Quadrature Demod

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This can be used to demod FM, FSK, GMSK, etc. The input is complex baseband, output is the signal frequency in relation to the sample rate, multiplied with the gain.

Mathematically, this block calculates the product of the one-sample delayed-&-conjugated input and the undelayed signal, and then calculates the argument (a.k.a. angle, in radians) of the resulting complex number:

Let x be a complex sinusoid with amplitude A>0, (absolute) frequency and phase sampled at f_s>0 so, without loss of generality,


is real, and so is , and hence only scales, therefore is invariant: = arg

This block does not support C++ output, so it cannot be used when the output language of a flowgraph in GRC is C++.


Gain setting to adjust the output amplitude. Set based on converting the phase difference between samples to a nominal output value. Default: "samp_rate/(2*math.pi*fsk_deviation_hz)".

Example Flowgraph[edit]

This flowgraph shows the Quadrature Demod block as a Frequency Shift Keying detector.

RTTY rcv.png

This flowgraph shows BFSK signal recovery using Quadrature Demod block

FSK2 mod demod.png

Transmitted and received bits using above example

BFSK tx rx.png

Source Files[edit]

C++ files
Header files
Public header files
Block definition