At 03:53 AM 12/19/2013, you wrote:
>On Dec 18, 2013, at 11:49 PM, Lindy Mayfield wrote:
>
> > it's still not clear, what duty cycle is.
>
> > The frequency is the time between pulses.
>
>Technically, the PERIOD is the time of one cycle of the waveform=20
>(same as the "time between pulses.")
>The FREQUENCY is the number of cycles you get in a particular time=20
>(one second.)
>They're intimate related; f =3D 1/p

To be absolutely clear here, the period is the time of one pulse plus=20
the time between two pulses.

     x-----x           x-----x
     |     |           |     |
     |     |           |     |
----x     x-----------x     x------

     <--- period ------>

     <-th->
           <-----tl---->

period =3D th + tl =3D 1/f
duty cycle =3D th/(th + tl) =3D th/period

If you put a low-pass filter on the output that's relatively low=20
cut-off frequency compared to f, you'll get a
more-or-less steady voltage that varies from 0 to 100% of the pulse=20
voltage depending on the duty cycle.
This is an easy way to make a DAC with a PIC PWM output, though it=20
runs out of gas if you need lots of bits at
high bandwidth. PWM also tends to be a better way rather than voltage=20
to control power, since the power into
a resistive load with be linearly proportional to duty cycle.


> > The duty cycle is the percentage of time the line is high.  So I=20
> guess that a perfect square wave would have a duty cycle of 50%?
>
>Yes.  Exactly.
>
>
> > PWM is simply changing the duty cycle from 1 - 99%
>
>0 to 100%, ideally.  But you have the right idea.  Your desired=20
>waveform (10us every 1s) has a duty cycle of 0.001%
>
>
> > Normally the frequency is 20 ms?
>
>20ms is a typical period for Radio Control signals (ie to=20
>servos.)  I didn't think that was what we were talking about.
>There are various reasons why RC signals aren't quite "typical pwm."

RC servo pulses are typically 1-2 msec long pulses repeated at 50Hz,=20
so every 20 msec.

It's the absolute duration of the pulse that determines the target=20
servo position- the time between pulses is not
very important (on a radio channel the dead time allows you to=20
time-multiplex a number of servos over a single RF signal with
the simple circuitry possible in the early days). During the dead=20
time you're off servicing other servos.





> > So when you say "high duty cycle" do you mean the time the line=20
> is high compared to low?  So like 75% would be high?
>
>Yes.  The 555 in "astable" (oscillator) mode tends to produce=20
>waveforms with greater than 50% "high" time.
>In monostable (pulse) mode it produces output "low" pulses when=20
>triggered.  Whether you set up a really extreme duty cycle=20
>oscillator (not too likely to work) or a double-555 configuration=20
>where a slow oscillator triggers a short pulse generator, the output=20
>waveform is going to have short "low" times and long "high" times,=20
>which is opposite what you said you wanted.
>
>
> > why do you need to invert a square wave?
>
>You wouldn't.  If you did the double-555 setup, you'd only need to=20
>invert the final output, not the (more square-shaped) wave between=20
>the two 555s.
>
>BillW
>
>
>
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