testing stability by feedback responce technique question - Page 1
Forum section: electronics / projects

author: john23
author_rank: Frequent Contributor
timestamp: January 03, 2025, 05:12:28 pm
content: Hello , I tried to check the stability of the circuit by opening the feedback loop and placing the pertubation as shown below.How ever I made two plots Vf anole and Vf,Vin in AC responce and I get quite different result.What is the proper way to test feedback responce?Thanks.

author: mawyatt
author_rank: Super Contributor
timestamp: January 03, 2025, 05:18:49 pm
content: Apparently you've shown the Loop as Open as there's no feedback connectivity. Place the test source in series with the feedback and plot the Voltage Ratio at each end of the test source.This Voltage Ratio gives the Total Open Loop System Transfer Function including the Gain and Phase Margin for which with Linear Analysis you can get the Closed Loop Response.BTW if you want to actually measure the Open Loop Response in a physical circuit, you'll find the "Floating Test Voltage Source" difficult to find. Most use a Wide-Band "Injection" Transformer with the secondary as the Floating Test Source and measure the voltage on each side of the transformer secondary with a DSO. This ratio then is the Open Loop System Voltage Response and can be plotted vs frequency to create a "Bode Plot".You can search and find "Bode Plot" which discusses this in detail and how to use the available Bode Function of the entry level DSOs now available. Very powerful and useful technique for those willing to learn and apply suchBest

author: mawyatt
author_rank: Super Contributor
timestamp: January 03, 2025, 06:54:31 pm
content: Another interesting technique to access the stability of feedback systems involves injecting a Time Domain signal rather than Frequency Domain as in the mentioned Bode Plots.A little history behind this is way back in 70s we were working with some rather complex electro-mechanical and electro-optical systems that weren't "happy" if one opened the loop to inject a signal. Since almost all systems can be distilled into a 2nd order system with good fidelity we figured we could access the System Performance from a Time Domain response of an approximate 2nd Order system. Since the impulse function has a S Domain transfer function of unity, this seemed like a good starting point as it should reveal the overall system transfer function without any math.We devised a "Pinger" which was a simple battery powered (floating) handheld probe that would inject an impulse into the System Under Test (SUT) and we could monitor the SUT Response with a scope (analog back then, sometimes with a camera to capture the trace) at some convenient SUT point.The Pinger was just a 555 timer with variable rate and a Dirac Doublet pulse generator with variable pulse width and amplitude. The Doublet was selected over a simple impulse as the impulse has an average DC value whereas the Doublet doesn't (equal amplitude narrow pulses slightly spaced in time but with opposite polarities) and thus doesn't introduce a DC term into the SUT which might cause issues.So the Pinger became a very popular item for testing various SUT in our labs way back and we could quickly access the SUT stability under various conditions. This technique seems to be lost in time as we rarely if ever see this mentioned or utilized, might be an interesting project to resurrect suchAnyway, thought some folks might be interested in this Time Domain technique to access Closed Loop System stability.Best

author: john23
author_rank: Frequent Contributor
timestamp: January 04, 2025, 06:05:50 am
content: Hello , so how do I plot correctly the feedback response ?I got two different plots.Thanks .

author: mawyatt
author_rank: Super Contributor
timestamp: January 04, 2025, 02:57:34 pm
content: "Apparently you've shown the Loop as Open as there's no feedback connectivity. Place the test source in series with the feedback and plot the Voltage Ratio at each end of the test source."From above, plot the voltage ratio across the test source where the test source is placed in series with the feedback.Here's a brief note on how this works, with VBP as the Test Source. Measure the voltage across VBP +- terminals and plot as a ratio which creates the Bode Plot of GH, where G is the Forward System "Gain" and H is the "Feedback". Note the test source VBP can be place on either side of the Feedback Function H and the result is the same.Best

author: iMo
author_rank: Super Contributor
timestamp: January 04, 2025, 03:49:26 pm
content: Quote from: john23 on January 04, 2025, 06:05:50 amPlotting Bode of a circuit is well understood stuff, described in many LTspice tutorials (there are several methods).What could be an issue, however, is the circuit itself..Could you tell us what is the circuit about and what do you expect from it?Does it work actually?

author: Someone
author_rank: Super Contributor
timestamp: January 04, 2025, 09:35:13 pm
content: Quote from: iMo on January 04, 2025, 03:49:26 pmCheck the users post history, there seems to be one big project they are working on but despite people like you asking for more context/explanation it is never given.