LMX2572: Phase Noise variation under low temperature

Hi Ninomiya-san,

What is the reference clock freq?

At what temperature was the LMX2572 initially calibrated and locked? For example, was it locked at 25C and then customer measured phase noise at different temperature?

Hi Fung-san,

The reference clock frequency is 50MHz.

The LMX2572 was locked again at each temperature.
Measurement step at each temperature was,
 1st, change temperature
 2nd, set synthesizer in order from the lowest frequency
 then, measure phase noise

Best Regards,

Koshi Ninomiya

4428.LMX2572 Phase Noise variation.xlsx

Hi Ninomiya-san,

The register settings look fine. Since 4943MHz and 4948MHz are very close to each other and you calibrated them at each temperature, I don't expect there will be big phase noise difference. 

How many devices did the customer test? Can they try set VCO_DACISET_FORCE = 1 and then vary VCO_DACISET value around 151 (decimal)?

Hi Fung-san,

The customer measured Phase Noise by the other LMX2572. It seems that the fluctuating frequency is different, but the same phenomenon has occurred.

Could you describe about the following registers?
 - VCO_DACISET
 - VCO_CAPCTRL

They tried the following measurement ways.
#1 measurement way (手順①):
  - Set synthesize under +35 degC
  - Measure Phase Noise under +35 degC
  - Change temperature to -30 degC
     *Not set synthesizer again under -30 degC
  - Measure Phase Noise under -30 degC

#2 measurement way (手順②):
  - Set synthesize under +35 degC
  - Measure Phase Noise under +35 degC
  - Change temperature to -30 degC
  - Set synthesizer again under -30 degC
  - Measure Phase Noise under -30 degC

Please check the attached their measurement results.

In case of the #2 measurement way (手順② -30℃), it looked that Phase Noise characteristics at around 5 to 80kHz offset frequency varied.
What could be the reason for such a change in Phase Noise characteristics?
If the parameter changes at this time, is there a way to read it?

Best Regards,

Koshi Ninomiya

LMX2572 Phase Noise variation 20100116.xlsx

Hi Ninomiya-san,

This looks weird to me, let me check this out in the lab and get back to you in next week.