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?

## Best Answer

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