Electronics > Beginners

-3dB and an LPF

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tggzzz:

--- Quote from: eev_carl on November 28, 2018, 02:42:20 pm ---
--- Quote from: mvs on November 28, 2018, 02:13:49 pm ---
--- Quote from: eev_carl on November 28, 2018, 01:54:52 pm ---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.

--- End quote ---
Your measurement is not correct. You have measured first positive peak, that is 0.1V higher then all others.

--- End quote ---


Thanks.  I measured a few peaks down and found the ratio was more like 1.4/2 and that's -3.09dB.

--- End quote ---

Your next task is to understand why that phenomenon occurred in simulation, and whether it would also be seen in a real circuit.

tggzzz:

--- Quote from: eev_carl on November 28, 2018, 03:03:58 pm ---
--- Quote from: tggzzz on November 28, 2018, 02:39:58 pm ---
--- Quote from: eev_carl on November 28, 2018, 01:59:34 pm ---
--- Quote from: tggzzz on November 28, 2018, 01:55:35 pm ---
--- 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.

--- End quote ---


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.

--- End quote ---

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.

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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?

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You 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.

mvs:

--- Quote from: eev_carl on November 28, 2018, 03:03:58 pm ---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?

--- End quote ---
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.

eev_carl:
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.

tggzzz:

--- Quote from: mvs on November 28, 2018, 03:23:55 pm ---
--- Quote from: eev_carl on November 28, 2018, 03:03:58 pm ---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?

--- End quote ---
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.

--- End quote ---

.. or rather it is a precision instrument as defined in the handbook - which is rather less precise than beginners imagine :)

A scope's prime function is to observe a signal's shape, and it does that well - within limitations.

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