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Balun for CC253x

Tommaso Polonelli
Intellectual 950 points

Other Parts Discussed in Thread: CC2530, CC2531, CC2511

I'm using a 2450BM15A0002 with a CC2530 and i having some problems with the impedence of the stripline. I know that the output impedance of the CC2530 is 69+j29 and that the balund match the impedace with a unbalanced output at 50 ohm. But what is the phi of the balun ?  Maybe the output impedace is 50+j0 ? I can't understand it from the datasheet.

 

  • 0
    TI__Guru**** 317180 points

    See http://www.ti.com/lit/an/swra297b/swra297b.pdf on how to use this. The output impedance for the balun is 50 ohm (stated in the datasheet)

  • 0 in reply to TER
    TI__Mastermind 39860 points

    Also, the balun is connected to an SMA connector, which, when doing conducted measurements, provide a 50 ohm load (spectrum analyzer, signal generator)

  • 0 in reply to Sverre
    Intellectual 950 points

    Thank you, but I'm using a PCB antenna, and I'm not understanding how you match the impedance on the stripline with the "matching network" (LC) in the TI demoboard, like cc2530 Mini Kit or CC2531 dongle. I can't replicate your calculations about it.

  • 0 in reply to Tommaso Polonelli
    TI__Guru**** 317180 points

    Could you point us to exactly which calculations in which document you are referring to? Hopefully it will be easier to get you a good anser if I'm able to comment on the correct equation.

  • 0 in reply to TER
    Intellectual 950 points

    I developed the CPW line using the TXLINE 2003. If the balun output is 50+j0 then the impedance close the antenna (AN0403) is only dependent from line lenght. For matching the 50+j0 of the antenna I need a LC "Impedance Matching". Now I using this http://home.sandiego.edu/~ekim/e194rfs01/jwmatcher/matcher2.html

    I tryed to do this using TI demoboard (swrr067) and (swrr076) , but I can't find the same values of L and C , my results is different, so I'm sure that there is a error on my method.

  • 0 in reply to Tommaso Polonelli
    TI__Guru**** 317180 points

    The antenna described in http://www.ti.com/lit/an/swra117d/swra117d.pdf is 50 ohm without matching components and the output of the balun is 50 ohm. Then you only need a 50 ohm strip-line between the two, no need for L/C. Just input the width/ length/ distance to ground/ thickness into TXLINE 2003 to find the dimensions for the line.

  • 0 in reply to TER
    Intellectual 950 points

    ok, but why there is a 0.5 pF in swrr067 and 2nH and 1,8pF in swrr076 close antenna ? both using a balun and a antenna without matching components

  • 0 in reply to Tommaso Polonelli
    TI__Guru**** 317180 points

    That is a good question actually... When I first answered you I had read only the start of the app note (swrr067) that claims that the antenna is 50 ohm without the need of matching components but when looking at the schematic some matching is used. Looking at in in the smith chart a 0.5pF shunt cap on 2.45 GHz does not shift the resonance frequency by much.

    My assumption is that the antenna was designed for 50 ohm but the limited ground plane on the CC2511 dongle caused the resonance to shift some and matching was needed.


    Generally speaking: When using a PCB antenna place holders for a pi network close to the antenna should be added. This is because the resonance frequency shift due to the size of the ground plane and the casing. Something in the near field to the antenna will change the resonance frequency. Then you have to use a VNA to measure the impedance of the antenna with casing in your application and add matching components to match it to 50 ohm. On 2.4 GHz using ideal components as in the web page you referred to will not give good results due to non ideal components and parasitics.