I had never tried it in spice. I tried direct coupled first because I didn't really know what I was doing with the LTspice transmission line.
But this much seems to work just fine. It is very idealistic. I was checking some math with a circuit simulator.
Version 4
SHEET 1 6120 3588
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TEXT -200 -976 Left 2 !.ac lin 5001 1e9 11e9
TEXT -192 -904 Left 2 ;Save only S-Parameters. It's not really necessary.
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TEXT 504 -1064 Left 2 !.net I(R_load00) Vsrc00 ; Rsrc & R_load determined from Vsrc and R_load
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TEXT 504 -1152 Left 2 !.param Rsrc00 = 50
TEXT 504 -1112 Left 2 !.param Rload00=50
Direct Coupled Transmission Line filter in LTspice
Started by ●March 16, 2023
Reply by ●March 16, 20232023-03-16
On Thu, 16 Mar 2023 15:47:15 -0700 (PDT), Simon S Aysdie <gwhite@ti.com> wrote:>I had never tried it in spice. I tried direct coupled first because I didn't really know what I was doing with the LTspice transmission line. > >But this much seems to work just fine. It is very idealistic. I was checking some math with a circuit simulator. >That's cool. How did you design it? What's the alternative to direct coupled?
Reply by ●March 16, 20232023-03-16
On Thursday, March 16, 2023 at 4:03:22 PM UTC-7, John Larkin wrote:> On Thu, 16 Mar 2023 15:47:15 -0700 (PDT), Simon S Aysdie > <gwh...@ti.com> wrote: > > >I had never tried it in spice. I tried direct coupled first because I didn't really know what I was doing with the LTspice transmission line. > > > >But this much seems to work just fine. It is very idealistic. I was checking some math with a circuit simulator. > > > That's cool. How did you design it? > > What's the alternative to direct coupled?I designed it mainly by following Daniels† and Minnis‡. The filter has one transmission zero (TZ) at dc and 3 unit elements (UE). With the Richards transformation, the UE are "half-order TZs" on the real axis at ±1. I used Kuroda transforms to turn the one dc TZ into the 4 equal shunt L. (Those are the shunt transmission lines.) That procedure makes the line impedances more practical. The "dual" would have series caps, which ends up meaning it would be a parallel-coupled (edge-coupled for PCB/planer) design. The caps require a different set of Kuroda xforms. "Edge coupled" has no ground via registration issue. I am not sure LTspice can do coupled lines which is needed for the dual. There is the .\examples\Educational\TransmissionLineInverter.asc helper file example that may help me get a coupled pair, if I can understand it, which I presently do not. I have AWR available, so I am just messing with LTspice out of curiousity. I was doing some math checking on an intentionally "simple example" because the combination of ±1 TZ of the UE affects the s21 phase, but not magnitude. The ±1 TZ UE combo does not affect the driving point immittance that the circuit is extracted from (at all). I found that interesting. I also used Filsyn for checking and affirming realization and the Kuroda xforms. This "bandpass" is actually a highpass filter that innately becomes bandpass because of the periodicity. The Filsyn transfer function output (rational/polynomial coefficients) for distributed HPF is quite badly wrong. † R. Daniels, Approximation Methods for Electronic Filter Design. New York, NY: McGraw-Hill, 1974. ‡ B. J. Minnis, Designing Microwave Circuits by Exact Synthesis. Norwood, MA: Artech House, 1996.
