# Cutoff frequency of low pass filter

#### Janani_Rajkiran

Joined Nov 1, 2016
46
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?

#### #12

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

#### shteii01

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

#### EM Fields

Joined Jun 8, 2016
583
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?
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.

#### shteii01

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