Low pass filter confusion

[pjohn]

Hi,

I am designing this very simple low pass filter, and also with the view to drop the Voltage to 3.3v from 24v.
Here are the specs: Cut off frequency: 297 Hz
I started initially with the following combination:

+24v
.
.
R1
.
................C....Ground
.
.
R2
.
.
Ground

Now the values: R1 = 390K
R2 = 62K
C = 10nF

I am getting the right values in simulation and every thing but I do not seem to make the relationship between these values and known theory that Cut off frequency is calculated as:

F = 1/2*3.14*R*C
If I take R as 390K and C as 10nF (both in series) cut off frequency is about 40 Hz. But as soon as I add (R2) 62K resistor in parallel with the RC Filter, the cut off frequency changes to 298 H. Which is what I require, but why????

I thought R2 is just acting as load and should not effect the cutoff.

Please help me clarify this confusion.
Thanks in advance

 

[FvM]

But as soon as I add (R2) 62K resistor in parallel with the RC Filter, the cut off frequency changes to 298 Hz.

Right, the RC time constant is formed by the output resistance of the voltage divider and the capacitor

 

[erikl]

but why????

Because R = R1 || R2 = 53.5kΩ