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

Charles Luke

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.

over 1 year ago

0 Evan Su over 1 year ago

TI__Genius 11255 points

Hi Charles,

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

Best,

Evan Su

0 Dean Banerjee over 1 year ago

TI__Mastermind 27641 points

Charles,

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

Regards,

Dean

0 Charles Luke over 1 year ago

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 Noel Fung over 1 year ago in reply to Charles Luke

TI__Guru* 85720 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 Charles Luke over 1 year ago 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 Dean Banerjee over 1 year ago in reply to Charles Luke

TI__Mastermind 27641 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.

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