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LMX2581: harmonic outputs are hi and affecting my demod image rejection

Charles Luke
Intellectual 270 points
Part Number: LMX2581

I am running differential outputs with 50 ohm pullups and driving into a demod chip that has input imped of 100 ohms. At 750M I'm measuring a high second harmonic level of -10dBc. I am only getting 18dB image rejection from the demod and I believe it is these harmonics that are creating the issue. Page 7 in the data sheet shows typical 2nd harmonic level of -25dBc. I'm using output A and output B is disabled. Is this performance to be expected? Is there anything I can do to reduce the harmonic content in the output? Would using the inductor give better harmonic performance? My system needs to tune from 740 to 2400M.

  • 0
    TI__Genius 12330 points

    Hi Charles,

    We have paged our device expert, he will get back to you next week.

    Best,

    Evan Su

  • 0
    TI__Mastermind 27661 points

    Charles,

    I measured this a while ago, but got much better results than -10 dBc.

    Regards,

    Dean

  • 0
    Intellectual 270 points

    Thanks for responding Dean,

    -15dBc is still a ways from the -25dBc in the data sheet . Also, not sure I would consider -15 much better than -10. How did you measure this? Single ended or differential. I measured single ended so that may account for some of it since differential should cancel some of the second harmonic..

    Same questions:

    Is there anything I can do to reduce the harmonic content in the output? Are there any hidden registers where I can change the bias on the output drivers? Would using the inductor give better harmonic performance?

  • 0 in reply to Charles Luke
    TI__Guru* 89110 points

    Hi Charles,

    Datasheet specifies the 2nd harmonic at 2700MHz. 

    Dean's data is showing similar harmonic at 2700MHz. At 750MHz, it is around, maybe 23dBc. 

    We measured output power and harmonic single-ended with the unused terminal (either P or M) AC-coupled to ground via 50Ω resistor.

    Harmonic is not controllable in register, there is no register we can use to tweak the harmonic.

    Your application requires 740M-2400MHz, inductor pull-up is not practical.

  • 0 in reply to Noel Fung
    Intellectual 270 points

    I misread his chart before. Yes, the blue line does look better. Any idea of why I might be seeing such a high 2nd harmonic relative to what you are measuring? Any suggestions on what might be causing this?

  • 0 in reply to Charles Luke
    TI__Mastermind 27661 points

    Charles,

    From the measurements I took, I used 50 ohm pull-up with no special modifications.  2nd harmonic relates to variation from an ideal 50% duty cycle. 

    My results showed that the optimal 2nd harmonic level was for around a setting of OUT_PWR=20.    So you might try tinkering with this field.