Electronics > Beginners

Meeasure capacitance wiht oscilloscope

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Wimberleytech:

--- Quote from: David Hess on February 13, 2020, 03:14:22 am ---
--- Quote from: Adrian_Arg. on February 13, 2020, 02:01:14 am ---
--- Quote from: David Hess on February 12, 2020, 11:30:00 pm ---My preferred method is to use a sine wave source and two channels of the oscilloscope to find the frequency where the phase lag is 45 degrees.
--- End quote ---

and how do you do that? :-//
--- End quote ---

Set the capacitor up in the same RC circuit but driven with a sine wave.  Attach the probes before and after the resistor.

Now you can measure the point where the output voltage is reduced to 0.707 of the input to find the -3dB bandwidth as the frequency is increased but it is more accurate to measure the point where the phase lags by 45 degrees and most DSOs can make this measurement automatically.  Once you know that, you can calculate the capacitors value from the resistance and frequency.

The time domain method using the exponential waveform works but measurements of time are generally easier and more accurate than measurements of voltage.

--- End quote ---
For kicks, I played around with both methods this morning.  My Siglent does XY but I have not found a way for it to do the phase calculation for me.  I have to adjust until the ratio of two vertical readings are 0.707.  You adjust freq, measure, calculate...adjust some more.  Tedious.
The time-domain method is a little less tedious because you are just measuring.

Adrian_Arg.:
ok, I could not find any video or frequency domain, or the phase  |O |O

Wimberleytech:

--- Quote from: Adrian_Arg. on February 13, 2020, 05:02:34 pm ---ok, I could not find any video or frequency domain, or the phase  |O |O

--- End quote ---
Make a simple RC circuit with the input into the resistor and the capacitor to ground.
With channel 1 of the scope, connect to the junction of the R and C
Input a sine wave to the dangling end of the resistor.
Input the same sine wave to channel 2 of the scope.
Set the scope in XY mode
Adjust the input frequency until you get a waveform as shown in the attached figure.
For a 45° phase shift. sin(45) = 0.707
Adjust the input further until C/D = 0.707
Multiply that frequency by 2π, multiply by the resistor value and then invert to get C

--That's how I did it anyway...never used this method before today--

Mortymore:
Decided to play with the 45º phase shift option, using C=820nF and R=100 Ohm

Scope CH2 only at R, and CH1 R+C (or measuring the generator output if you wish to see it that way)

The generator set for sinusoidal 4Vpp and frequency was set until the scope shows the desired 45º shift between the 2 scope inputs



An in XY mode



C= 1/(2*Pi*f*R) = 1 / (2*Pi*1900*100) = 837.66*10^-9 or 837nF

Wimberleytech:

--- Quote from: Mortymore on February 13, 2020, 11:30:35 pm ---Decided to play with the 45º phase shift option, using C=820nF and R=100 Ohm

Scope CH2 only at R, and CH1 R+C (or measuring the generator output if you wish to see it that way)

The generator set for sinusoidal 4Vpp and frequency was set until the scope shows the desired 45º shift between the 2 scope inputs


An in XY mode


C= 1/(2*Pi*f*R) = 1 / (2*Pi*1900*100) = 837.66*10^-9 or 837nF

--- End quote ---
Cool!  What scope is that?

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