Hi to all. RF is not really my strong point , so I could use some advice here. I have a simple Collpits oscillator running at 433.92Mhz. It has a strong output signal at 433Mhz , but unfortunately also at 868Mhz. In fact the signal at 868Mhz is also very strong. I would like to attenuate this harmonic , and the others that I can't see(My Spectrum analyser only goes to 1Gig), which are definitely there. I want to put in a 5th order low pass filter. I have Chris Bowick's RF design book , which shows some easy ways to design a filter for this. The problem is I don't know how to calculate the output impedance of the oscillator , which is obviously needed in the calcs. I'm assuming the load , which is a 1/4 length piece of wire to be about 75 ohms , give or take. The data sheet I have for the BRF93a only shows S11 at 400 and
500MHz , so I have to interpret the impedance at 433Mhz , but it gives ma a starting point. This is also at Vce of 8V and 25ma , which is not exactly where I'm sitting , but I think is close enough. I get a s11 of 0.211 , angle -137deg. Plotting this on a Smith chart shows an impedance of 35.3-J10.5 ohms. I assume this to be roughly the output impedance of the transistor. I'm not even sure if I'm on the right track at all :0( Assuming this is correct , Do I use this as the source impedance in my calculations , or do I then have to take into account the other components around the transistor.(I assume so) It would then be (attached is a schematic in LTSpice) (35.3-j10.5 + 47) // (L1 // (C1+c2)) . Hope this makes sense. It's the output impedance of the transistor in series with the emitter resistor. All this in parallel with the parallel combination of the inductor and the 2 caps in series , phew..... What I need to know is am I on the correct path at all , or am I missing the boat all together :0( Any pointers would be great. Cheers RobVersion 4 SHEET 1 880 680 WIRE -16 16 -160 16 WIRE 96 16 -16 16 WIRE 176 16 96 16 WIRE 176 32 176 16 WIRE -16 80 -16 16 WIRE 96 96 96 80 WIRE -160 144 -160 16 WIRE 176 144 176 112 WIRE 288 144 176 144 WIRE 432 144 288 144 WIRE 448 144 432 144 WIRE 560 144 528 144 WIRE 576 144 560 144 WIRE 672 144 656 144 WIRE 704 144 672 144 WIRE 784 144 768 144 WIRE 176 160 176 144 WIRE 288 176 288 144 WIRE -16 208 -16 160 WIRE 112 208 -16 208 WIRE 784 224 784 144 WIRE 432 240 432 144 WIRE 560 240 560 144 WIRE 672 240 672 144 WIRE 176 272 176 256 WIRE 288 272 288 240 WIRE 288 272 176 272 WIRE -16 288 -16 208 WIRE 176 288 176 272 WIRE 288 288 288 272 WIRE -160 400 -160 224 WIRE -16 400 -16 336 WIRE -16 400 -160 400 WIRE 176 400 176 368 WIRE 176 400 -16 400 WIRE 288 400 288 352 WIRE 288 400 176 400 WIRE 432 400 432 304 WIRE 432 400 288 400 WIRE 560 400 560 304 WIRE 560 400 432 400 WIRE 672 400 672 304 WIRE 672 400 560 400 WIRE 784 400 784 304 WIRE 784 400 672 400 WIRE 176 416 176 400 FLAG 176 416 0 FLAG 96 96 0 SYMBOL res 160 272 R0 SYMATTR InstName R1 SYMATTR Value 47 SYMBOL ind 160 16 R0 SYMATTR InstName L1 SYMATTR Value 68n SYMBOL cap 272 176 R0 SYMATTR InstName C1 SYMATTR Value 1.5p SYMBOL cap 272 288 R0 SYMATTR InstName C2 SYMATTR Value 3.3p SYMBOL voltage -160 128 R0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName V1 SYMATTR Value PULSE(0 12 0 10n) SYMBOL res -32 64 R0 SYMATTR InstName R2 SYMATTR Value 10k SYMBOL cap 80 16 R0 SYMATTR InstName C5 SYMATTR Value 100n SYMBOL cap 544 240 R0 SYMATTR InstName C6 SYMBOL cap 656 240 R0 SYMATTR InstName C7 SYMBOL ind 544 128 R90 WINDOW 0 5 56 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName L2 SYMBOL ind 672 128 R90 WINDOW 0 5 56 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName L3 SYMBOL res 768 208 R0 SYMATTR InstName R3 SYMATTR Value 75 SYMBOL cap 768 128 R90 WINDOW 0 0 32 VBottom 0 WINDOW 3 32 32 VTop 0 SYMATTR InstName C8 SYMATTR Value 1n SYMBOL cap 416 240 R0 SYMATTR InstName C3 SYMBOL npn 112 160 R0 SYMATTR InstName Q1 SYMATTR Value BFR93a TEXT 0 -16 Left 0 !.tran 0 100u 0 TEXT -88 360 Left 0 ;433.92Mhz SAW RECTANGLE Normal 0 336 -32 288