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'[PIC]: PWM stabilty'
2003\04\11@095847 by Micro Eng

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16f877 running 10bit PWM. Output is tied to a 10K and .01uF cap.

With the registers holding a static value, would you expect the output
voltage to be stable? Its acting as if the lsb's are toggling.

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2003\04\11@105738 by Ned Konz

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On Friday 11 April 2003 06:58 am, Micro Eng wrote:
> 16f877 running 10bit PWM. Output is tied to a 10K and .01uF cap.
>
> With the registers holding a static value, would you expect the
> output voltage to be stable? Its acting as if the lsb's are
> toggling.

What frequency PWM? You're going to see some ripple on the output of
the filter, anyway. What does it look like on a scope?

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2003\04\11@111027 by Alan B. Pearce

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>> 16f877 running 10bit PWM. Output is tied to a 10K and .01uF cap.
>>
>> With the registers holding a static value, would you expect the
>> output voltage to be stable? Its acting as if the lsb's are
>> toggling.
>
>What frequency PWM? You're going to see some ripple on the output of
>the filter, anyway. What does it look like on a scope?

That is my reaction too. That RC does not sound like it will filter too much
even if the PIC is running at 20MHz, with a 10 bit PWM.

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2003\04\11@163720 by Olin Lathrop

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> 16f877 running 10bit PWM. Output is tied to a 10K and .01uF cap.
>
> With the registers holding a static value, would you expect the output
> voltage to be stable? Its acting as if the lsb's are toggling.

The filter rolloff is about 1.6KHz, so a little ripple should be expected
depending on what your PWM frequency is.


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2003\04\11@170928 by Dmitriy A. Kiryashov
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Hi Olin.

Olin Lathrop wrote:
>
> > 16f877 running 10bit PWM. Output is tied to a 10K and .01uF cap.
> >
> > With the registers holding a static value, would you expect the output
> > voltage to be stable? Its acting as if the lsb's are toggling.
>
> The filter rolloff is about 1.6KHz, so a little ripple should be expected
> depending on what your PWM frequency is.

How 1.6KHz was obtained from 10K and 0.01uF ? ( what the formula )


WBR Dmitry.

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2003\04\11@172913 by Dwayne Reid

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At 04:42 PM 4/11/03 +0400, Dmitriy A. Kiryashov wrote:
>Hi Olin.
>
>Olin Lathrop wrote:
> >
> > The filter rolloff is about 1.6KHz, so a little ripple should be expected
> > depending on what your PWM frequency is.
>
>How 1.6KHz was obtained from 10K and 0.01uF ? ( what the formula )

1 / (2 * Pi * R * C)

dwayne

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2003\04\11@173016 by Bob Blick

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Dmitriy A. Kiryashov said:

> How 1.6KHz was obtained from 10K and 0.01uF ? ( what the formula )

Hi Dimitri,

f = 1 / (2*pi*R*C)

f = hertz
pi = 3.14159265....
R = ohms
C = farads

That gives you the half-power (-3db) point.

Cheers,

Bob

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2003\04\11@181201 by Olin Lathrop

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> How 1.6KHz was obtained from 10K and 0.01uF ? ( what the formula )

The rolloff frequency (-3dB point) of a single pole R/C filter is

 F = 1 / (2Pi * R * C)

where F is the frequency in hertz, R the resistance in ohms, and C the
capacitance in farads.  This also means that

 F * R * C = 1 / 2Pi

This is such a handy formula that I keep 1,000,000 / 2Pi = 159,155
permanently in register 7 of my calculator.  I can start with the register
7 value and divide it by any two of the frequency in herz, the resistance
in ohms, or the capacitance in microfarads to get the third value.  Very
handy.


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2003\04\12@003942 by Dmitriy A. Kiryashov

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Got it. I was confused with cut off frequency ( not  -3 db )

Thanks guys. ;)


WBR Dmitry.



Bob Blick wrote:
{Quote hidden}

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2003\04\12@003947 by Dmitriy A. Kiryashov

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Thank you Olin. ;)

WBR Dmitry.


