--- title: Why Does My Bode Plot Look Like That?!? description: Blog post answers why the results of a loop gain measurement on a Bode Plot look unexpected and how to resolve. image: https://www.venableinstruments.com/hubfs/funkyplot%202.jpg --- [![Venable Instruments](https://www.venableinstruments.com/hs-fs/hubfs/VenLogoInst%20%5BConverted%5D-1.png?width=115&height=45&name=VenLogoInst%20%5BConverted%5D-1.png "Venable Instruments")](https://www.venableinstruments.com) Search ## Why Does My Bode Plot Look Like That?!? *By Ford Mays - February 16, 2024* Occasionally, when measuring loop gain, you see something that is quite unexpected. You have tried most all the [tricks](https://www.venableinstruments.com/blog/adjusting-injection-levels) and [tips](https://www.venableinstruments.com/blog/servo-loop) ([injection site location](https://www.venableinstruments.com/blog/injection-signal-location-and-levels-in-loop-gain-measurements), [injection levels](https://www.venableinstruments.com/blog/feedback-loop-injection-levels-for-measuring-loop-gain)) to get a good smooth plot, but it the loop gain plot still ‘looks funny’. Maybe it is an unexpected ‘hump’ in the gain or ‘weird’ phase behavior. At this point, you want to understand what is causing this, and the first step is to break the loop into its constituent parts, the modulator (plant) and feedback (compensation). By doing this, we can see what is in our control (the feedback circuit) or what is not (the modulator). This is discussed in detail in the [Tuttle’s Topics](https://www.venableinstruments.com/blog/tuttles-topics) blog series. Measuring the modulator transfer function and the feedback transfer function is very much like measuring the loop gain, we just have to move the probes. Here is a connection diagram for measuring the modulator:   ![Modulator](https://www.venableinstruments.com/hs-fs/hubfs/Modulator.png?width=1190&height=590&name=Modulator.png) Figure 1. Modulator Transfer Function Measurement The modulator transfer function is the gain (and phase), where A=Vo/Vi (CH2/CH1). To measure the feedback transfer function, move the Channel 2 input to the other side of the injection resistor (V1), and the feedback transfer function is CH1/CH2. Now you will be able to see which part of the loop is contributing the unexpected gain or phase shift.  You might also check and make sure that any over-voltage or over-current protection features are not 'kicking in', as they can cause unexpected results in the plots. Should you experience unexpected results, splitting the loop gain into its constituent parts, the modulator (plant) and feedback (compensation) can often be very insightful.  However, there may be times when you need "another set of eyes" to look at an irregular plot, and we are here to help.  Please [contact us here](https://www.venableinstruments.com/support/technical-support) or call (512-949-3141) or email ([fmays@venableinstruments.com](mailto:fmays@venableinstruments.com)) me for assistance. ![iStock-157336072](https://www.venableinstruments.com/hs-fs/hubfs/Web%20Images/iStock-157336072.jpg?width=1000&name=iStock-157336072.jpg "iStock-157336072") #### Distributed Power: What Causes These Systems to Oscillate Distributed power systems can oscillate, especially when driving one power supply with another power supply. Download this white paper to learn what causes distributed power systems to oscillate and how you can prevent oscillation. [![Download White Paper](https://hubspot-no-cache-na2-prod.s3.amazonaws.com/cta/default/4867466/ba09c31a-becf-4e1b-b896-9a0c990c4fc5.png)](https://hubspot-cta-redirect-na2-prod.s3.amazonaws.com/cta/redirect/4867466/ba09c31a-becf-4e1b-b896-9a0c990c4fc5) Comments ### Sign Up for our Blog We promise that we won't SPAM you. #### About Venable Venable provides scalable energy storage and power systems test solutions for precise voltage, current, and frequency measurements, partnering with engineers to ensure battery and power systems around the world will meet stringent field performance demands.  #### Recent news [![Venable Training - Stabilizing Power Supply Control Loops](https://www.venableinstruments.com/hubfs/Venable%20Training%20Front%20Image.jpg) **Venable Training - Stabilizing Power Supply Control Loops** 20 August 2025 ](https://www.venableinstruments.com/blog/stabilizing-power-supply-control-loops) [![The Ins and Outs of System Level Input/Output Impedance Measurements](https://www.venableinstruments.com/hubfs/Imput%20Impedance%20Test%20Set%20UP.jpg) **The Ins and Outs of System Level Input/Output Impedance Measurements** 23 June 2025 ](https://www.venableinstruments.com/blog/impedance-measurements) #### Subscribe Subscribe to our blog to receive emails and updates on scalable power and energy systems test solutions from Venable Instruments.  #### Contact us Venable Instruments 8656 SH 71 West Cuesta Centre, Building E Austin, TX 78735 Office: 512-949-3100 Fax: 512-949-3151