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LMX2595: Doubler output with Subharmonics

Taketo Sato
Genius 10295 points
Part Number: LMX2595
Other Parts Discussed in Thread: LMX2594

Hello team,

I have a question about LMX2595.

My customer is evaluating its EVM and found -30dBc 1/2 subharmonics on doubler output in case VCO is selected in OUTB_MUX. However, the harmonics is improved when changing OUTB_MUX value from VCO to SysRef.

It looks channel divider is not disabled or powered down when selecting VCO in OUTB_MUX.

I thought the channel divider is powered down if both OUTA_MUX and OUTB_MUX are set as VCO, in case of LMX2594.

Questions are:

1) When we use LMX2595 with doubler output(OUTA_MUX=VCO2X with VCO2X enable), will it power up or enable the channel divider even if OUTB_MUX=VCO?

2) When we doesn't use OUTB but doubler output on OUTA, is OUTB_MUX=SysRef recommended?

It helps to reduce not only subharmonics but also power consumption, so I would like to know if there will be any problem with OUTB_MUX=SysRef setting when we don't use OUTB.

Best regards,

  • 0
    TI__Guru** 107110 points

    Hi Sato-san,

    Whenever OUTB_MUX = Divider or SYSREF, the channel divider will be powered up.

    If OUTA_MUX = VCO or VCO doubler, when OUTB_MUX = VCO, the channel divider will be powered down. In order to save power consumption, we recommend make OUTB_MUX = VCO or High-Z if it is not being used.

    See datasheet Table 9 for details. 

    I guess the reason why sub-harmonic gets improved when channel divider is enabled is because the VCO signal gets split up to two paths, one goes to the VCO doubler and the other goes to the channel divider. As a result, the signal going to the VCO doubler is a little bit smaller and therefore the sub-harmonic is smaller.

  • 0 in reply to Noel Fung
    TI__Genius 10295 points

    Hello Noel-san,

    Thanks for your prompt reply.

    However customer's observation is different from your explanation and datasheet.

    They confirmed power consumption is also improved with OUTB_MUX=SysRef, compared to power consumption in case of OUTB_MUX=VCO. It sounds to me that OUTB_MUX=SysRef setting will power down channel divider rather than a case of OUTB_MUX=VCO setting.

    Could you please confirm OUTA_MUX= VCO2X performance with both OUTB_MUX=SysRef case and OUTB_MUX=VCO?

     

    Sorry for my typo. I correct as follows.

    found -30dBc 1/2 subharmonics on doubler output in case VCO is selected in OUTB_MUX. -->  found -30dBc 1/2 subharmonic of VCO on doubler output in case VCO is selected in OUTB_MUX.

     Best regards,

  • 0 in reply to Taketo Sato
    TI__Guru** 107110 points

    Hi Sato-san,

    OK, I will check on next Monday.

  • 0 in reply to Noel Fung
    TI__Guru** 107110 points

    Hi Sato-san,

    Right, I measured big current reduction when OUTB_MUX is SYSREF, let me check with the design team and get back.

  • 0 in reply to Noel Fung
    TI__Genius 10295 points

    Hello Noel-san,

    Thanks for your confirmation. please confirm this with design team and let us know their feedback.

    Best regards,

  • 0 in reply to Noel Fung
    TI__Genius 10295 points

    Hello Noel-san,

     

    What I would like to confirm is below. I would appreciate if you could confirm these with design team.

    1) When we use LMX2595 with doubler output(OUTA_MUX=VCO2X with VCO2X enable), will it power up or enable the channel divider even if OUTB_MUX=VCO?

    2) When we doesn't use OUTB but doubler output on OUTA, is OUTB_MUX=SysRef also recommended?

     

    Best regards,

  • 0 in reply to Taketo Sato
    TI__Guru** 107110 points

    Hi Sato-san,

    I've just got the information from the design team, the channel divider status also depends on VCO_PHASE_SYNC and SYSREF_EN bits.

    OUTA_MUX OUTB_MUX VCO_PHASE_SYNC SYSREF_EN Channel divider
    Doubler SYSREF 0 X OFF
    Doubler SYSREF 1 0 OFF
    Doubler VCO 1 1 OFF

    So, our observation is correct, when OUTB_MUX=SYSREF, the channel divider is power down because VCO_PHASE_SYNC and SYSREF_EN are both 0.

    Our recommendation is, set OUTB_MUX=SYSREF.

  • 0 in reply to Noel Fung
    TI__Genius 10295 points

    Hello Noel-san,

    Thank you for confirming with the design team.

    I understand OUTB_MUX=SYSREF is recommended when we don't use OUTB.

    Will channel divider be powered off when OUTA_MUX=Doubler, OUTB_MUX=VCO, VCO_PHASE_SYNC=0 and SYSREF_EN=0?

    Best regards,

  • 0 in reply to Taketo Sato
    TI__Guru** 107110 points

    Hi Sato-san,

    When OUTA_MUX=Doubler, OUTB_MUX=VCO, VCO_PHASE_SYNC=0 and SYSREF_EN=0, channel divider will be powered up.

  • 0 in reply to Noel Fung
    TI__Genius 10295 points

    Hello Noel-san,

    Thank you for confirmation.

    However the channel divider power on/off behavior is different from what datasheet Table 9 says.

    Could you correct the information when you revise datasheet next time?

    Best regards,

  • 0 in reply to Taketo Sato
    TI__Guru** 107110 points

    Hi Sato-san,

    Yes, we have recorded this error and will fix it in next datasheet revision. 

  • 0 in reply to Noel Fung
    TI__Genius 10295 points

    Hello Noel-san,

    Is there any way to power off the channel divider when OUTA_MUX=Doubler and OUTB_MUX=VCO?

    Customer has such use case, so I would like to know if there is any way to do so.

    Best regards,

  • 0 in reply to Taketo Sato
    TI__Guru** 107110 points

    Hi Sato-san,

    in this case, we need to put both EN_VCO_PHASE_SYNC and EN_SYSREF = 1. 

  • 0 in reply to Noel Fung
    TI__Genius 10295 points

    Hello Noel-san,

    Datasheet Table 21 says below.

    >SYSREF_EN should be 1 if and only if OUTB_MUX = 2 (SysRef).

    Can I think that setting SYSREF_EN=1 with OUTB_MUX=VCO will not cause any issue? 

    Best regards,

  • 0 in reply to Taketo Sato
    TI__Guru** 107110 points

    Hi Sato-san,

    I tend to believe the true table given by the design is correct, datasheet will have to be modified accordingly. 

    At the moment, please follow the design team's suggestion, we will fix the datasheet as soon as possible.