Low pass filtering PWM signals?

[konrad109]

Hi, I'm new to the RC UAV thing.

I'm curious why a PWM signal needs to be low pass filtered?

Is the idea that you don't want sharp ups and downs but want to smooth out the signal into a straight line that just a lower voltage value based on the duty cycle?

Woudln't it be possible to send a straight up PWM signal out of a microcontroller to a power transistor or op-amp? At a high enough frequency wouldn't the output still look pretty much like a DC value at a lower voltage than the full blown ON voltage?

[Ceros]

I think its time you deserved an answer, Konrad

In simple terms the answer to your question - "Woudln't it be possible to send a straight up PWM signal out of a microcontroller to a power transistor or op-amp? At a high enough frequency wouldn't the output still look pretty much like a DC value at a lower voltage than the full blown ON voltage?" - is both yes & no.

It will come down to the specific application. Low pass filters in PWM circuits can be there to serve one or more of many different purposes - some examples below:

1) random noise removal - especially if there is an analogue section in the cirucit somewhere

2) removal of high freq switiching componants

3) ... more specifically, when you mention "DC" and "op-amp", low pass filters in such types of PWM circuits often constitute part of the sine-wave generation.

....... so before trying to help you, may I ask: just what is it are you trying to get to the bottom of. What is the application or the problem you are faced with? This will help me to be more specific in my reply.

Thanks