Hi Francesco, the PPM output is the full 5 volt logic level from the Atmel processor, which is far too high for a microphone input
Typically a mic produces just a few millivolts, so I'd suggest a divider of 1:1000, say 100k and 100 ohms as a starting point. The low impedance will help keep the noise down.
I saw your question on RCG, you could send PPM info via bluetooth, even at the default 9600 baud you could send several 16 bit values within the 20ms frame. You'd need to extract the channel values, send each as 2 bytes over BT then at the receiving end convert these values back to PPM. Hasi's code does both jobs so most of the work is done!
Its worth mentioning that you cant measure the PPM level with a multimeter, its a pulse waveform, you need an oscilloscope
At best a meter will show the average voltage, thats 8 x 300uS / 20000uS x 5v = around 0.6v
Your meter could be missing alternate frames so maybe 0.3v
I wonder if your 'mic input' is actually a 'line input', that would normally be a couple of hundred millivolts. The output of the Atmel chip can only be zero or one, ground or (nearly) 5v as its a digital output.
Does it feed the RF module directly, no resistor, diode or anything inline?
Its possible that the RF module is clamping the level, in which case you need some resistance to drop the
voltage difference - hard clamping can damage the chip (its like partially shorting an output to ground).
If you unplug the RF module does the PPM level from the encoder rise? again remember you cant measure the peak amplitude of a pulse waveform with a multimeter...
Re Bluetooth, remember you need at least one of the two BT modules to be master AND slave capable.
Some such as the HC06 are slave only so two cannot talk to each other
Cheers
Phil