This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

  • TI Thinks Resolved

LMX2572LP: VCO4 weird behavior

Prodigy 60 points

Replies: 11

Views: 491

Part Number: LMX2572LP

Hello
I've choose LMX2572LP for my project and have met weird situation.
I use frequency band - 137..175 MHz, arbitrary FSK I2S modulation with symbol rate - 9600 bps, and sampling rate - 230.4 kHz (24 samples per symbol).

It works perfect if selected VCO isn't VCO4. I've made some screenshots to show the problem. Conditions are mentioned under each picture.

First  - 148MHz, VCO forced to VCO3.

.

pic.1  NVCO=3, I2S_On,  FSK_EN=1, INPIN_IGNORE=0, SPAN=500kHz

----------------

pic.2   NVCO=3, I2S_On,  FSK_EN=1, INPIN_IGNORE=0, SPAN=100kHz

-----------------

pic.3    NVCO=3, I2S_Off, FSK_EN=0, INPIN_IGNORE=1, SPAN=500kHz

----------------

pic.4  NVCO=3, I2S_On,  FSK_EN=0, INPIN_IGNORE=1, SPAN=500kHz

===========

Second four pictures - the same condition, 148 MHz, but VCO auto calibrated to VCO4

pic.5   NVCO=4, I2S_On,  FSK_EN=1, INPIN_IGNORE=0, SPAN=500kHz

------------------

pic.6 NVCO=4, I2S_On,  FSK_EN=1, INPIN_IGNORE=0, SPAN=100kHz

--------

pic.7  NVCO=4, I2S_Off, FSK_EN=0, INPIN_IGNORE=1, SPAN=500kHz

-------------

pic.8  NVCO=4, I2S_On,  FSK_EN=0, INPIN_IGNORE=1, SPAN=500kHz

As you can see - if VCO4 is used - there are very high sampling rate spurs and common noise floor, even if FSK disabled, but host I2S pins remain active,

And these noise and spurs disappear, only if host I2S pins set inactive.

Sadly most of my frequency band  falls into VCO4. 

So the question is - how can I realise my FSK I2S in my frequency band?

  • In reply to Boris Krotkov:

    Boris,

    They have been notified and if they can come up with a clever solution, we will let you know.

    Regards,
    Dean

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.