Hi John, I have to implement a modified version of costas loop which is to be used to compensate for frequency offset present between LO's of two USRP's (RFX900). So, I think I should make a new such signal processing block now.
Thanks, Mahboob ________________________________ From: John Wilson [[email protected]] Sent: 02 August 2010 02:33 To: Rahman, Muhammad Mahboob Ur Cc: discuss-gnuradio Subject: Re: [Discuss-gnuradio] PLL and VCO questions Hey, You can't make a feedback system in the Python flowgraph. You can, however use feedback in an individual signal processing block. If you need to use a PLL look at the Costas loop block which locks to RF signals with complex baseband components eg. QPSK, SSB etc. John On Mon, Aug 2, 2010 at 2:52 AM, Rahman, Muhammad Mahboob Ur <[email protected]<mailto:[email protected]>> wrote: Hi all, I have made up a simple PLL block in GRC using a multiplier phase detector, low pass filter (FIR) and VCO to lock to a 1KHz reference. When I run the simulation, it gives me a run time error "flow graph has loops!". Does it mean that I cannot make any kind of feedback system in the gnuradio or is there something else to ponder? Also, I am adding "1" to the output of the low pass filter so that when the phase error (output of low pass filter) becomes zero in the locked state, the VCO then runs at its quiescent frequency of 1KHz (I have set the sensitivity of VCO as 2*pi*1K rad/s/V). Is it the right way to use VCO? Below is the error message: ------------------------------------------------------ ------------------------------------------------------ Generating: "/home/mahboob/Desktop/top_block.py" Executing: "/home/mahboob/Desktop/top_block.py" >>> gr_fir_fff: using SSE Traceback (most recent call last): File "/home/mahboob/Desktop/top_block.py", line 71, in <module> tb.Run(True) File "/usr/lib/python2.6/dist-packages/grc_gnuradio/wxgui/top_block_gui.py", line 73, in Run if start: self.start() File "/usr/lib/python2.6/dist-packages/gnuradio/gr/top_block.py", line 97, in start self._tb.start() File "/usr/lib/python2.6/dist-packages/gnuradio/gr/gnuradio_swig_py_runtime.py", line 1455, in start return _gnuradio_swig_py_runtime.gr_top_block_sptr_start(self) RuntimeError: flow graph has loops! _______________________________________________ Discuss-gnuradio mailing list [email protected]<mailto:[email protected]> http://lists.gnu.org/mailman/listinfo/discuss-gnuradio
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