RF Design Help: Re-Learn and Combine 2 Networks

[dnyberg2]

I'm having to re-learn RF design after years of not doing it. Can someone lead me in the right direction? If I have two RF networks, say two simple LC filters and I need to feed them both in parallel with the same source simultaneously, what are the steps? To make it easy, say each filter is 50 ohms in and out, filter one is a low pass filter, filter two is a high pass filter. I need to join the two inputs together, fed by the same source, so does that mean they look like 25 ohms in parallel? How do you make sure that the one filter which works fine doesn't have a bad effect on the second filters response?

 

[0xDEADBEEF]

Yes the transmission line should see 25 Ohm. But if you want to be sure you can calculate the whole thing with complex resistances. If I remember correctly a balun transformer with a winding ratio of 1:2 will do the impedance matching, if you want to split the line in two.

 

[dnyberg2]

Yes the transmission line should see 25 Ohm. But if you want to be sure you can calculate the whole thing with complex resistances. If I remember correctly a balun transformer with a winding ratio of 1:2 will do the impedance matching, if you want to split the line in two.

I wonder if you could sketch that topology and get it to me?

 

[Averagesupernova]

Look into power combiners/splitters. Sounds to me like that is what you need.

 

[skeptic2]

Approximately what frequency is it for and are the two filters for a single frequency or for a range of frequencies? If for a range of frequencies, how broad a range? Also how many elements are in the low pass and high pass filters?

 

[dnyberg2]

Each filter is only a 3rd order filter at the moment. One is set to 10.7 MHz and the other is set to 49 MHz. This experiment revolves around understanding and trying to create diplexers and duplexers. The source impedance of the 49 MHz is around 20 ohms, the source impedance of the 10.7 is 300 ohms, the single I/O port needs to be 50 ohms.