question about Low Pass Filter

[walid1]

Hi
for the LPF shown below, the cuoff frequency is at -3dB or -6dB and why?
thanks

View attachment 40973

 

[audioguru2]

The 4.7nF capacitor creates a lowpass filter with the unknown value of the source resistance.
The 10k resistor and 6.8nF capacitor create another lowpass filter.

If the cutoff frequency of both filters is the same then their total is -6dB because each filter is -3dB.

 

[walid1]

The 4.7nF capacitor creates a lowpass filter with the unknown value of the source resistance.
The 10k resistor and 6.8nF capacitor create another lowpass filter.

If the cutoff frequency of both filters is the same then their total is -6dB because each filter is -3dB.

If the cutoff frequency of both filters is  not the same? is it sitll -6dB
thanks

 

[audioguru2]

If the cutoff frequency of both filters is  not the same is it sitll -6dB?

No.
You need to calculate the amount of attenuation. One filter might pass the cutoff frequency of the other filter then the attenuation is only -3dB.

 

[Kevin Weddle]

Phase shift oscillators produce 180 degrees phase shift using cascaded RC stages. Maybe the components are chosen so that each stage is independent of the other and reduces the loading effect, which would keep the phase shift at 90 degrees at best.

 

[ARUN CHOWDARY]

please indicate the polarities of the capacitors

 

[Hero999]

please indicate the polarities of the capacitors

Why? 4.7nF and 6.8nF capacitors will be non-polarised anyway so that's a silly question.