Member Since: July 2, 2013

Country: Slovak Republic

  • I do not think that it is ~50 ohm. RFM12B datasheet:

    • RF Power Amplifier (PA) The power amplifier has an open-collector differential output and can directly drive different PCB antennas with a programmable output power level. An automatic antenna tuning circuit is built in to avoid costly trimming procedures and the so-called “hand effect”.
    • LNA The LNA has approximately 250 Ohm input impedance, which functions well with the proposed antennas If the RF input of the chip is connected to 50 Ohm devices, an external matching circuit is required to provide the correct matching and to minimize the noise figure of the receiver. The LNA gain can be selected in four steps (between 0 and -20dB relative to the highest gain) according to RF signal strength. It can be useful in an environment with strong interferers.

    Note 4: See reference design with 50 Ohm Matching Network for details

    Note 5: See reference design with Resonant PCB Antenna (BIFA) for details

    Note 6: Optimal antenna admittance/impedance:

    RFM12B     Yantenna [mS]           Zantenna [Ohm]             Lantenna [nH]
    433 MHz          2 – j5.9                  52 + j152                     62
    868 MHz        1.2 - j11.9                7.8 + j83                     15.4
    915 MHz        1.49 - j12.8                 9 + j77                     13.6

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