# Question on control-loop block design **URL:** \ **Category:** GNU Radio\ **Created:** [April 28, 2015, 2:51pm UTC](https://www.ruby-forum.com/t/question-on-control-loop-block-design/241750 "2015-04-28T14:51:59Z")\ **Posts on this page:** 5\ **Page:** 1
**Author:** ![musicdenotation](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/ea666f/32.png) [@musicdenotation](https://www.ruby-forum.com/u/musicdenotation)\ **Post date:** [April 28, 2015, 2:51pm UTC](https://www.ruby-forum.com/t/question-on-control-loop-block-design/241750/1 "2015-04-28T14:51:59Z")
Hello, I’ve been looking into the design details of the control-loop block. I stumbled upon the blog post of Tom ([Control Loop Gain Values — Rondeau Research](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](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](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? Thanks Ruben ---
**Author:** ![laber](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/l/67e7ee/32.png) [@laber](https://www.ruby-forum.com/u/laber)\ **Post date:** [May 11, 2015, 5:44pm UTC](https://www.ruby-forum.com/t/question-on-control-loop-block-design/241750/2 "2015-05-11T17:44:50Z")
On Tue, Apr 28, 2015 at 8:50 AM, [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote: > 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? > > Thanks > Ruben 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. Tom ---
**Author:** ![laber](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/l/67e7ee/32.png) [@laber](https://www.ruby-forum.com/u/laber)\ **Post date:** [May 11, 2015, 6:02pm UTC](https://www.ruby-forum.com/t/question-on-control-loop-block-design/241750/3 "2015-05-11T18:02:57Z")
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. Rich Sent from my iPhone ---
**Author:** ![laber](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/l/67e7ee/32.png) [@laber](https://www.ruby-forum.com/u/laber)\ **Post date:** [May 11, 2015, 6:36pm UTC](https://www.ruby-forum.com/t/question-on-control-loop-block-design/241750/4 "2015-05-11T18:36:50Z")
On Mon, May 11, 2015 at 12:00 PM, Richard B. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote: > Rich > > Sent from my iPhone Yep, thanks, Richard! Tom ---
**Author:** ![laber](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/l/67e7ee/32.png) [@laber](https://www.ruby-forum.com/u/laber)\ **Post date:** [May 12, 2015, 9:13am UTC](https://www.ruby-forum.com/t/question-on-control-loop-block-design/241750/5 "2015-05-12T09:13:46Z")
Ok, makes sense. Thanks for the feedback R From: removed\_email\_address@domain.invalid [[mailto:removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid)] On Behalf Of Tom R. Sent: Monday, May 11, 2015 6:35 PM To: Richard B. Cc: Merz Ruben, INI-INO-ECO-MXT; GNURadio D.ion List Subject: Re: [Discuss-gnuradio] Question on control-loop block design On Mon, May 11, 2015 at 12:00 PM, Richard B. \ 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. Rich Sent from my iPhone Yep, thanks, Richard! Tom On May 11, 2015, at 8:42 AM, Tom R. \ wrote: On Tue, Apr 28, 2015 at 8:50 AM, \ wrote: Hello, I’ve been looking into the design details of the control-loop block. I stumbled upon the blog post of Tom ([Control Loop Gain Values — Rondeau Research](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](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](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? Thanks Ruben 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. Tom