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LMX2571: Full setup vs express setup

Alex Strelbytskyy
Expert 1910 points
Part Number: LMX2571

Hello,

i'm using LMX2571 in frequency hopping mode with FSK I2S functions. It looks like switching noise is smaller when I reset the synthesizer (bit RESET in R0) and program all registers, than when I use the express setup (only the relevant registers are programmed and output power is on/off (OUTBUF_TX_PWR_F1, OUTBUF_TX_EN_F1). The datasheet says that the RESET function "Resets all the registers to the default values.", what internal functions (internal state machines) are involved in the RESET procedure? Can be still used express setup of synthesizer with achieving small noise levels?

Thanks,

Alex  

  • 0
    TI__Guru* 81580 points

    Hi Alex,

    I want to understand what "switching noise" you are seeing.

    Is the following correctly describe your operation?

    Assume you have it locked to a certain frequency channel and have the FSK enabled and running. Now you want to change the frequency channel, you did:

    1. stop FSK

    2. reduce output power (OUTBUF_TX_PWR_F1)

    3. disable output (OUTBUF_TX_EN_F1)

    4. change N divider value

    5. Program R0 to calibrate the VCO to the new frequency channel

    6. enable output and adjust output power

    7. enable FSK

    At which step above did you see switching noise?

    RESET will reset all the register values to their silicon default values, including hidden registers. Some of these values are internal LDOs or VCO related but the silicon default values are not the best setting so we need to program them to a different value after Vcc power up. 

  • 0 in reply to Noel Fung
    Expert 1910 points

    Hello Noel,

    The problem is getting worse because we have PA with significant gain. We have more questions:

    1) Looks like the LMX2571 is very sensitive to Load variation when used to drive an Power amplifier.

    Is there any setting to reduce the load pull / push variation on the output?

    2) It looks like it has some noticeable at RFOutTx pin, even when RFBuffer is disabled and power is zero.

    Thanks,

    Alex

     

  • 0 in reply to Alex Strelbytskyy
    TI__Guru* 81580 points

    Hi Alex,

    I think the output buffer provides around 50dB isolation when it is off, so yes, you will see some signal coming out from the buffer. More isolation will have to be implemented externally. I know we have customers using this synthesizer with a 5W PA without problems.

    Do you have a pre-amp between the synthesizer and the PA? Is the synthesizer shielded? 

  • 0 in reply to Noel Fung
    Expert 1910 points

    Thank you, Noel,

    We don't have pre-amp In the current design, we will shield the synthesizer in the next revision of hardware.

    Is it possible to give more details on the first RF question?

    1) Looks like the LMX2571 is very sensitive to Load variation when used to drive an Power amplifier.

    Is there any setting to reduce the load pull / push variation on the output? What could you recommend?

    Thanks,

    Alex 

  • 0 in reply to Alex Strelbytskyy
    TI__Guru* 81580 points

    Hi Alex,

    We have several buffer stages after the VCO, so the isolation is already fixed. 

    May be the isolation will be different between push-pull and open-drain output. OUTBUF_RX_TYPE is used to configure the buffer type. See datasheet 8.1.8 for details.

    Not sure, but you can also try set R59 (a hidden register) to 0x0087. The default value is 0x0084.