Period doesn't match for frequency sweeping using probe

Hello GNURadio community,

I am trying to generate a sweeping frequency signal. I use a probe to detect a Triangle signal with 1 s period, and measure the level using function probe.

Then I set the frequency of the signal source to 10e6 * func_probe, hoping it starts to sweep from -5e6 to 5e6 Hz with 1 s period. It does sweep between -5e6-5e6 Hz, but with much faster period than 1 s.  Could you please help me to figure out where is the problem for the period mismatch？

Thank you very much!

Wei