LMX2595: rb_VCO_DACISET value

Taketo Sato

Part Number: LMX2595

Hello team,

My customer has a question about LMX2595. Could you please confirm this?

rb_VCO_DACISET value reaches upper limit of 511 and Vtune voltage becomes about 0.8V when changing frequency from around 13GHz(higher frequency in VCO SEL=5) to other frequency(e.g. 9GHz, 10GHz and 11GHz) with No Assist mode.

In this condition, VCO_DACISET_STRT=300 is used as recommended in datasheet.

If changing the VCO_DACISET_STRT to 200, then customer can get normal rb_VCO_DACISET value and Vtune voltage.

Questions are

Is the rb_VCO_DACISET=511 condition expected as normal operation?
Although reducing VCO_DACISET_STRT to 200 seems improve the situation, is it right solution?
If above can be solution, could you let me know recommended VCO_DACISET_STRT value for each frequency?

Best regards,

over 5 years ago

0 Noel Fung over 5 years ago

TI__Guru** 107450 points

Hi Sato-san,

What is the reference clock frequency, CAL_CLK_DIV and ACAL_CMP_DLY values?

0 Taketo Sato over 5 years ago in reply to Noel Fung

TI__Genius 10325 points

Hello Noel-san,

203.125MHz REF is used for 13GHz output and 156.25MHz REF is used for 10GHz output. They use R=2, N=128, CAL_CLK_DIV=1, ACAL_CMP_DLY=15.

Set VCO_SEL=7, VCO_DACISET=300 and VCO_CAPCTRL_STRT=183 and perform below with No assist mode.

Output 13GHz(REF=203.125MHz, R=2, N=128, CAL_CLK_DIV=1, ACAL_CMP_DLY=15 then FCAL_EN=1)
Confirm VCO_SEL=5 by rb_VCO_SEL
Output 10GHz(change to REF=156.25MHz then FCAL_EN=1)
Confirm rb_VCO_DACISET

Best regards,

0 Noel Fung over 5 years ago in reply to Taketo Sato

TI__Guru** 107450 points

Hi Sato-san,

Can you try with the following default settings in no assist mode?

VCO_SEL = 4,

VCO_DACISET_STRT = 300,

VCO_CAPCTRL_STRT = 1.

0 Taketo Sato over 5 years ago in reply to Noel Fung

TI__Genius 10325 points

Hello Noel-san,

Thank you for suggestion.

Customer tried with the setting(VCO_SEL=4, VCO_DACISET_STRT=300, VCO_CAPCTRL_STRT=1) in no assist mode, customer could see improvement on around 10GHz. However they still see 511 as VCO_DACISET value around 9GHz and 11GHz.

Is VCO_IDACSET=511 condition normal and proper as LMX2595 operation?

Best regards,

0 Noel Fung over 5 years ago in reply to Taketo Sato

TI__Guru** 107450 points

Hi Sato-san,

511 is not right.

Please try also with CAL_CLK_DIV=2. I think the problem is still related to calibration speed, although they have already set ACAL_CMP_DLY to 15.

0 Taketo Sato over 5 years ago in reply to Noel Fung

TI__Genius 10325 points

Hello Noel-san,

By setting CAL_CLK_DIV=2, customer confirmed this resolves the strange rb_VCO_DACISET value.

It appears calibration speed contributes to the phenomenon.

Best regards,

Clock & timing

Clock & timing forum

LMX2595: rb_VCO_DACISET value