Carrier Recovery, Frequency and Phase offset in (CP)FSK I wanted to ask what exactly is meant for phase recovery of signal (at least intuitively). I have a working example of an acoustic fsk modulation and demodulation software but i don't know how to apply these techniques to improve the detection of the signal.  The center frequency is 30Khz and demodulation is done in passband.

In the following i will briefly explain how it works:

1- Send a chirp, used from the receiver to find the beginning of the signal

2- Wait for a fixed amount of time, known also to the receiver

3- Send the modulated  signal

4- Send a final chirp

The receiver after i get the  correlation v with the chirp, i buffer data, isolate the signal until the end of the second chirp.

Now at the processing part my doubts are the following:

If the two nodes run the same clock i would know perfectly where the signal starts from the end of the first  chirp, after the fixed period of pause. In this case the signal i try to demodulate would have no phase shifting, correct?!

If my clocks run differently, i will reconstruct a different signal, right? If yes, why is shifted in phase? Or what does it mean shifted in phase in such case. Can you point me out how to adjust, if possible, this offset or to know where to sample the signal?  Is this effect something like a Doppler?

Please consider that my demodulation is in passband and i don't know if there exists any methods which would not require baseband transformation. 

Thanks in advance

Regards
If the two nodes run the same clock i would know perfectly where the signal starts from the end of the first chirp, after the fixed period of pause. In this case the signal i try to demodulate would have no phase shifting, correct?!


Wrong. The signal travels a distance. The distance it travels inherently means you get a phase shift – a full period ($2\pi$) for every wavelength (that's kind of the definition of wavelength, isn't it).

The signal gets reflected. Reflection often also leads to a phase shift. 

The system with which you produce and measure the physical signal has a phase-shifting behaviour, too.

The only case where you'd not see a time shift is when all these effects add up to a multiple of $2\pi$ (because periodicity).


  If my clocks run differently, i will reconstruct a different signal, right? If yes, why is shifted in phase? 


It's shifted in frequency. Frequency is the derivative of phase. That means that the phase shift is now a linear function of time.


  Can you point me out how to adjust, if possible, this offset or to know where to sample the signal? 


To correct a statically phase-shifted signal, you'll need phase correction. To correct a frequency-shifted shifted signal, you'll need frequency correction.

In analog thinking, you typically employ phase locked loops (PLLs) of first order (static phase correction only) or higher order (frequency shift, or even frequency drift).


  Is this effect something like a Doppler?


The Doppler effect leads to a shift in frequency, too. But it's a different thing.