Thanks attila, but I discovered this is a backward incompatibility in the Waveforms application. I have over 20 workspace files from 2019 that worked well in Waveforms version 3.12.2. I found other errors trying to run those workspace files in version 3.18.1, so went back to version 3.12.2. Now the workspace file in this example behaves as shown below, with no problem going from 500 Hz to 200 Hz. For me, advancing to the latest version isn't worth finding and reporting and doing workarounds for the backward incompatibility problems. (I am a retired software Quality Assurance engineer for Hewlett-Packard test equipment. I think more regression testing and emphasis on backward compatibility should be a higher priority with your software engineers.)