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22nd October 2019, 13:40 #1
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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?
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.

22nd October 2019, 15:00 #2
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Re: What is the cutoff frequency of the capacitively loaded Transmission line

22nd October 2019, 15:14 #3
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22nd October 2019, 15:20 #4
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Re: What is the cutoff frequency of the capacitively loaded Transmission line

22nd October 2019, 15:30 #5

22nd October 2019, 16:16 #6
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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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23rd October 2019, 00:04 #7
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23rd October 2019, 01:03 #8

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24th October 2019, 21:03 #9
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24th October 2019, 22:11 #10

25th October 2019, 07:38 #11
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25th October 2019, 14:50 #12
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Re: What is the cutoff frequency of the capacitively loaded Transmission line
You must not use Sparameter, 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

28th October 2019, 22:10 #13
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28th October 2019, 22:25 #14
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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.

28th October 2019, 23:37 #15
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Re: What is the cutoff frequency of the capacitively loaded Transmission line
Then when to use this formula fc=1/(pi*sqrt(Ltl(Ctl+C/l)))? which scenarios, line with infinite length ?
   Updated   
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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29th October 2019, 12:43 #16
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29th October 2019, 13:09 #17
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