[Discuss-gnuradio] Loops in graphs? Do we need them?

[Achilleas Anastasopoulos]

Hi there,

my motivation for this question/comment is the email sent to the list 
regarding the lack of support for loops in gnuradio.

The examples given were PLLs, AGCs, etc that are common in communication 
systems, but cannot be implemented under the current architecture
of gnuradio.

It seems to me that in DSP there is little need to
implement these subsystems.
For instance, a PLL is a non-linear feedback filter
that tracks the phase of an input sinusoid (approximately).
In a DSP implementation we can recover the phase of the complex IQ 
signal and operate on it using simple FIR or IIR filters.
The reason (imho) of the existence of PLLs (and similar non-linear 
feedback loops) is that in old analog systems, such an option was not 
there (ie, taking the arctan of a signal).

Bottom line question: if I substitute a PLL by a device consisting of 
linear (FIR or IIR) filters and a device that evaluates the argument of 
a complex IQ signal, am I loosing anything in performance or am I 
actually gaining since I do not even have to worry about the weird 
non-linear effects of the PLL?

Best
Achilleas