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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.
Quote from: eev_carl on November 28, 2018, 12:48:51 pmHi,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.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)
I have a simple passive LPF and was trying out the loss at fc.
Quote from: eev_carl on November 28, 2018, 12:48:51 pmI have a simple passive LPF and was trying out the loss at fc.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.
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.
Quote from: tggzzz on November 28, 2018, 01:55:35 pmQuote from: eev_carl on November 28, 2018, 12:48:51 pmI have a simple passive LPF and was trying out the loss at fc.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.I also breadboarded the circuit and got a similar ratio (520mv / 700mv) for Vin/Vout.Does this FFT analysis look correct? I see a 0dB value at 159Hz but am not sure how this relates to the -3dB of a cutoff frequency.
Quote from: eev_carl on November 28, 2018, 01:54:52 pmI 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.Your measurement is not correct. You have measured first positive peak, that is 0.1V higher then all others.
Quote from: eev_carl on November 28, 2018, 01:59:34 pmQuote from: tggzzz on November 28, 2018, 01:55:35 pmQuote from: eev_carl on November 28, 2018, 12:48:51 pmI have a simple passive LPF and was trying out the loss at fc.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.I also breadboarded the circuit and got a similar ratio (520mv / 700mv) for Vin/Vout.Does this FFT analysis look correct? I see a 0dB value at 159Hz but am not sure how this relates to the -3dB of a cutoff frequency.If you are using a solderless breadboard, then expect to spend more time debugging the solderless breadboard than your circuit. For alternatives, see http://bristol.hackspace.org.uk/wiki/doku.php?id=pcb#avoiding_solderless_breadboardsThose spikes lead me to believe your graph is a measurement rather than an analysis. Since we have no idea what you measuring, nor how, there's no point in us speculating about what the graph shows.
Quote from: mvs on November 28, 2018, 02:13:49 pmQuote from: eev_carl on November 28, 2018, 01:54:52 pmI 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.Your measurement is not correct. You have measured first positive peak, that is 0.1V higher then all others.Thanks. I measured a few peaks down and found the ratio was more like 1.4/2 and that's -3.09dB.
Quote from: tggzzz on November 28, 2018, 02:39:58 pmQuote from: eev_carl on November 28, 2018, 01:59:34 pmQuote from: tggzzz on November 28, 2018, 01:55:35 pmQuote from: eev_carl on November 28, 2018, 12:48:51 pmI have a simple passive LPF and was trying out the loss at fc.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.I also breadboarded the circuit and got a similar ratio (520mv / 700mv) for Vin/Vout.Does this FFT analysis look correct? I see a 0dB value at 159Hz but am not sure how this relates to the -3dB of a cutoff frequency.If you are using a solderless breadboard, then expect to spend more time debugging the solderless breadboard than your circuit. For alternatives, see https://entertaininghacks.wordpress.com/2020/07/22/prototyping-circuits-easy-cheap-fast-reliable-techniques/Those spikes lead me to believe your graph is a measurement rather than an analysis. Since we have no idea what you measuring, nor how, there's no point in us speculating about what the graph shows.I'm measuring gain in decibels at the cutoff frequency. Another poster pointed out that I was looking at the first peak and that subsequent peaks were lower, bringing me to that -3dB value I was asking about.On the breadboard side, I'm forming the ratio with different scope settings (peak-to-peak, RMS, average). I've also tried cursors but don't see the peaks getting lower the way I did with LT Spice. Since it's such a simple circuit, I just clipped everything together and ditched the breadboard, but I'm still getting a ratio of 1.48/2.02 (Vpp). Is there a better scope setting to measure Vpp?
Quote from: eev_carl on November 28, 2018, 01:59:34 pmQuote from: tggzzz on November 28, 2018, 01:55:35 pmQuote from: eev_carl on November 28, 2018, 12:48:51 pmI have a simple passive LPF and was trying out the loss at fc.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.I also breadboarded the circuit and got a similar ratio (520mv / 700mv) for Vin/Vout.Does this FFT analysis look correct? I see a 0dB value at 159Hz but am not sure how this relates to the -3dB of a cutoff frequency.If you are using a solderless breadboard, then expect to spend more time debugging the solderless breadboard than your circuit. For alternatives, see https://entertaininghacks.wordpress.com/2020/07/22/prototyping-circuits-easy-cheap-fast-reliable-techniques/Those spikes lead me to believe your graph is a measurement rather than an analysis. Since we have no idea what you measuring, nor how, there's no point in us speculating about what the graph shows.
On the breadboard side, I'm forming the ratio with different scope settings (peak-to-peak, RMS, average). I've also tried cursors but don't see the peaks getting lower the way I did with LT Spice. Since it's such a simple circuit, I just clipped everything together and ditched the breadboard, but I'm still getting a ratio of 1.48/2.02 (Vpp). Is there a better scope setting to measure Vpp?
Quote from: eev_carl on November 28, 2018, 03:03:58 pmOn the breadboard side, I'm forming the ratio with different scope settings (peak-to-peak, RMS, average). I've also tried cursors but don't see the peaks getting lower the way I did with LT Spice. Since it's such a simple circuit, I just clipped everything together and ditched the breadboard, but I'm still getting a ratio of 1.48/2.02 (Vpp). Is there a better scope setting to measure Vpp?You may improve your measurement by selecting high resolution mode and/or selecting another V/div range. But in general scope is not a precision instrument.
Just use AC simulation instead: