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[Discuss-gnuradio] Quadrature demodulator update

From: Ian Wraith
Subject: [Discuss-gnuradio] Quadrature demodulator update
Date: Tue, 11 Feb 2003 08:33:15 +0000

     Just a quick update on my little quadrature demodulator project
which all being well will enable me to do some GNU Radio development. It
takes a low level 10.7 MHz IF from a receiver amplifies this with a
MSA-0886 MMIC then demodulates this with a RF2711 quadrature demodulator
IC from RF Micro Devices. The I & Q outputs of the RF2711 turned out to be
rather low level so I used a pair of OPA340 single supply op amps to
amplify them to a reasonable level. These signals are then feed into the
stereo inputs of my soundcards line input.

 Last night I finally got to test all this using Audacity running under
RH8. I used Audacity to record the I & Q signals in stereo at 44 KHz
and then to display them. While doing this I took a couple of screenshots
in case any of you are interested. This ..


is a MPT1327 (a analogue trunked control channel 1200 baud MSK) signal
which looks nice and clean. While this ..


is a TETRA DQPSK signal. From what I have read this will look much better
when cleaned up by a root raised cosine filter. Annoyingly I forgot to mark
which of the displayed channels is the I & Q signal.

 Next when time allows its time to start playing with the software.




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