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-3dB and an LPF

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eev_carl:
Hi,

I have a simple passive LPF and was trying out the loss at fc.  I'm using the equation 20log(Vout/Vin) but wasn't getting the -3dB as I expected and was wondering if I had applied this incorrectly or was missing something like the effect of the phase shift.

My Vin peak-to-peak is 2V and Vout is 1.5V.  I get 20log(1.5/2) = -2.5dB.  The source of the formula is here: https://www.electronics-tutorials.ws/filter/filter_2.html .

Thanks,
Carl

ogden:
LTspice can do calculation for you. Easy way: make sure simulation have at least 100 sine periods (.tran 10s) and run FFT which gives signal magnitudes in dB.

aneevuser:

--- Quote from: eev_carl on November 28, 2018, 12:48:51 pm ---Hi,

I have a simple passive LPF and was trying out the loss at fc.  I'm using the equation 20log(Vout/Vin) but wasn't getting the -3dB as I expected and was wondering if I had applied this incorrectly or was missing something like the effect of the phase shift.

--- End quote ---

Are you sure that you're measuring the 3dB point? The two signals should have a 45 degree phase shift at that frequency, and I'm not sure that your image shows that (though it could be my eyesight)

eev_carl:

--- Quote from: aneevuser on November 28, 2018, 01:27:55 pm ---
--- Quote from: eev_carl on November 28, 2018, 12:48:51 pm ---Hi,

I have a simple passive LPF and was trying out the loss at fc.  I'm using the equation 20log(Vout/Vin) but wasn't getting the -3dB as I expected and was wondering if I had applied this incorrectly or was missing something like the effect of the phase shift.

--- End quote ---

Are you sure that you're measuring the 3dB point? The two signals should have a 45 degree phase shift at that frequency, and I'm not sure that your image shows that (though it could be my eyesight)

--- End quote ---

I was measuring the ratio (1.5V/2.0V) at 159Hz which is the cutoff frequency.  I expected this to =3dB but was getting -2.5dB.

tggzzz:

--- Quote from: eev_carl on November 28, 2018, 12:48:51 pm ---I have a simple passive LPF and was trying out the loss at fc.

--- End quote ---

You don't have a simple passive LPF, you have a simulation of a simple passive LPF.

Use the simulator's different types of analysis; in this case AC (i.e. frequency domain) simulation is enlightening.

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