LMX2571: TX PLL spur changes

Kazuki Itoh

Genius 10730 points

Part Number: LMX2571

Hi team,

My customer is struggling with the TX spur of LMX2571.

The ref LO has ~80dBc spur at 10MHz +/- 80kHz.

Looking at the PLL output (f0=78MHz – 94MHz), the f0+/- 80kHz spur increases to 60dBC (+20dBC).

The spur level changes every time setting R0 register.

It changes to 60dBC or 70dBC randomly.

Could you let me know the possible reason that the LO spur level increases with PLL?

What is the possible cause of the spur level change with the R0 register setting?

Best regards,

Itoh

over 2 years ago

0 Noel Fung over 2 years ago

TI__Guru** 107320 points

Hi Itoh-san,

What is the input, output and fpd frequency of LMX2571?

0 Kazuki Itoh over 2 years ago in reply to Noel Fung

TI__Genius 10730 points

Hi Noel-san,

Please find the figure below.

Regards,

Itoh

+1 Noel Fung over 2 years ago in reply to Kazuki Itoh

TI__Guru** 107320 points

Hi Itoh-san,

Spurs increase is normal, as the output frequency is 7.8 to 9.4 times higher than the input.

Unless the spurs at the input is not stable, I expect the spurs at the output should also be stable. For example, you should get stable output spurs when the output frequency is 80MHz (integer channel).

At fractional channels, if fractional spurs is also 80kHz, then it is possible that the spurs level may varies.

Another possibility happen when the required VCO frequency is at the boundary between two adjacent VCO core. Every calibration may select different VCO.

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LMX2571: TX PLL spur changes