LMK05028: Oscillator Input (XO_P/N) on the datasheet P28.

Hirotaka Matsumoto

Part Number: LMK05028

Hi all

Would you mind if we ask LMK05028?

There is the description on the datasheet P28.
"For optimal performance, the XO frequency should be at least 48 MHz and have a non-integer frequency relationship with the VCO frequencies so the APLLs operate in fractional mode."

We would like to understand correctly.

1. XO frequency should be at least 48 MHz, so we should choice the XO which is more than 48MHz's one.
2. The relation between XO and VCO should be a non-integer frequency relationship.
For example, we choice followings;
XO : 100MHz
PLL1 VCO : 4915.2MHz, PLL2 VCO :5791.5MHz
Output frequency : 148.5MHz
(REF input : 15625Hz)
Is our recognition correct?

And then, why it should have a non-integer frequency relationship with the VCO frequencies?
Is it to improve phase noise?

Kind regards,

Hirotaka Matsumoto

over 4 years ago

+1 Amin Eshraghi over 4 years ago

TI__Expert 6140 points

Hi Hirotaka -san,

1. 48 MHz XO will work perfectly fine. It's preferable to use that than a 100 MHz XO (will require some manual editing to generate a configuration).

2. XO frequency and VCO frequency cannot be an integer relationship because the DPLL needs to have room to update the ndivider in both directions in order to correct the ppm error. So for example if a 48 MHz XO is used, the 4800 MHz VCO cannot be used. That results in an integer only n div (100, 48 MHz * 100 = 4800 MHz), which then wouldn't allow DPLL to do ppm correction and lock the output to the reference input since there's no fractional n divider value.

Thanks and regards,

Amin

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LMK05028: Oscillator Input (XO_P/N) on the datasheet P28.