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 2pi1K rad/s/V). Is it the right way to use VCO?
Below is the error message:
gr_fir_fff: using SSE
Traceback (most recent call last):
File “/home/mahboob/Desktop/top_block.py”, line 71, in
line 73, in Run
if start: self.start()
File “/usr/lib/python2.6/dist-packages/gnuradio/gr/top_block.py”, line
97, in start
line 1455, in start
RuntimeError: flow graph has loops!