# What is the cutoff frequency of the capacitively loaded Transmission line

1. ## 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? 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.  Reply With Quote

2. ## Re: What is the cutoff frequency of the capacitively loaded Transmission line Originally Posted by circuitslave 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. Originally Posted by circuitslave but I don't see the same when I simulated.
What value do you set for reference impedance of ports ?  Reply With Quote

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3. ## Re: What is the cutoff frequency of the capacitively loaded Transmission line Originally Posted by pancho_hideboo What value do you set for reference impedance of ports ?
50 ohm in both ports  Reply With Quote

4. ## Re: What is the cutoff frequency of the capacitively loaded Transmission line Originally Posted by circuitking 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.  Reply With Quote

5. ## Re: What is the cutoff frequency of the capacitively loaded Transmission line Originally Posted by circuitslave 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 ?  Reply With Quote

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6. ## 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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7. ## Re: What is the cutoff frequency of the capacitively loaded Transmission line Originally Posted by FvM 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  Reply With Quote

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8. ## Re: What is the cutoff frequency of the capacitively loaded Transmission line Originally Posted by circuitslave I used an RLGC file and the Z0 for line is more than 50 ohm
What quantity do you see in simulation ?  Reply With Quote

9. ## Re: What is the cutoff frequency of the capacitively loaded Transmission line Originally Posted by pancho_hideboo What quantity do you see in simulation ?
I see cutoff Frequency of 119.43 GHz in simulation  Reply With Quote

10. ## Re: What is the cutoff frequency of the capacitively loaded Transmission line Originally Posted by circuitslave 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 ?  Reply With Quote

11. ## Re: What is the cutoff frequency of the capacitively loaded Transmission line Originally Posted by pancho_hideboo No.

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

12. ## Re: What is the cutoff frequency of the capacitively loaded Transmission line Originally Posted by circuitslave I check S21.
You must not use S-parameter, since you can not understand RF and very basic things at all.  Reply With Quote

13. ## Re: What is the cutoff frequency of the capacitively loaded Transmission line Originally Posted by FvM 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?  Reply With Quote

14. ## 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.  Reply With Quote

15. ## Re: What is the cutoff frequency of the capacitively loaded Transmission line Originally Posted by FvM 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 - - - Originally Posted by pancho_hideboo 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.  Reply With Quote

16. ## Re: What is the cutoff frequency of the capacitively loaded Transmission line  Reply With Quote

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

I would consider the -3 dB point as the cutoff frequency. And as you mentioned there is no simple equation to calculate it.  Reply With Quote

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