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Re: [Discuss-gnuradio] Question on control-loop block design

From: Tom Rondeau
Subject: Re: [Discuss-gnuradio] Question on control-loop block design
Date: Mon, 11 May 2015 12:35:15 -0400

On Mon, May 11, 2015 at 12:00 PM, Richard Bell <address@hidden> wrote:
From what I remember, there shouldn't be a functional difference. The difference is the amount of feedback delay in the loop. You always need to make sure one delay exists in any feedback path you can take, to avoid race conditions. After that, you want to minimize the loop delay. The one accumulator with delay in the feedback path ensures stability. The other accumulator keeps the delay out of the feedback path because it minimizes the loop latency. 

Hope that helps. 


Sent from my iPhone

Yep, thanks, Richard!


On May 11, 2015, at 8:42 AM, Tom Rondeau <address@hidden> wrote:

On Tue, Apr 28, 2015 at 8:50 AM, <address@hidden> wrote:

I've been looking into the design details of the control-loop block. I stumbled upon the blog post of Tom (http://www.trondeau.com/blog/2011/8/13/control-loop-gain-values.html with the derivation https://static.squarespace.com/static/543ae9afe4b0c3b808d72acd/543aee1fe4b09162d0863397/543aee1fe4b09162d08633ac/1313345573084/control_loop_derivation.pdf) and also this article from TI: http://www.ti.com/lit/an/slyt169/slyt169.pdf.

Now looking at the schematics of the control loop in Tom's post and the TI article (figure 5), you notice a slight difference after the loop filter. In the TI article, there is delay (z^-1) followed by an accumulator. In the schematics of Tom's post, the delay block is directly integrated in the loop. Can somebody comment on these differences?


Hi Ruben,

This is a good question, and I don't want to let it drop. I just don't have a good answer for your right now. If I can find the time, I'll try to go through things more carefully to understand why there is the added delay. Just keeping the thread alive in case someone else can jump in and answer it.

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