Why does Vout show negative values above 300kHz in RC low pass filter (180Ω, 220nF)?

I was connect a 180 ohm resistor in series with 220 nano farad capacitor and input signal with sine wave 10 volt pp ( peak to peak ) . The frequency is measure from 1 Hz to 1M Hz. I am measure unit with 1 decade log scale ( means measure 1 Hz to 10 Hz, then from 10 Hz to 100 Hz. Every time apply 10 division to frequency . Also say like 10^1. ). Then I measure the voltage drop across capacitor . Or you can say I apply prob from oscilloscope on the leg of capacitor which connect to resistor ( middle of the circuit ) as my Vout . after that I have been plotted this graph by using ( 20 log | vout / vin | ) as y-axis and ( log f ) as x-axis.

Question :
Why after 300k Hz , the Vout are showed negative value? According to the formula ( 20 log | Vout / Vin | ) , This have caused my graph acting upward . Usually at the big inning of y-axis of single pole low pass filter are constant before cut off frequency and after cut off frequency the graph starting inversely proportional to the x-axis .

Pls I need help . Thank you

I hope someone can help me

Sorry sir B) this forum only for pro. Mr . Jason Teoh are too noob didn't qualified to ask a question. Even thought he ask, there will never be a person to answer.


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