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# How to match LTC5562 output (OUT+, OUT-) port?

Dear Analog Device Support Team,

I need to match LTC5562 output to 50 Ohm via a 2:1 balun at 6.8 GHz frequency. I have checked the datasheet. Many frequency bands have a suggestion how to match. Matching circuit contains some lumped elements (inductors, capacitors). Differential OUT port impedances are given in Table 5. I've tried to figure it out how you have calculated the element's values but math just doesn't work out. My professor advised the following. Differential port's impedance is twice higher than single port's impedance. After this consideration, let's ‘half’ the circuit horizontally. L2, L3 is a DC choke so let's skip it. After this we get a two-element-matching which is so simple, it can be checked by a software tool like Smith v3.10. Considering the frequency bands, I have checked Table 6. but I can not understand how these element values are calculated. If I am not mistaken, in case of a 4:1 balun OUT port is matched to 200 Ohm and after the balun you get 50 Ohm. I've tried to re-calculate the circuit element values but I can not make the math work out. (Paralel – Serial impedance transformation has not worked even).

As I mentioned, I need to match the OUT port to 6.8 GHz (narrow band) to 50 Ohm. OUT port difference impedance is approximately 350.6 - j69.8 Ohm. I will use 2:1 balun (NCS2-83+) and I am able to design stub matching with transmission lines lumped elements could be minimized.

So I have two questions.

1. How did you define the values in Table 6?
2. What do you suggest in my situation when I need 50 Ohm matching at 6.8 GHz?

Kind regards,

David Molnar

Parents
• Hello David.

ad 2/ I assume, you want to use mentioned RF-balun 1:2. Parallel model of the diff. impedance (350.6 - j69.8)Ohm can be recalculated to serial model (13.4-j67.2)Ohm. And this impedance must be transformed to 100Ohm. As a first step add 0.77nH serial impedance => 2 inductors 0.39nH at both diff. pins. You get serial impedance (13.4-j34)Ohm Ξ parallel (100-j39)Ohm. Then you need to add parallel impedance-inductor 0.92nH in order to compensate imagin. part. I would recommend you to check the impedance of selected RF balun, cause there could be also residual imagin. part, which is to be compensated.

Regards.

• I see this calculation method and it makes things clear. Thank you very much for your help! I really appreciate it.