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TRF7970A: TX power is not stable during LPCD mode?

Wayne Huang
Genius 16585 points
Part Number: TRF7970A


Dear Champs, 

I am asking this for our customer.

The user has used TRF7970a for couple of years but found field issue recently and it's urgent to help them clarify.

They use TI-suggested LPCD (Low Power Card Detection) method to detect card like below.

They found TX power is not stable (occurrence frequency around 1% - 2%), so that ADC reading will not correct (become too large falsely) and this turns out to be large and unexpected battery consumption.

They have tested ~30 pcs of TRF7970a and found same results.

Questions:

1. Is it normal that TX power is not stable on TRF7970a during LPCD?

2. If yes, is there any way to improve it?

3. By software, there are options for full power and half power? Is there any other option, say 3/4 power? The user is thinking to adjust the power to mitigate this issue, but it's a trade-off between power and distance. This is why they are asking for more power options to tune this.

1. 

  • 0
    TI__Mastermind 24140 points

    Hello Wayne,

    the depicted approach is just a common example and not a suggested solution.

    I guess ideally this approach would be independent from the TRF7970A TX power. It is a card presence detection mechanism with a separate antenna, which enables the TRF7970A. The TRF7970A should then operate the same way as without the detection method, as long as the detection circuitry is turned off once the TRF7970A is enabled. Otherwise the resonator antenna signal could disturb the TRF7970A antenna signal.

    Best regards,

    Andreas.

  • 0 in reply to Andreas Mueller
    Prodigy 10 points

    Hi 

    We directly measured the transmit power of TRF7970a, sometimes the transmit power is not the same as normal. The power is about 0.5~1dBm higher.

  • 0
    TI__Mastermind 24140 points

    Hello Judd,

    I fit is a static 0.5 to 1 dBm transmit power difference, it might result from setup changes, such as using a different cable, bending a cable too much, contact issues in the connector. If the transmit power changing dynamically during operation, then it might result from unstable supply voltage.

    Best regards,

    Andreas.