reason to choose K = 5?
Then, they calculate R and C, I understand that, but after that they calculate how many dB the
20kHz signal from the PWM is cut-off (I don't really understand what they mean) and they get -
14dB but according to my math I get -10.43dB with:
F = 20kHz
R = 4K
C = 0.01uF
Solution
As the note says, K should be greater than 1 and an arbitrary value is selected for K. Choosing a
different value for K will change fPWM, the frequency of PWM. ie., if K = 6 then fPWM will be
24KHz.
That is a printing mistake. The original equation is,
G(dB)20KHz=?10