Post Go back to editing

Impedance matching of ADL5391 output

Hi, I saw a couple of Q&A treads about matching networks for ADL5391 and in this thread

I found some information about input broadband matching of the device and excel file with S11. But there was no information about output matching.

Does output circuit illustrated in figure 23 suit to broadband signal ( I plan to put signals with bandwidth 1-2 Ghz on both inputs X and Y)?

If it suits and input matching with 56.2 Ohm resistor is broadband too then why in datasheet it is said that "The evaluation board is matched for lower frequency operation" on page 11?

Also it's hard to read figures 15 and 16 with S11 and S22. Could your provide a file with S22?

Parents Reply Children
  • Apologies.  I am looking into your questions and hope to reply with answers as soon as possible.

  • It looks like the S-parameters on the product webpage has S22 included.  Please see the Readme.txt file.

    The output circuit should load the output with 50 Ohms differential between 1 & 2 GHz. 

    I think the evaluation board is matched better at the lower frequencies; you cannot have a perfect match at all frequencies when the Input/Output Z is changing over frequency.

  • Thanks a lot for your responce!

    I've found a zip file with S-parameters on the ADL5391 webpage.

    What does it mean  "the output with 50 Ohms differential"? Should I replace R4 resistor from 100 Ohms to 50 Ohms and then output network suits more for 1-2 GHz bandwith? Are there any principles how to adjust output resistors R4, R5, R6 for users application?

    Please, correct me if I'm wrong: evaluation board is matched better at the lower frequencies only because of coupling capacitors C7, C18, C4, C20, C14, C2. The evaluation board will become matched better at the higher frequencies if I replace them with more suitable for my bandwith (1-2 GHz).

  • The output of the ADL5391 will "see" 50 Ohms differential when looking into the R4, R5, R6 and the TC1-1-13M (balun) network.  The balun is a 1:1 impedance ratio so it presents 50 Ohm to R4, R5, R6, when J1 is loaded with 50 Ohms.  The whole network (R4, R5, R6 and the TC1-1-13M) makes 50 Ohms differential across WPLS and WMNS.  Note that this does not match the output to 50 Ohms, but rather loads the output with 50 Ohms.

    The capacitors that you mention are for coupling into and out of the part, and decoupling/AC grounds for the baluns.  They don't really do anything to help the matching; their impedance just decreases as frequency increases. 

  • Your answer clerify many things, thanks! So, as I see, evaluation board is "matched for lower frequency operation" because of internal structure and properties of ADL5391. And if I'm right it is impossible to achieve better gain on high frequencies (Figure 6) with another input and output matching networks.

    Correct me, please, if I'm wrong.