srobertjames
New Member
I'm prototyping an Infrared audio communications system. I'm having trouble demodulating PWM, and was hoping one of you pros could help.
The system is designed to convert a 20Hz-3kHz signal into PWM, by modifying the duty cycle of a 35 kHz square wave. The square wave is sent via Infrared.
What's working:
* Transmitter modulates duty cycle based on input voltage.
* Receiver receives an IR, uses I-to-V and comparator to restore the square wave. I clearly see the square wave, with the fluctuating duty cycle, on my scope.
What's not working:
* Demodulating the PWM back into the signal. I thought sending it through a simple Low Pass Filter (f3db about 10kHz) would do it - but my scope only shows noise.
So, my question is, how can I demodulate the PWM properly? Why won't a simple LPF work?
The only other idea I have is to use an op amp based integrator, resetting it at every falling edge of the pulse, and then put it through an LPF. But that's a bit on the edge of my complexity level, and I'm not even sure it will work.
Anyone who can help this analog beginner design & build this circuit is greatly appreciated.
The system is designed to convert a 20Hz-3kHz signal into PWM, by modifying the duty cycle of a 35 kHz square wave. The square wave is sent via Infrared.
What's working:
* Transmitter modulates duty cycle based on input voltage.
* Receiver receives an IR, uses I-to-V and comparator to restore the square wave. I clearly see the square wave, with the fluctuating duty cycle, on my scope.
What's not working:
* Demodulating the PWM back into the signal. I thought sending it through a simple Low Pass Filter (f3db about 10kHz) would do it - but my scope only shows noise.
So, my question is, how can I demodulate the PWM properly? Why won't a simple LPF work?
The only other idea I have is to use an op amp based integrator, resetting it at every falling edge of the pulse, and then put it through an LPF. But that's a bit on the edge of my complexity level, and I'm not even sure it will work.
Anyone who can help this analog beginner design & build this circuit is greatly appreciated.