Olin Lathrop wrote:

>   F * R * C = 1 / 2Pi
>
> This is such a handy formula that I keep 1,000,000 / 2Pi = 159,155
> permanently in register 7 of my calculator.  I can start with the register
> 7 value and divide it by any two of the frequency in herz, the resistance
> in ohms, or the capacitance in microfarads to get the third value.  Very
> handy.

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2003\04\12@044355 by Mike Singer

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Dmitriy A. Kiryashov wrote:
{Quote hidden}

  Hey Dima,
keep on asking about formula.

  I suspect that RC circuit discharges according
to the exponential low. (When input voltage is constant,
of cause.) It was so long ago at a school I was told
about this.
  The original posting was about ripple on capacitor
that is fed from PWM output over resistor. When the ripple
is small compared to amplitude of voltage over resistor
then the ripple curve should look exponentially, I think.
  By definition R and C should discharge to 1/e value
over 1 second. Where did guys get 2*Pi, I don't know.
  To estimate the ripple very roughly, we may keep in
mind the next: over 0.1*RC PWM cycle voltage change
would be very aprox V/2.7/10 (0.2v when V=5v)

  Don't blame me hard if I'm wrong: Have been developing
some software under MS tools over last years.

  Mike Singer.

--------------------
P.S. I use not to use calculators at all when estimating
such timings with non-sinusoidal signals.
Very, very handy(or non-handy, hands-free) :-)

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2003\04\12@084021 by Olin Lathrop

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> Got it. I was confused with cut off frequency ( not  -3 db )

The -3dB point is often called the "cutoff frequency", although I prefer
the term rolloff frequency.  The former implies a sudden transition, which
this certainly is not.


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2003\04\12@084849 by Olin Lathrop

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>    By definition R and C should discharge to 1/e value
> over 1 second. Where did guys get 2*Pi, I don't know.

That is correct.  Note that when the R*C time constants is significantly
longer than the period of the incoming square wave, the result approaches
a triagle wave (the exponential becomes a line because it doesn't have
time to change slope).

>    To estimate the ripple very roughly, we may keep in
> mind the next: over 0.1*RC PWM cycle voltage change
> would be very aprox V/2.7/10 (0.2v when V=5v)

Right idea, but the details are a bit off.  Assuming square wave input,
the output will always be "near" 2.5V.  Also, you have 2 exponentials per
cycle, one for each half of the square wave.  I haven't checked your
calculations, but these two factors should make the ripple 1/4 of what you
computed.


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2003\04\14@221524 by Mike Singer

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  I wonder, if the best of the best student and
fast-learner could calculate the ripple when PWM-ing
RC? By the way, headhunters love to ask this type
very basic questions.

  Mike.




Marc Nicholas wrote:
{Quote hidden}

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2003\04\15@010344 by Ned Konz

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On Saturday 12 April 2003 12:42 am, Mike Singer wrote:
>    By definition R and C should discharge to 1/e value
> over 1 second. Where did guys get 2*Pi, I don't know.

If you look at the frequency response of the RC filter, you get an
expression with the frequency in radians per second in the
denominator.

To get from cycles/second (Hz) to radians/second you need 2 * pi...

So the 3dB down point (in Hz) of the RC filter is at 1/(2*pi*RC)

or at 1/RC, if you want to talk in terms of radians/second.

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2003\04\15@200801 by Mike Singer

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Ned Konz wrote:
> On Saturday 12 April 2003 12:42 am, Mike Singer wrote:
> >    By definition R and C should discharge to 1/e value
> > over 1 second. Where did guys get 2*Pi, I don't know.
>
> If you look at the frequency response of the RC filter, you get an
> expression with the frequency in radians per second in the
> denominator.
>
> To get from cycles/second (Hz) to radians/second you need 2 * pi...
>
> So the 3dB down point (in Hz) of the RC filter is at 1/(2*pi*RC)
>
> or at 1/RC, if you want to talk in terms of radians/second.


This isn't sinusoidal signal, at first.
At second, what should be done with all these things
to figure out the value of a ripple?

  Mike.

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