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CC112x - with LNA and SAW

RingMain
Prodigy 80 points
Other Parts Discussed in Thread: CC1020, CC1120

Hi. Where in the reference schematic (CC112xEM_420_470) would I insert a  SAW and LNA, both with 50ohm in/out?

Thanks

 

  • 0
    TI__Mastermind 20960 points

    Can you give detail of what you intend to do?

    Thanks.

  • 0 in reply to FI
    Prodigy 80 points
    Hi. I just want to increase RX sensitivity, as I’ve done previously with CC1020, where I inserted a SAW followed by LNA after the antenna switch. But CC112x reference design has a different arrangement for switching between TX and RX and it’s not clear to me where I can insert the SAW and LNA, both of which are 50 ohm in/out, without affecting circuit’s impedance.
  • 0 in reply to RingMain
    TI__Mastermind 20960 points

    this should give an idea of how to integrate your particular solution. You can place the SAW filter after the LNA/PA

    http://www.ti.com/tool/cc112xsky65367em-rd

    if you have a specific LNA/PA and SAW in mind let us know and we can suggest a configuration.

  • 0 in reply to FI
    Prodigy 80 points

    Thank you Farrukh. I am familiar with the link you sent me, but taking that approach (which is for 169MHz application) would require component value changes for my 420-470MHz application, plus an antenna switch which I'm hoping to avoid with CC112x. Can I send you a schematic?

  • 0 in reply to RingMain
    TI__Mastermind 20960 points

    sure. Send the schematic and i will get back to you on this.

  • 0 in reply to FI
    Prodigy 80 points

    Thanks. The attached schematic is from TI's reference design (CC112xEM_420_470). I've highlighted capacitor C181 which taps off RX signals at point A. Ideally (unless you suggest otherwise) I'd like to remove C181 pin 2 from point A and instead connect my LNA/SAW circuit to point A. Then C181 would be connected to the output of my LNA/SAW circuit. However, for this scheme to work the impedance at point A needs to be 50 ohms as my LNA/SAW circuit has 50 ohms in/out. Any thoughts? Alternative solutions would involve an antenna switch, and component value changes, both of which I'd like to avoid if I can.

    Thanks

  • 0 in reply to RingMain
    TI__Mastermind 20960 points

    Ideally you could place the LNA where you have suggested and SAW instead of L173, C177, L174, and C175 to knock down harmonics.
    I believe the impedance at point A is 50ohm but i will verify this with simulation and get back to you.

  • 0 in reply to FI
    Prodigy 80 points

    Thank you for the speedy replies.

    Replacing the filter components with the SAW is a good idea, although the SAW then needs to handle the required TX power from CC112x.

    It would be great to know if point A is already at 50 ohms.

    Thanks.

  • 0 in reply to RingMain
    TI__Mastermind 20960 points

    I have attached the S11 file measured at point A by lifting L173 and setting the CC1120 in RX mode.
    Now this includes the effect of L172 and C176 portions of the network. This will help you with impedance calculations.


    Also, for your particular application, you may have to tweak the balun to have impedance looking into the LNA/SAW to be 50ohm. Please see this AN068 for reference.

    1440.CC1120_S11_RX_MODE.txt
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    !Agilent Technologies,E5071B,MY42301378,A.09.10
    !Date: Mon Jan 27 10:56:58 2014
    !Data & Calibration Information:
    !Freq S11:SOLT2(ON)
    # Hz S RI R 50
    1000000 1.007842e+000 -3.018761e-002
    8495000 9.794376e-001 -2.223932e-001
    15990000 8.988328e-001 -3.995303e-001
    23485000 7.813489e-001 -5.482725e-001
    30980000 6.372680e-001 -6.600929e-001
    38475000 4.786313e-001 -7.347236e-001
    45970000 3.257477e-001 -7.691408e-001
    53465000 1.814904e-001 -7.804286e-001
    60960000 4.629389e-002 -7.650353e-001
    68455000 -7.779147e-002 -7.319126e-001
    75950000 -1.870427e-001 -6.846595e-001
    83445000 -2.841654e-001 -6.242324e-001
    90940000 -3.661583e-001 -5.559456e-001
    98435000 -4.378267e-001 -4.793573e-001
    XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX