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Original question:

LMX2594: the lock problem and the spur of lmx2594

LMX2594: spur when switching outputs drivers

Yaacov Shoef
Prodigy 50 points
Part Number: LMX2594

Hi,

on my application output_A is used for high freq (>5G) and output_B is used for lower freq. On some units, sometimes, when I switch from high freq to low freq i.e. switching from a state of out_A  active and out_B not active to the opposite state (out_A inactive and out_B active) , or vice versa, then I got a spur on about 400kHz offset (on many carriers) . the switching between states is done by writing the desired state to register 44.  I'v found that always when activating both outputs then the spur appears .  On this situation the way I'v found to cancel the spur is to deactivate both out_A and out_B and after a delay of >1 mSec to activate the desired output. Do you have an explanation to this? Do you have another solution without a delay?   

Regards,

Yaacov.

  • 0
    TI__Genius 11900 points

    Hello Yaacov,

    Are you programming only R44, basically powering down one output and powering up the other in the same register write? Do you see a difference when you power down output_A and then power up output_B in the next register write?

    Does the spur disappear after some time or continue to be present at 400 kHz? What are your FOSC, FPD and N-divider values?

    Thanks,

    Vibhu

  • 0 in reply to Vibhu Vanjari
    Prodigy 50 points

    Hello Vibhu,

    Yes, there is a difference between  power down output_A and then power up output_B in the next register write vs doing it at one writing to R44. On this way + adding a delay of 1ms between the two writings - the spur does not appear. But without a delay between the two writings it appears.

    The spur does not disappear after some time, and it continue even when changing frequency.

    The Fosc is 50MHz and also the Pfd is 50MHz. the N could be any value, specifically I saw the spur on 2300 MHz, 6300, 12550 but there are more frequencies.

    Another point is that when the unit is cold i.e. during some minutes after power up, there is no spur, but after warming to about 35c degrees and then apply afrequency change – the spur appears. But on a good unit there is no spur even on 55c.  

     

    Regards,

    Yaacov.

  • 0 in reply to Yaacov Shoef
    TI__Guru** 108480 points

    Hi Yaacov,

    This is a weird behavior we have ever met, we don't have an explanation or workaround at the moment. We will have to debug to see what is happening.

    Because we have very limited lab access, I am afraid we are not able to get back to you in a short period of time. We will put this post in a "waiting" state, once we have an update, we will keep you post again. 

    Thank you for your post.