# Cutoff frequency of low pass filter

Discussion in 'General Electronics Chat' started by Janani_Rajkiran, Dec 9, 2016.

1. ### Janani_Rajkiran Thread Starter Member

Nov 1, 2016
46
0
Hi!

I need to calculate the cut-off frequency of a low pass filter.

there is a resistor divider network from the input after which the low pass filter is present

When i simulate it using LTspice the cut-off frequency is much lower than the calculated value

when I remove the Resistor divider network and supply the RC Circuit directly then the cut-off frequency is the same as the calculated value.

Which value is correct the calculated value or the simulated value with the resistor divider?

2. ### #12 Expert

Nov 30, 2010
18,076
9,690
With no circuit diagram, no component values, and no frequency values for the result, how are we supposed to know?

3. ### shteii01 AAC Fanatic!

Feb 19, 2010
4,667
745
Simulated is correct.

4. ### EM Fields Active Member

Jun 8, 2016
583
154
They're both correct; the problem is that you forgot to add the voltage divider's resistance contribution to the filter's RC.

On the attached graphic, note that the cutoff frequency of the filter with a high-resistance divider is lower than the cutoff frequency of the same filter with a lower resistance divider.

5. ### shteii01 AAC Fanatic!

Feb 19, 2010
4,667
745
Assuming all resistors are identical.
$V_{out}=\frac{0.5}{sCR+1}V_{in}$