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CC1200 matching network 433 MHz with separate TX/RX paths

maximilian.boettcher
Prodigy 160 points
Other Parts Discussed in Thread: CC1200, CC1120

Hi,

I've designed a 30dBm RF-Modul in 433MHz band using your CC1200.

Because I'm using an additional FEM from Skyworks, I had to split up the RX/TX path.

During the first test, I measured just 27.8dBm max. instead of 30dBm out of the FEM (saturated). So I'm wondering if I've done something wrong with the impedance matching between CC1200 and the FEM. Would you be so kind and have a look at my circuit (to be found in the appendix)?

Many thanks!

SCH2-eps-converted-to.pdf
  • 0
    TI__Mastermind 27145 points

    Hi,

    Did you test the Skyworks Amplifier alone by feeding an external signal?  This would give an indication, where the problem is. I reviewed the data sheet of SKY65366-11 and it shows 30.2dBm typical for the frequency range of 412~424MHz.

    Thanks,

    PM

  • 0
    TI__Mastermind 31610 points

    Have you tried to increase the voltage slightly to the FEM ?

    With the 915 MHz and 169 MHz designs we have with Skyworks FEMs; the supply voltage to the FEM is critical and at 3.3V and some voltage drop on the test board, the power at the FEM is around 3.1V and this causes a lower output power when programmed to 30 dBm. I presume this might be the case for you as well.

     Good to measure the actual voltage level at the FEM supply pads as well.

    Regards, Richard

  • 0 in reply to RGW
    Prodigy 160 points

    Thanks, I should try this. Right now, I'm using 3.6V from a separate step-down converter - but I could increase it to 3.8 V.

    The output power is controlled via the VPC-Pin. From 2.10 V upwards, the FEM is saturated. Even if I increase the output power of the CC1200 from 10 dBm to 13 dBm, nothing changes. So indeed, the problem could be caused more by the FEM than by the transceiver.

    BTW, do you have a reference design for the CC1200 with to seperate paths for TX/RX in the 440 MHz band? 

  • 0 in reply to maximilian.boettcher
    TI__Mastermind 31610 points

    Take a look at the CC1120 30 dBm Skyworks EM design at 915 MHz: http://www.ti.com/lit/zip/swrr110 to see how the Rx and Tx paths are separated.

    The CC1200 and CC1120 are quite similar so the RF discrete network to the FEM will be the same but use the components values from the 433 MHz design: http://www.ti.com/lit/pdf/tidr233 instead.

    Regards, Richard

  • 0 in reply to RGW
    Prodigy 160 points

    Thanks for answering.

    I know, that's the TI forum, but just in addition to the first mentioned power problem with the Skyworks FEM:

    I measured a pretty bad VSWR of ~1:5 in RX mode into the FEM (I guess until the LNA input), but the matching network looks quite good (should give an VSWR of 1:1.2).
    Besides, I noticed that when I press my finger on top of the FEM the output power increases a little bit (but up to 2dB).

    Could a connectivity problem cause this behavior?  Do you have ever experienced such thing? Thanks for sharing your opinion!

  • 0 in reply to maximilian.boettcher
    TI__Mastermind 31610 points

    It sounds suspicious if the output power increases up to 2 dB when pressing down on the FEM.

    I would remove and assemble again or "re-flux" under the FEM to see if it helps. Is there sufficient solder under the FEM?

    Have you seen this document from Skyworks: www.skyworksinc.com/uploads/documents/101752i.pdf

    Regards, Richard