FSK - newbie questions

Hi,

I’m currently working on implementing a 2-level FSK receiver with gnu
radio and have
got a simple demo running which uses bandpass filters to isolate the
mark and space freqs
and finds the difference in their amplitudes.

The input to the system is currently from the sound card, from an normal
SSB radio reciever
but we were planning to switch over to USRP style hardware in the future
but I was just trying
to get gnu radio working with out current hardware to smooth the
transition.

Ideally I would like the software to work with the new hardware with
little modification, so although
my input now is just a float stream, I think I should be working with
quadrature signals from the start. In
which case passing the sound card input through a Hilbert filter will
give me a quadrature signal,
is this correct? Also do you think this is the way to go about it?

Also I have noticed in some of the MSK examples use the quadrature
demodulator block, is this applicable
to non-continuous phase FSK and if so is it more robust than the filter
method?

Also, the next problem is bit recovery, I don’t really understand the
correlator block, could anyone
point me towards some documentation that would be grand. Am I supposed
to be using this or the
clock recovery block?

Sorry for the naivety of these questions, only just starting up the long
comms learning curve.

Many Thanks,
Fergus Noble

Fergus Noble wrote:

Also I have noticed in some of the MSK examples use the quadrature
demodulator block, is this applicable
to non-continuous phase FSK and if so is it more robust than the filter
method?

I have used the quadrature demodulator block on GMSK (which is also
non-continuous phase) with great success.

Also, the next problem is bit recovery, I don’t really understand the
correlator block, could anyone
point me towards some documentation that would be grand. Am I supposed
to be using this or the
clock recovery block?

The clock recovery block will resample the data stream at the data
clock’s frequency, yielding a stream of “soft bits”. These can then
be fed to a bit-slicer to produce a stream of binary '1’s and '0’s.

@(^.^)@ Ed

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