phase in a simple RC circuit

[yakirsudry]

I have tried to measure the phase between the source voltage (a sine voltage) to the voltage on the capacitor and resistor.
by my understanding the phase between the source and the resistor is
arctg(1/(wRC)) (which i actually received)
and between the source and the capacitor the phase is
arctg(1/(wRC))-pi/2

but this result I dont receive. I get something different that when w->infinity the phase goes to zero

can someone first approve my understanding of the theory and try to explain it?

 

[dknguyen]

Don't you mean voltage and current? THe voltage lags on the current by 90 degrees on a capacitor. Or are you talking about something else?

 

[yakirsudry]

I mean the phase shift between the entering voltage (the source) to the received voltage on the resistor or the capacitor
for the resistor I get good results
phase=arctg(1/(wRC))
but not for the capacitor which in my understanding should be
phase=arctg(1/(wRC))-pi/2

 

[dknguyen]

What is your circuit? Is RC in series or parallel?

 

[yakirsudry]

series circuit.. judt s function generator (generating a sine wave), R ,C and osciloscope

 

[Mikebits]

It would be hard to measure that small of phase shift on a scope.

Are you saying you have:

Gen--->R---C---scope?

 

[yakirsudry]

it's not so hard because the scope generates a file that can be used in matlab to get the result.
the circuit is connected correctly if this is your question.
this is the graph I receive
**broken link removed**

it's close to the expected one except the part that raises back up.. I think it should keep go down to -pi/2

 

[MrAl]

Hi,

If i understand you correctly, you want this:

PH=-atan(w*C*R)

and this PH is the phase angle between input voltage and output voltage.

If you also state your R and C values and the frequency we can
verify you are getting the right value for PH.

 

[yakirsudry]

no.
I excpect
ph=arctg(1/(wRC))-pi/2

R=12 kOhm
C=1 nF

I must say that the amplitude graph goes correctly (low pass filter)
but the phase doesn't for some reason.

 

[MrAl]

Hi,

Do you have the scope across the capacitor?

Like this:

   -----R------+------
IN             C       OUT
   ------------+------

 

[Mikebits]

Try this. Duplicate your circuit by a magnitude of say ten. Then measure phase. I still question the scopes ability to distinguish small phase changes. At minimum, have you checked scopes specification for phase measurement?

 

[MrAl]

no.
I excpect
ph=arctg(1/(wRC))-pi/2

R=12 kOhm
C=1 nF

I must say that the amplitude graph goes correctly (low pass filter)
but the phase doesn't for some reason.

If that's a low pass RC filter than you expect the wrong thing.
The phase is -atan(w*R*C), not what you are showing.
Check my circuit drawing above and tell us if that is the circuit,
or else draw the circuit yourself so we know.

 

[yakirsudry]

yes.. this is kind of it...
so what is the phase I should receive when measuring on the cpacitor and when measuring on the resistor?

 

[MrAl]

Hi again,


With the scope ground connected to the signal generator ground and
the scope probe connected to the junction of the resistor and
capacitor (this will be looking across the capacitor) you will see the
phase as -atan(wRC). This is the normal way to look at the output
of a low pass filter anyway.

If you would like to post your resistor value (ohms) and capacitor
value (farads or microfarads) and choose some frequency f and
tell us that too and we can calculate the phase and then you can
compare to your own readings.