PWM-DC Low Pass Filter

Thread Starter

ampotvin

Joined Nov 20, 2012
1
Hello Everyone, first time user here.
I have some questions about a Butterworth sallen-key topology low pass filter for PWM TO Analog conversion. The PWM signal is the input to the low pass filter, and has a changing frequency around 45Hz (from roughly 42-47Hz)(this is for a DC motor control feedback circuit). I got this filter from a friend and I calculated the Q factor and cut-off frequency. (0-5V is PWM signal)

The cutoff frequency fc =2.8Hz and the Quality factor Q=4.2

I understand why such a low cut off frequency but my question is why does the Q factor have to be so large? From my understanding a Butterworth works best with a Q=1/2. Does a higher Q factor allow for a quicker response to changes in frequency?

When graphed, the high Q value shows a large amount of oscillation, and when i tried ti simulate a lower Q value, there is less oscillation. Can anyone suggest why a high Q value is good in this situation.
 
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