Hilbert transform (90 degree phase shift) for Quadrature Detector

[ACiDuser]

Hello everyone!

I'm trying to implement FM quadrature detector. 

I was unable to find in the internet proper math equations for FM Quadrature Detection method, because everywhere there are explained 

electronic circuits. 

I follow this manual: 

 http://www.radio-electronics.com/info/receivers/fm_demod/fm_demodulation.php

 As for 90 degree phase shift network I use Hilbert transform function.

 Then I multiply pure signal and 90 degree shifted one and pass through Low-pass filter to eliminate double frequency component.

 This method seems to work fine If my deviation is set to 1. When I increase it I get incorrect form of demodulated signal.

 It seems like output signal corruption has linear dependence on deviation.

 In the above article I noticed this : "For the quadrature detector, the signal is split into two components. One passes through a network that provides a basic 90 degree phase shift, plus an element of phase shift dependent upon the deviation and into one port of a mixer. The other is passed straight into another port of the mixer."

So I guess it is the problem why I have incorrect output. 

Could anyone explain how to obtain this "element which is dependent of phase shift upon the deviation" ?

I attached my VI's below (FM-hilbert.vi is the main one, others are Sub-VI's)

Thanks in advance 

[Ben_S]

ACiDuser,

Hello! I'm no expert at Modulation and Demodulation theory, but your application is very interesting. I've taken a look at your code and from what I can tell it looks like you've codded your current application correctly. If you have any questions on implimenting anything in LabVIEW please let me know.

I was looking at this wikipedia article on Detectors and it mentioned another possible way of making a Quadrature Detector using XOR gates and a Square Wave. Have you considered trying that approach?

Best of luck on your FM Quadrature Detector!

[ACiDuser]

Hi Ben S,

Thanks for your response. I would try, but I don't know how XOR Gate works with two input signals. And even I'm not sure If there is used a Square wave,  not Pulse width modulated waveform. Could you explain this? Thanks 

[Baji]

Find the information about square wave and XOR gate

http://en.wikipedia.org/wiki/XOR_gate

http://en.wikipedia.org/wiki/Square_wave

[ACiDuser]

http://en.wikipedia.org/wiki/Detector_(radio)

 In this article for a method with square wave: there is said that square wave is with the same frequency as FM signal, but there isn't said how this signal is produced, is it a local oscillator (VCO) or calculated from input FM? Not known. For a XOR gate there is said that signals are compared by the time they reached zero voltage. This means that Two-zero crossing detectors are used. I already have zero-crossing detection algorithm that demodulates FM, but I can do it using only one detector, why should use two instead? This sure will decrease performance of code. In addition - zero crossing detection will return false results if there is any noise in my signal. So as there is written "quadrature detector" is one of the common FM detection method - there should be more effective solution. Please, correct me if i'm wrong 🙂