The LTSpice sim below is a diplexer which presents a broadband 50-ohm termination to a diode ring mixer and provides a 1:4 impedance step-up to a
200 output load over > 2 MHz bandwidth centred around 30 MHz.The problem is, for optimal input match, L2 needs to be ~6u8 which makes C2 ~3p6. Both unrealistic values. Try clicking on Vin, run the sim, then change L2 to 1u and run it again.
I found the optimal value of L2 by stepping the parameter. I would like to how how to calculate it. Also, is there any way to improve the performance with more realistic component values?
TIA
Version 4 SHEET 1 3612 1156 WIRE -528 128 -704 128 WIRE 112 128 -528 128 WIRE 304 128 112 128 WIRE -704 144 -704 128 WIRE -528 224 -528 128 WIRE 304 240 304 128 WIRE -976 256 -1056 256 WIRE -784 256 -896 256 WIRE -704 256 -704 224 WIRE -704 256 -784 256 WIRE -192 272 -400 272 WIRE 128 272 -112 272 WIRE -704 288 -704 256 WIRE -1056 336 -1056 256 WIRE -704 384 -704 368 WIRE -528 384 -528 288 WIRE -528 384 -704 384 WIRE -400 384 -400 272 WIRE -400 384 -528 384 WIRE -320 384 -400 384 WIRE -80 384 -240 384 WIRE 128 384 128 272 WIRE 128 384 -16 384 WIRE -1056 464 -1056 416 WIRE 128 464 128 384 WIRE 304 464 304 320 FLAG -1056 464 0 FLAG 128 464 0 FLAG 304 464 0 FLAG 112 128 Vout FLAG -784 256 Vin SYMBOL voltage -1056 320 R0 WINDOW 123 18 99 Left 0 WINDOW 39 0 0 Left 0 SYMATTR Value2 AC 2 SYMATTR InstName V1 SYMATTR Value "" SYMBOL res -880 240 R90 WINDOW 0 0 56 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName RS SYMATTR Value 50 SYMBOL res -208 288 R270 WINDOW 0 32 56 VTop 0 WINDOW 3 0 56 VBottom 0 SYMATTR InstName RD SYMATTR Value {1/(1/50-1/200)} SYMBOL ind2 -720 128 R0 SYMATTR InstName L1A SYMATTR Value 36n SYMATTR Type ind SYMBOL ind2 -720 272 R0 SYMATTR InstName L1B SYMATTR Value 36n SYMATTR Type ind SYMBOL cap -512 288 R180 WINDOW 0 24 64 Left 0 WINDOW 3 24 8 Left 0 SYMATTR InstName C1 SYMATTR Value 170p SYMBOL ind -336 400 R270 WINDOW 0 32 56 VTop 0 WINDOW 3 5 56 VBottom 0 WINDOW 39 -23 56 VBottom 0 SYMATTR InstName L2 SYMATTR Value {L2} SYMATTR SpiceLine Rser=2 SYMBOL cap -80 400 R270 WINDOW 0 32 32 VTop 0 WINDOW 3 0 32 VBottom 0 SYMATTR InstName C2 SYMATTR Value {1/2/2/PI/PI/F/F/L2} SYMBOL res 288 224 R0 SYMATTR InstName RL SYMATTR Value 200 TEXT -1024 144 Left 0 !.ac dec 1000 1e7 1e8 TEXT -712 424 Left 0 !K1 L1A L1B 1 TEXT -384 168 Left 0 !.param L2 6u86\n.param F {1/2/PI/sqrt(4*36e-9*170e-12)}