Electronics > Beginners
-3dB and an LPF
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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