# Very simple circuit on spice

I have a very simple low-pass filter with an AC source a capacitor and a resistance in series. Now I set C and R so that R*C=100 but when I plot the diagram whit db(out), it looses 3dB at 156Hz instead of 100Hz.

Vin  in    0  DC 0.0  AC 1.0 0.0
C1   out 0   1uF
R1   in     out   1kOhm

ac dec 50 0.1Hz 100kHz


I'm just starting with this things, can someone help me?

-
RC is a time constant, not a frequency. In your case: tau = RC = (1 uF)(1 kohm) = 1 ms. This time constant doesn't correspond to the cutoff frequency directly, only to the point where the signal reaches 1-1/e or 63% in magnitude. The cutoff frequency is where your signal is at 50%. This is at 1/(2π*tau). – tyblu Dec 4 '10 at 2:41
yeah I meant 1/RC thanks, was missing the conversion w=2*pi*f anyway – gurghet Dec 4 '10 at 7:19

The 3db cut-off frequency is $\dfrac{1}{2 \cdot \pi \cdot RC}$ = 159 Hz, which is quite close to your 156 Hz.