Jeroen Belleman has designed some spiffy absorptive lowpass filters that look resistive at one port.
Mini-circuits has some new filters that are resistive on both ports:
I have some samples coming to test. What are the input and output shunt resistors for? What is their value? We'll see.
So I thought I'd take a whack at it myself. I wanted something simpler than Jeroen's and cheaper than Minicircuits, and something I could make from parts in stock. The idea to to remove ringing and junk from a very fast but pretty ugly pulse generator output, giving up a little speed for beauty.
Version 4 SHEET 1 960 680 WIRE 32 128 -16 128 WIRE 96 128 32 128 WIRE 224 128 176 128 WIRE 272 128 224 128 WIRE 432 128 352 128 WIRE 480 128 432 128 WIRE 592 128 480 128 WIRE 816 128 688 128 WIRE 864 128 816 128 WIRE 224 160 224 128 WIRE 592 160 560 160 WIRE 720 160 688 160 WIRE -16 176 -16 128 WIRE 432 192 432 128 WIRE 864 192 864 128 WIRE 560 208 560 160 WIRE 720 208 720 160 WIRE -16 304 -16 256 WIRE 224 304 224 224 WIRE 432 320 432 272 WIRE 864 336 864 272 WIRE 432 416 432 384 FLAG -16 304 0 FLAG 224 304 0 FLAG 432 416 0 FLAG 560 208 0 FLAG 720 208 0 FLAG 864 336 0 FLAG 32 128 GEN FLAG 480 128 OUT FLAG 816 128 EOL SYMBOL res 80 144 R270 WINDOW 0 79 60 VTop 2 WINDOW 3 69 59 VBottom 2 SYMATTR InstName R1 SYMATTR Value 50 SYMBOL res 416 176 R0 WINDOW 0 -56 31 Left 2 WINDOW 3 -56 71 Left 2 SYMATTR InstName R2 SYMATTR Value 50 SYMBOL cap 208 160 R0 WINDOW 0 -52 44 Left 2 WINDOW 3 -58 79 Left 2 SYMATTR InstName C1 SYMATTR Value 6.8p SYMBOL cap 416 320 R0 WINDOW 0 -61 17 Left 2 WINDOW 3 -68 50 Left 2 SYMATTR InstName C2 SYMATTR Value 6.8p SYMBOL ind 256 144 R270 WINDOW 0 80 56 VTop 2 WINDOW 3 71 56 VBottom 2 SYMATTR InstName L1 SYMATTR Value 33n SYMBOL voltage -16 160 R0 WINDOW 0 19 117 Left 2 WINDOW 3 -50 195 Left 2 WINDOW 123 0 0 Left 2 WINDOW 39 0 0 Left 2 SYMATTR InstName V1 SYMATTR Value PULSE(0 1 1n 800p 800p 8n) SYMBOL tline 640 144 R0 WINDOW 0 -2 -72 Bottom 2 WINDOW 3 -3 -62 Top 2 SYMATTR InstName T1 SYMATTR Value Td=10n Z0=50 SYMBOL res 848 176 R0 WINDOW 0 57 40 Left 2 WINDOW 3 56 75 Left 2 SYMATTR InstName R3 SYMATTR Value 1K TEXT 592 384 Left 2 !.tran 50n TEXT 464 296 Left 2 ;Simple Absorptive Lowpass Filter TEXT 552 336 Left 2 ;JL May 24, 2016
The test case is to set the terminator R3 high, above 50 ohms, to let the txline reflect a pulse back into the filter, and see how much it bounces around. This is pretty good, just a few percent echo.
This filter was designed in about a half hour purely by instinct and fiddling, with parts values selected from what we have in stock; no explicit math involved. That works for simple stuff like this but blows up for not-much-more complex networks where the fiddling tends to diverge. That becomes the "lost in space" syndrome.