[Discuss-gnuradio] PLL and VCO questions

[Rahman, Muhammad Mahboob Ur]

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:

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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!