PWM and Variable Pulse Width
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Posts: 46,084
Can someone explain the difference between PWM(Pulse Width
Modulation) and Variable Pulse Width Modulation? I have looked on
google and cannot find clear answers. And is there a way to
interpret Variable Pulse Width with the BSX2?
Modulation) and Variable Pulse Width Modulation? I have looked on
google and cannot find clear answers. And is there a way to
interpret Variable Pulse Width with the BSX2?
Comments
to measure the high and low portions of an incoming signal, then do your
analysis.
+
+ +----
| | |
----+ +
+
tOn tOff
Period = tOn + tOff
Duty Cycle (%) = tOn * 100 / Period
-- Jon Williams
-- Parallax
Original Message
From: webboyben [noparse]/noparse]mailto:[url=http://forums.parallaxinc.com/group/basicstamps/post?postID=Fa2hKzeA4QsqNIOH2AbO6zuhTuNZAUsaoi9UXfnpMJx1RJI7dTrZ_A4bi8XHMLJi-rm_ZCAhINC4m2EU]webboyben@h...[/url
Sent: Wednesday, January 07, 2004 10:34 AM
To: basicstamps@yahoogroups.com
Subject: [noparse][[/noparse]basicstamps] PWM and Variable Pulse Width
Can someone explain the difference between PWM(Pulse Width
Modulation) and Variable Pulse Width Modulation? I have looked on
google and cannot find clear answers. And is there a way to
interpret Variable Pulse Width with the BSX2?
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> Can someone explain the difference between PWM(Pulse Width
> Modulation) and Variable Pulse Width Modulation?
"Pulse modulation" in general refers to the creation of an intermittent
signal. We do this to carry information on the pulse relationships, as in
telemetry, or to increase the peak radiated power while keeping within a
limiting average power, as in radars. In a simple telemetry example, we
might make the pulse length proportional to the temperature a sensor
detects, while the interval between pulses is made proportional to the
barometric pressure a different sensor is detecting. In radars, the pulse
modulation is chosen to enhance the return from targets of interest, to
improve penetration of areas of precipitation, or to increase the radiated
power of individual pulses while holding the average power to within the
equipment limitations.
The pulse repetition rate would determine whether a Stamp could be used
without an off-board receiver chip to interpret something like that
telemetry example. I don't think a BS2sx would handle a very high PRF, but
something like a temp/pressure sensor pair obviously would not require very
many transmissions per second. One? Ten at worst? In the radar example, the
emitted pulses have no intentional intelligence encoded in their
relationships, so in that application and many others the term "interpret"
is not meaningful. A Stamp could be used to "interpret" such a pulse train
as a voltage to which the average power corresponds. You would do that with
RCtime in general, but I'm not sure I can picture an application.
The Pulse Width Modulation command used on the stamp uses that general
principle as a form of digital to analog conversion. Like a miniature
version of the radar example, the objective is the average power of a long
train of pulses, not the length of any particular pulse. With any kind of
capacitance in the load, the effect of a PWM signal is that the load
perceives the average power, rather than the peaks or the intervals when no
power at all is transmitted. This gives you a simple way to increase the
perceived voltage of an output when in fact the Stamp is either providing
ground or +5V at all times.
As I remember, the Stamps are coded (in the firmware I mean) to provide
successive pulses of pseudo-random width with the specified average power.
This prevents a resonance that otherwise would occur with some loads if the
Stamp output a steady pulse train with each pulse the same width.
Gary