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.
$V_{out}=\frac{0.5}{sCR+1}V_{in}$