Questions about Escobedo PWM - help me understand how it works.

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Ok, so I see that the signal is going into the 386 which provides enough amplification to, I think, cause inverter 1 on the 40106 to flip high and low.

Then the output of nverter 1 (the squared up signal?) is sent into the input of inverter 2.

What is the cap doing though? And the pot? Filtering the output of inverter 1?

How does the network between 1 & 2 lead to the setting of the width of the pulse from inverter 2?

Thanks for any light you can shed on this.

[Processaurus]

A Hi pass RC filter, like the one made with the cap and pot, attenuate low frequency AC and block DC voltage.  If you make the cap smaller, or the resistor (pot in this case) smaller, less lows get through and vice versa.  Imagine the inverter turns on and stays on, the initial turn on +9v transient gets through the cap to the next inverter's input, triggering it, but as the cap charges up, it lets less current through, and the voltage on the 2nd inverter's input drops until it is zero.  At some threshold point in its ramp down, the inverter decides it is no longer a hi logic signal and switches its output immediately, like a comparator.

The series cap+shunt resistor RC filter between logic gates is a handy way to turn a steady state thing into a pulse, there is the exact same thing happening on this relay driver that came up recently, to make a steady DC voltage representing bypass momentarily pulse a latching relay's set and reset coils when it changed state.

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So let me re-state to see if I understand - there is a highpass filter, and when there is less bass getting through, the pulsewave is as a result is narrower, because the waves getting through are less "wide" (being only the higher frequencies and having the "wider" waves removed)?

[Processaurus]

Maybe think of it less as blocking bass as blocking DC.  When you switch DC off/on, for a moment it is AC.  The AC gets through the cap, and for how long depends on the resistor (pot, here).

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                 1st inverter output

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                 Voltage at 2nd inverter input



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                           output


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