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    What is the cutoff frequency of the capacitively loaded Transmission line

    Hi, I connected a circuit like shown below. Let mtlines have length l, per unit inductance Ltl and capacitance Ctl, and capacitance C, what should be the formula for cutoff frequency?
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    In various books it is given that the cutoff frequency should be fc=1/(pi*sqrt(Ltl(Ctl+C/l))) but I don't see the same when I simulated. Thanks in advance.

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    Re: What is the cutoff frequency of the capacitively loaded Transmission line

    Quote Originally Posted by circuitslave View Post
    In various books it is given that the cutoff frequency should be fc=1/(pi*sqrt(Ltl(Ctl+C/l)))
    Generally wrong, although I don't know what book you read.

    Your circuit is well-known capacitive discontinuity in uniform line.

    Quote Originally Posted by circuitslave View Post
    but I don't see the same when I simulated.
    What value do you set for reference impedance of ports ?



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    Re: What is the cutoff frequency of the capacitively loaded Transmission line

    Quote Originally Posted by pancho_hideboo View Post
    What value do you set for reference impedance of ports ?
    50 ohm in both ports



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    Re: What is the cutoff frequency of the capacitively loaded Transmission line

    Quote Originally Posted by circuitking View Post
    In various books it is given that the cutoff frequency should be fc=1/(pi*sqrt(Ltl(Ctl+C/l))) but I don't see the same when I simulated. Thanks in advance.
    That's probably for an infinite cascade of embedded capacitors. I'm not sure how well one capacitor would reproduce this result. I would recommend to try additional periods and see of anything changes.



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    Re: What is the cutoff frequency of the capacitively loaded Transmission line

    Quote Originally Posted by circuitslave View Post
    50 ohm in both ports
    So large mismatch at port, although I don't know what you set for "mtline".
    What quantity do you see in simulation ?



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    Re: What is the cutoff frequency of the capacitively loaded Transmission line

    If mtline is setup as an ideal 50 ohm transmission line, the answer is very simple and you should be able to derive it yourself. Just a first order low pass with tau = c*Z0/2.


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    Re: What is the cutoff frequency of the capacitively loaded Transmission line

    Quote Originally Posted by FvM View Post
    If mtline is setup as an ideal 50 ohm transmission line,
    I used an RLGC file and the Z0 for line is more than 50 ohm



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    Re: What is the cutoff frequency of the capacitively loaded Transmission line

    Quote Originally Posted by circuitslave View Post
    I used an RLGC file and the Z0 for line is more than 50 ohm
    What quantity do you see in simulation ?



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    Re: What is the cutoff frequency of the capacitively loaded Transmission line

    Quote Originally Posted by pancho_hideboo View Post
    What quantity do you see in simulation ?
    I see cutoff Frequency of 119.43 GHz in simulation



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    Re: What is the cutoff frequency of the capacitively loaded Transmission line

    Quote Originally Posted by circuitslave View Post
    I see cutoff Frequency of 119.43 GHz in simulation
    No.

    What quantity do you see to evaluate cutoff Frequency in your EDA Tool Play ?
    Last edited by pancho_hideboo; 24th October 2019 at 22:17.



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    Re: What is the cutoff frequency of the capacitively loaded Transmission line

    Quote Originally Posted by pancho_hideboo View Post
    No.

    What quantity do you see to evaluate cutoff Frequency in your EDA Tool Play ?
    I check S21.



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    Re: What is the cutoff frequency of the capacitively loaded Transmission line

    Quote Originally Posted by circuitslave View Post
    I check S21.
    You must not use S-parameter, since you can not understand RF and very basic things at all.
    https://www.edaboard.com/showthread.php?385709#2
    https://www.edaboard.com/showthread.php?385709#7



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    Re: What is the cutoff frequency of the capacitively loaded Transmission line

    Quote Originally Posted by FvM View Post
    If mtline is setup as an ideal 50 ohm transmission line, the answer is very simple and you should be able to derive it yourself. Just a first order low pass with tau = c*Z0/2.
    What happens if the Line is not 50 ohm. is the same equation applicable?



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    Re: What is the cutoff frequency of the capacitively loaded Transmission line

    Surely not.You get reflections at the ports, the line length also matters, and the frequency characteristic is no longer monotone. Theres's no simple cutoff frequency formula applicable.



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    Re: What is the cutoff frequency of the capacitively loaded Transmission line

    Quote Originally Posted by FvM View Post
    Theres's no simple cutoff frequency formula applicable.
    Then when to use this formula fc=1/(pi*sqrt(Ltl(Ctl+C/l)))? which scenarios, line with infinite length ?

    - - - Updated - - -

    Quote Originally Posted by pancho_hideboo View Post
    You must not use S-parameter
    If the port impedances are 50 ohm (i.e, in matched condition) voltage gain is equal to power gain (S21). When the circuit is Linear, Pout(dBm)= S21(dB)+Pin(dBm). So it doesn't matter which one I am looking at, all should give same cut off frequency.



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    Re: What is the cutoff frequency of the capacitively loaded Transmission line

    An example of Zline > Zport is shown below. How do you design cut-off frequency in this case?

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Name:	line.PNG 
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    Re: What is the cutoff frequency of the capacitively loaded Transmission line

    Quote Originally Posted by FvM View Post
    How do you design cut-off frequency in this case?

    Click image for larger version. 

Name:	line.PNG 
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    I would consider the -3 dB point as the cutoff frequency. And as you mentioned there is no simple equation to calculate it.



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