Author Topic: Question about PWM frequency  (Read 1017 times)

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Offline joe61Topic starter

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Question about PWM frequency
« on: September 18, 2011, 09:43:56 AM »
I'm reading this Application Note from Atmel, which has the following statement:

"The output filter smoothens (sic) the output signal and removes the high-frequency PWM carrier signal"

I don't understand this. What is the carrier signal that's being removed? I thought the carrier was simply the period between two successive duty cycles. How would that get removed?

Thanks

Joe

Offline mstacho

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Re: Question about PWM frequency
« Reply #1 on: September 18, 2011, 10:52:13 AM »
from what I could tell all they're saying here is that it reproduces the audio signal, not the ON-OFF signal of the PWM.  In other words, they said in words what you understood intuitively -- that when you pass PWM through a filter, you get an approximation to the original analog signal back out.

In all honesty I have no idea why they even bothered to put that in.  It's the short form version of why PWM works :-P

MIKE
Current project: tactile sensing systems for multifingered robot hands

Offline joe61Topic starter

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Re: Question about PWM frequency
« Reply #2 on: September 18, 2011, 11:56:17 AM »
from what I could tell all they're saying here is that it reproduces the audio signal, not the ON-OFF signal of the PWM.

Sorry if I'm being dense, but what is the on/off signal? For example, if the duty cycle is 0, what signal would be there?

Thanks

Joe

Offline Soeren

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Re: Question about PWM frequency
« Reply #3 on: September 18, 2011, 12:00:49 PM »
Hi,

"The output filter smoothens (sic) the output signal and removes the high-frequency PWM carrier signal"

I don't understand this. What is the carrier signal that's being removed? I thought the carrier was simply the period between two successive duty cycles. How would that get removed?
It's the PWM sample frequency that's being removed (like in any form of digital audio).
Proper digital audio use a PWM frequency of at least 300kHz, to represent 0..20kHz faithfully.

I have to disagree a bit with the datasheet though, as a steep cut off of anything above 3kHz (which a 6kHz sample frequency will do) is a bit low and while there are language differences, it won't work well for Danish (and will probably be a challenge in most languages).
I have done a wee bit of research in how low pass filtering of voice elements will change them into sounding like other sounds and the only reason that a 3kHz wall might work is because we use context and other clues to guess the words.
For a more understandable speech, you need to keep at least up to 6kHz (and for precision, count on 8..16kHz).
Regards,
Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
Please remember...
Engineering is based on numbers - not adjectives

 


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