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

From: Achilleas Anastasopoulos
Subject: [Discuss-gnuradio] Loops in graphs? Do we need them?
Date: Mon, 07 Feb 2005 18:30:22 -0500
User-agent: Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.4) Gecko/20030624 Netscape/7.1 (ax)

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?


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