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Phase shift

Arouse1973

Adam
Dec 18, 2013
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Why don't you have a look and you tell me what you think might work. Download LT Spice and copy my diagram and have a go yourself. It's more fun that way.
Adam
 

Ricperes

Jul 20, 2015
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i already have the Lt spice, and have tested the diagram. I was just wondering why you could find some alternative to LT 1006 and so tomorrow would buy. But thank you anyway
 

Arouse1973

Adam
Dec 18, 2013
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Why not using a simple allpass stage (one opamp only) with a phase shift of -60deg (identical to +120deg) ?
https://en.wikipedia.org/wiki/All-pass_filter

Hi LvW, I thought to my self LvW will say all pass filter :). I used this approach because the building blocks can be broken down and easily understood for a beginner. You never know he might find all pass filters easier to understand.

And yes any op-amp can be used your right ak. I wanted the OP to do a bit of work himself, he appears to be a bit reluctant to help himself for some reason.

Adam
 

LvW

Apr 12, 2014
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that's a good ideia, can you show me how I can scale the R and C for a phase shift 120º please?
The circuit with three equal resistors R is given in the wiki reference.
The transfer function is H(s)=(1-sT)/(1+sT) with T=RC.
And the phase is phi=- 2*arctan(2*Pi*f*T).
Hence, for any fixed frequency the value of T can be found for a desired phase phi. Note that -60deg is equivalent to +120deg.
 

Ricperes

Jul 20, 2015
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The circuit with three equal resistors R is given in the wiki reference.
The transfer function is H(s)=(1-sT)/(1+sT) with T=RC.
And the phase is phi=- 2*arctan(2*Pi*f*T).
Hence, for any fixed frequency the value of T can be found for a desired phase phi. Note that -60deg is equivalent to +120deg.

So in the image that you show me in wikipedia are 3 R ( Rx, Rx, R) you say that are Rx=Rx=R ? So i want 120º degrees , it's 120 degrees convert to radians and do phi= -2 arctan( 2*pi*50* R*C) ?
 

LvW

Apr 12, 2014
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Oh - I just have realized that the wiki circuit differs from the circuit I had in mind. But it is not a problem.
* The wiki circuit has a CR highpass at the non-inv. input node and the phase is phi=180 - 2*arctan(2*Pi*f*T).
*
Replacing the highpass by an RC lowpass (interchange of R and C) the phase is as given in my answer#28.

Concerning your last question: Yes - three equal resistors.
 

Ricperes

Jul 20, 2015
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Phi = tan^-1(XC/R) or Phi = 360*fo*dt. But this will only get you close, you will have to adjust the resistor values to get it right. When I did the calculations the circuit didn't produce the desired phase shift so I had to manually change them to get it right, you will need to adjust the values yourself.
Adam

Hello , I made the schematic that you give me and the result is very good, but after that i want to ask you two things.

First: which the equation to obtain the value of C and R? I tested this Phi = tan^-1(XC/R) and i don't have a goo result, maybe i'm doing something wrong.

Second: this assembly was made by you do i need some literature to reference in my work.

Thanks for your help.
 

Arouse1973

Adam
Dec 18, 2013
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It was something I thought would work just of the top of my head. I don't have anything written down. The formula I gave was to calculate the phase shift. Show us what you did and what was wrong.
Adam
 

Ricperes

Jul 20, 2015
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It was something I thought would work just of the top of my head. I don't have anything written down. The formula I gave was to calculate the phase shift. Show us what you did and what was wrong.
Adam

th formula is phi=tan^-1 (Xc/R) , i know that Xc= 1/(2pi*F*C) replacing phi=tan^-1 ( 1/(2pi*F*C) / R) it is right?


So "phi" is the phase shift , i want 120 degrees. replacing in formula 120 =tan^-1 ( 1/(2pi*F*C) / R) that's it?
 

Ricperes

Jul 20, 2015
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It was something I thought would work just of the top of my head. I don't have anything written down. The formula I gave was to calculate the phase shift. Show us what you did and what was wrong.
Adam

Need help please Arouse1973
 

Ricperes

Jul 20, 2015
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With what? I thought you had it all sorted sorry. Please explain in detail what you are still having issues with.
Adam

Thanks for your help one more time.

My doubt is how you dimension de values of C and R to get 120º phase shift.... I had tried but didn't understand de calculations . Can you post here the calculations that you made please?
 

Arouse1973

Adam
Dec 18, 2013
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Here is my offering for calculating the capacitor value for approx. 60 degrees phase shift. Hopefully you can see how this works. This is an example only, you need to input your own values if different.

Thanks
Adam

XC_Example.PNG
 

Ricperes

Jul 20, 2015
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Here is my offering for calculating the capacitor value for approx. 60 degrees phase shift. Hopefully you can see how this works. This is an example only, you need to input your own values if different.

Thanks
Adam

View attachment 25351


I will try this equations but in this shematic that you posted here each ampop offer 60º degrees phase shift, right?
 

Arouse1973

Adam
Dec 18, 2013
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I will try this equations but in this shematic that you posted here each ampop offer 60º degrees phase shift, right?

You said you wanted to know how to work out the capacitor value. I gave you an example. You do realise you can't get 120 degrees from one RC network, you have to add more than one. So you make two 60 degree stages and put them together.
Adam
 

Ricperes

Jul 20, 2015
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You said you wanted to know how to work out the capacitor value. I gave you an example. You do realise you can't get 120 degrees from one RC network, you have to add more than one. So you make two 60 degree stages and put them together.
Adam

Arouse1973 sorry my doubts but, I had tried to do this and it's ok, i had good results. So when i'm trying to scale with the equations that you given me I can't....
Please tell me what i'am doin wrong, so my Vin is a sinusoide with 50hz frequeny, and i don't unerstand what R3 and R4 are.

Thanks one more time
 

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