-3dB and an LPF - Page 1
Forum section: electronics / beginners

author: eev_carl
author_rank: Regular Contributor
timestamp: November 28, 2018, 12:48:51 pm
content: 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

author: ogden
author_rank: Super Contributor
timestamp: November 28, 2018, 01:16:56 pm
content: 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.

author: aneevuser
author_rank: Frequent Contributor
timestamp: November 28, 2018, 01:27:55 pm
content: Quote from: eev_carl on November 28, 2018, 12:48:51 pmAre 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)

author: eev_carl
author_rank: Regular Contributor
timestamp: November 28, 2018, 01:54:52 pm
content: Quote from: aneevuser on November 28, 2018, 01:27:55 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.

author: tggzzz
author_rank: Super Contributor
timestamp: November 28, 2018, 01:55:35 pm
content: Quote from: eev_carl on November 28, 2018, 12:48:51 pmYou don't have a simple passive LPF, you have asimulationof a simple passive LPF.Use the simulator's different types of analysis; in this case AC (i.e. frequency domain) simulation is enlightening.

author: eev_carl
author_rank: Regular Contributor
timestamp: November 28, 2018, 01:59:34 pm
content: Quote from: tggzzz on November 28, 2018, 01:55:35 pmI 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.

author: mvs
author_rank: Frequent Contributor
timestamp: November 28, 2018, 02:13:49 pm
content: Quote from: eev_carl on November 28, 2018, 01:54:52 pmYour measurement is not correct. You have measured first positive peak, that is 0.1V higher then all others.

author: tggzzz
author_rank: Super Contributor
timestamp: November 28, 2018, 02:39:58 pm
content: Quote from: eev_carl on November 28, 2018, 01:59:34 pmIf you are using a solderless breadboard, then expect to spend more time debugging the solderless breadboard than your circuit. For alternatives, seehttp://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.

author: eev_carl
author_rank: Regular Contributor
timestamp: November 28, 2018, 02:42:20 pm
content: Quote from: mvs on November 28, 2018, 02:13:49 pmThanks. I measured a few peaks down and found the ratio was more like 1.4/2 and that's -3.09dB.

author: eev_carl
author_rank: Regular Contributor
timestamp: November 28, 2018, 03:03:58 pm
content: Quote from: tggzzz on November 28, 2018, 02:39:58 pmI'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?

author: tggzzz
author_rank: Super Contributor
timestamp: November 28, 2018, 03:06:48 pm
content: Quote from: eev_carl on November 28, 2018, 02:42:20 pmYour next task is to understand why that phenomenon occurred in simulation, and whether it would also be seen in a real circuit.

author: tggzzz
author_rank: Super Contributor
timestamp: November 28, 2018, 03:14:02 pm
content: Quote from: eev_carl on November 28, 2018, 03:03:58 pmYou are measuring far more than "gain in decibels at the cutoff frequency" - the graph goes from 0.1Hz to 20kHz with a measurement at all those frequencies.Now I could hazard a guess at your experimental setup (signal source, UUT, measurement tools and techniques), but that's a waste of my time.

author: mvs
author_rank: Frequent Contributor
timestamp: November 28, 2018, 03:23:55 pm
content: Quote from: eev_carl on November 28, 2018, 03:03:58 pmYou may improve your measurement by selecting high resolution mode and/or selecting another V/div range. But in generalscope is not a precision instrument.

author: eev_carl
author_rank: Regular Contributor
timestamp: November 28, 2018, 03:30:01 pm
content: Thanks all. My components were off. My R was actually 9.88k and C was 94.5nF. This gave me a fc of 170Hz, not 159Hz so my function generator wasn't set correctly. The ratio, measured as V peak-to-peak was 1.42/2.02 and this brought me to -3.06dB.

author: tggzzz
author_rank: Super Contributor
timestamp: November 28, 2018, 03:34:43 pm
content: Quote from: mvs on November 28, 2018, 03:23:55 pm.. or rather it is a precision instrumentas defined in the handbook- which is rather less precise than beginners imagineA scope's prime function is to observe a signal's shape, and it does that well - within limitations.

author: Berni
author_rank: Super Contributor
timestamp: November 28, 2018, 03:51:54 pm
content: Just use AC simulation instead:

author: tggzzz
author_rank: Super Contributor
timestamp: November 28, 2018, 04:24:24 pm
content: Quote from: Berni on November 28, 2018, 03:51:54 pmAs mentioned abovePlus he could give the resistor a 1% tolerance and the capacitor a 10% tolerance, and run a monte carlo analysis to see a range of possible cutoff frequencies: