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WL1805MOD: Tee filter calculation

Thomas BISIAUX
Prodigy 20 points
Part Number: WL1805MOD

Hi,

I'd like to understand how the suggested Tee filter - on the output 2G4_ANT2_W of the WL1805MOD - is calculated.

(see https://datasheet.octopart.com/WL180...t-33026339.pdf ; page 30)

As the input impedance of the antenna is said to be 50Ω, and the same goes for the output impedance of the pin 2G4_ANT2_W, what is this filter used for if not for impedance matching ?

Here is a frequency response simulation of the actual Tee filter :

How do you get the 8pF, 10pF and 1.1nH values ? Are they critical or could they be slightly shifted (by a few %) ?

Thanks in advance for your help

  • 0
    TI__Mastermind 35795 points
    Thomas,
    To confirm , you are using TI WL1805MOD module and not a 3rd party/partner module ?

    Thanks
    Saurabh
  • 0 in reply to S
    Prodigy 20 points

    Hello,

    Yes I'm using TI WL1805MOD module and TDK ANT016008LCD2442MA1 Antenna.

    Thanks

  • 0 in reply to Thomas BISIAUX
    TI__Genius 12260 points
    Hi Thomas,

    the 10pF is the DC block required for the WL1805MOD. So if you look back into the 10pF capacitor what you would see is 50 ohms (or in terms of RL our requirement of -10dBm).

    Now the 1.1nH and 8pF network are the matching network to match to 50 ohms when looking into the antenna. This is the network that is used to tune toe 50 ohms of the antenna.

    When determining the values you can do this via a s-parameter test box and manually change components until you get a good RL (S11) looking into your antenna over your frequency range of interest, or via simulation.

    If going down the simulation route you need to include the PCB parasitics and this will effect the final matching components as well. The method we do is use ADS to simulate the PCB parasitics, have modes of the matching components, and S-parameters of the antenna. We can then use ADS to optimize and determine the values of the L and C for the match. We can then build it up and measure to correlate our results.

    I hope this helps.

    Thanks,
    Riz