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Differential impedance of CC2540 under differnt power mode

YING CHENG
Intellectual 260 points
Other Parts Discussed in Thread: CC2540, CC2640, CC2530

The following sentence is from datasheet of CC2540

Differential impedance as seen from the RF port (RF_P and RF_N) 70 + j30 Ω toward the antenna

I want to know whether impdeance is still 70+j30ohm under power mode 1, 2 and 3 or not. Please clarify impedance value under power mode 1, 2 and 3.

– Power Mode 1 (3-μs Wake-Up): 235 μA
– Power Mode 2 (Sleep Timer On): 0.9 μA
– Power Mode 3 (External Interrupts): 0.4 μA

thanks a lot~

  • 0
    TI__Mastermind 18780 points

    The impedance for the RF pins change with the state of the chip (TX/ RX) and the output / input signal level. When operated in receive, the LNA gain is adjusted according to the input signal level and is thus not constant. The PA impedance is further different than the receive impedance, it changes with the output power level chosen and is not linear. For the Power Mode 1, 2 and 3, the radio is off. The impedance given in the datasheet is a compromise that will give you the optimal performance with respect to output power, sensitivity, current consumption and harmonic suppression. As always, we recommend following the reference design. If not using the radio, you can let the RF pins float. 

  • 0 in reply to CHS
    Intellectual 260 points
    Thanks for your feedback! When chip is working in power mode 1, 2 and 3, from CC2540 side view, we can consider impedance of RFN and RFP is very high. Is it right?
  • 0 in reply to YING CHENG
    TI__Mastermind 18780 points
    Can you explain what you need these numbers for? "Optimum Load Impedance" as we give in the datasheetis, as said, the impedance looking into the balun from the RF_N and RF_P ports. The PA is non-linear and it does not make sense to give the impedance at the RF pins. In addition, you have both an LNA and Power Amp which you need to match. Further, load impedance is only one parameter that influences the overall circuit performance. Phase/amplitude mismatch on balanced signal, filter characteristics and the impedance at harmonic frequencies are a few examples on parameters that influence performance. In sleep, the radio is off.

    Are you trying to tune the antenna? In that case you should refer to the 50 ohm point after the filter/balun in the reference design and simulate from there. Are you trying to make your own design? In that case you should follow the reference design. We can review the design to minimize the risks.
  • 0 in reply to CHS
    Intellectual 260 points

    Thanks for your feedback

    I can explain more why we need to know the impedance of CC2640.

    In my board, both CC2540 and CC2530 are mounted.

    And these two RF chips are close each other. In real application, Zigbee function is always active, BLE is in sleep mode for most time.

    Hereafter, you will see the simulation structure for two antennas. Through simulation, we can know the coupling between two antennas, because the impact is huge on S11 bandwidth of antenna#1 when BLE feeding line is terminated with 50ohm or left as open. When antenna#1 will be transmitting, we do not know the status of RF chain #2 (rx, tx, sleep). That is why we need to know the impedance at transceiver output (RF port towards chip itself).

    Is it clear for you now? Thanks a lot

  • 0 in reply to CHS
    Intellectual 260 points
    Soory I failed to attache simulaiton strucuture in my message. In my simulation, antenna#1 is for Zigbee function and antenna#2 is for BLE function. The air spacing between two antennas is arournd 25mm.
  • 0 in reply to YING CHENG
    TI__Mastermind 18780 points

    Continued through mail support