sudden transition in phase response

Hi there

I have attached the magnitude and phase response of a low pass filter. The phase response is linear thus delaying all signals by the same amount. However, it is observed that there is a certain transition (circled) upward in the response.

What causes this occurrence?? I realize it happens at the valley between the pass band ripple and the stop band ripple and would like to know why..

Thanks!

I hate to say this as there was another never ending thread* on this very subject, but the point with the abrupt phase change is a resonace point.

Let the arguments begin.

JimB

* This one:
https://www.electro-tech-online.com/threads/misuse-of-the-term-resonance.117703/

It is just that the software you are using modulos at 2Π radians.