Does a resistor between the TX differential output change the output impedance? Please see the attached pic. What impact would the resistor contribute to the TX output circuit?
The resistor shown in the schematic is seen by AD9361 Tx output as a load. How much of an impact the resistor has on the load impedance depends on the other load components.
The other load component involve Bias-Tee and Transformers, all capacitive and inductive components, supposed to be well matched to 50 ohm.
I would recommend configuration a) in Figure 15 of AD9361 RF Port Interface document. It is a preferred approach to interfacing to Tx output. It uses a center tapped transformer which provides impedance matching and DC bias.
But our product is expected to operate in a wide band, we can not find a wide band transformer with center tap. We have to use RF chokes to do DC biasing. So we would like to know what would happen if we use the resistor shown in the pic attached above.
Tx ouput was designed for 50ohm differential output impedance and that is the load it needs to see for a matched case.
The 50ohm bias tees will only be 50ohm if their terminations are 50ohm. The bias tees are terminated with a 25ohm on one side and by an unknown impedance on the other side presented by the transformer.
You need to work from the single ended load on the transformer primary to the Tx output in order to determine what is the impedance seen by the Tx output. Once you know what this impedance is you can compensate with an additional component like a resistor shown in your schematic.
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