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I get what PWM is and how it can be done using a 555 astable circuit. However I feel I don't completely get what PPM is how its circuit works. I understand the VCO circuit, but I don't see how it can be used as PPM modulator simply by varying UTP and LTP using the pin5. I don't see much difference between PWM and PPM in below circuits. Both seem to produce varying width pulses. Shouldn't the pulse width be fixed in PPM and just the position change ? Could somebody please help me understand how the VCO in second figure acts as a PPM modulator ?

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    \$\begingroup\$ It looks like people mean different things by PPM. That's not what I'd call PPM, though an argument could be made that it has elements of PPM about it. It's a modulator that 'does what it does, whatever you call it'. \$\endgroup\$ – Neil_UK Jun 12 '18 at 7:57
  • \$\begingroup\$ Thank you @Neil_UK so this is indeed a weird circuit for PPM. I'll try googling other variations and see if it makes sense.. :) \$\endgroup\$ – AgentS Jun 12 '18 at 10:21
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    \$\begingroup\$ The second case is in fact fairly conventional PPM, the polarity of the signal in this particular circuit merely happens to be inverted compared to many conceptual presentations, and then a bit oddly labeled. \$\endgroup\$ – Chris Stratton Aug 4 at 20:27
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The negative going pulse is in different positions so it's ppm.

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    \$\begingroup\$ Care to elaborate? \$\endgroup\$ – Voltage Spike Aug 4 at 5:17
  • \$\begingroup\$ This is basically correct; but it would be more complete to say that the width of the negative going pulse is constant, while its position changes. Part of the issue originates with the odd wording of the diagram in the question; it would be more typical to say that the information is encoded in the pulse spacing while the width (in this case of the negative portion) is constant. Maybe it's just that humans are used to looking at active-high pulses, while this circuit happens to produce active-low ones. \$\endgroup\$ – Chris Stratton Aug 4 at 20:23

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