Forums - Impedance of transmission lines before and after balun:

3 posts / 0 new
Last post
Impedance of transmission lines before and after balun:
Join Date: 30 Oct 17
Posts: 12
Posted: Wed, 2018-07-25 07:53


We're developing a board with the QCA4010, and are production-testing the boards. (EVM measurements). They seem to fail this test and the Qualcomm application engineer blames our use of 100ohm differential transmission lines between the QCA4010 and the balun. However, the documentation (80-y9905-8_c_rb04_hardware_reference_guide.pdf section 3.3) mentions that everything should be 50ohm, without mentioning whether this is the differential or single-ended impedance. The layout pictures clearly show similar tracewidths before and after the balun, which tells us that the impedance used is actually 100ohm differential chip-balun and 50ohm single-ended balun-antenna. Can anyone shed some light?

  • Up0
  • Down0
Join Date: 20 Jun 17
Posts: 3
Posted: Wed, 2019-06-19 02:45

Hi michael.burm,

Welcome to Qualcomm's documentation! ;)

I know this response is way too late for you but maybe this helps someone else.

Please note that depending on your PCB stack differences and differences in layout between your PCB and the SP241 or MP01/02 PCBs, the EVM doesn't decessarily pass or have  good marging. If you have slightly different PCB stack (likely) than the eval boards or your layout is slightly different, the front-end matching components on the reference design don't necessarily give you desired EVM results. So, you need to tune the circuit. Your Qualcomm support engineer (if you can find one) will give you an indication of the target impedances (they don't have a document stating the  target impedance or at least I wasn't given one when requested).

The differential impedance needed is 100ohm. The impedance of the track between balun and antenna depends on the output of your balun but it's most likely 50ohms. If you use reference design balun (at least the discrete one), it is 50hms.

Generally, the documentation on these chips is vastly lacking on HW and SW sides. Perhaps these chips are really meant for big OEM vendors who've got muscle to demand information (big contracts). The lack of documentation isn't great for small design house or hobbyist by any means.

Kind regards



  • Up0
  • Down0
Join Date: 12 May 19
Location: Jakarta
Posts: 1
Posted: Wed, 2019-06-19 08:06


  • Up0
  • Down0
or Register

Opinions expressed in the content posted here are the personal opinions of the original authors, and do not necessarily reflect those of Qualcomm Incorporated or its subsidiaries (“Qualcomm”). The content is provided for informational purposes only and is not meant to be an endorsement or representation by Qualcomm or any other party. This site may also provide links or references to non-Qualcomm sites and resources. Qualcomm makes no representations, warranties, or other commitments whatsoever about any non-Qualcomm sites or third-party resources that may be referenced, accessible from, or linked to this site.