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[Discuss-gnuradio] Pulse shaping filter advice needed

From: Ian Wraith
Subject: [Discuss-gnuradio] Pulse shaping filter advice needed
Date: Mon, 04 Aug 2003 08:25:58 +0100

     I am continuing my work on a PI/4 DQPSK demodulator module and
thought I was making some progress with the symbol timing (I am using
the Gardner algorithm now). However I have now noticed a problem with
the pulse shaping filter that I could do with some help with.

 My system works like this - I have a homebuilt quadrature demodulator
being fed with the 10.7 MHz IF of my receiver. The quadrature demodulators
I & Q channel outputs go into the stereo line input of my onboard sound
card. In software a small test program connects the GrAudioSource module
to my module.

 Now the first thing my module does is what all the books recommend which
is to put the I & Q channels through a root raised cosine pulse shaping
filter with a beta of 0.35. I designed the filter using this web page ..


with the following parameters ..

filtertype=Raised Cosine

You will see the corner is set to 9 KHz. As the web page recommends this be
set to half the baud rate. As my symbol rate is 18 KHz  I set this to 9 KHz.
The trouble is when I look at the raw data and the filtered data in
graphical form it looks to me as though the data is being pulse shaped to
to low a frequency i.e the pulses are to wide. You can see what I mean by
looking at ..


Remember the symbol rate is 18 KHz and the waveform is being sampled at
44.1 KHz. On the raw data you will see each symbol is made up of 2 or so
data points which I think is correct. However if you look at the graph
below of the same data that has been filtered you will see the pulses
are much wider - far to wide I believe.

 Could anyone tell me where I am going wrong here ? Am I misunderstanding
the filter parameters or is my mistake more basic ?




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