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LMX2694-SEP: maximum output power vs OUTx_PWR parameter

Inho Hwang
Prodigy 20 points
Part Number: LMX2694-SEP
Other Parts Discussed in Thread: LMX2594

Hi I have question about datasheet of LMX2694.

Registers R44 and R45's OUTx_PWR are allocated 6 bits and the Power-on Reset value is displayed as 0x22. However, the Description field displays 0x0 as the minimum current and 0x1F as the maximum current. Additionally, the Pout in the Electrical characteristic is displayed as 2dBm (8GHz) @ OUTx_PWR=31 (0x1F).
1. Which is the correct maximum value for OUTx_PWR, 0x1F or 0x3F?
2. Is the reset value of OUTx_PWR 0x22 or (22)dec?
3. Can you explain the relationship between the OUTx_PWR current setting and the output power? For example, if OUTx_PWR=31 and Pout is 2dBm, does Pout decrease by 3dB when OUTx_PWR=15?

  • 0
    TI__Guru** 110920 points

    Hi Inho,

    OUTx_PWR is 6-bit long, so the register values are between 0x0 and 0x3F. Higher register values increases the current to the output driver. Higher current means higher output power. However, the peak output power is usually happen at 0x1F or below. Also this characteristic varies over frequency. For example, you may get the max. output power at setting = 0x1F for 8GHz output. However, the peak power setting for 14GHz output could be 0x14.

    yes, reset value of OUTx_PWR is 0x22 (34 decimal)

  • 0 in reply to Noel Fung
    Prodigy 20 points

    Thank you for your quick reply.


    However, if the current is maximum at OUTx_PWR=0x1F and minimum at 0x00, I still don't understand under what circumstances the values ​​0x20~0x2F are set.

    I have another question: I want to use an inductor pull-up to achieve maximum power at a fixed frequency of 8GHz. With an SRF of 14GHz and an inductor value of 3nH (j150.8 ohm), the combined impedance is 45 + j15. Can I achieve maximum power without using a resistor pad behind the decoupling capacitor?

    Thanks

    Inho Hwang

  • 0 in reply to Inho Hwang
    TI__Guru** 110920 points

    Hi Inho,

    Apologies, forgot to tell you that settings 32 through 47 are repeating settings 16 to 31, so we will see a dip at setting 32, similar to below. 

    With inductor pull-up, we can get higher output power than 50Ω pull-up. We don't have inductor pull-up test data on this device, on the sister part, LMX2594, we can see pretty good power boost with inductor pull up.

    At 45 + j15, VSWR is 1.4:1, RL is 16dB, this is not bad, the resistor pad may not necessary. However, I still recommend to placeholder this pad in your schematic. 

  • 0 in reply to Noel Fung
    Prodigy 20 points

    Thank you for your quick reply.

    I understand the OUTx_PWR register setting.

    Thanks

    Inho Hwang.