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LMX2594EVM: Lower output power near 13.5GHz

XIN LEE
Prodigy 70 points
Part Number: LMX2594EVM
Other Parts Discussed in Thread: LMX2594

Hi,

I accidently observed a much lower output power near 13,5GHz by using inductor pull-up method, see picture below:

I'm wondering where it comes from. The first reflection is the resonance from the combination of the inductor and the decoupling capacitor on th power rail, but it seems not after test...

1nH inductor : B82496C3129Z000

1.2nH inductor : MHQ0402PSA1N0BT000

50Ohm : FC0402E50R0BST1

Could someone help me on this issue?

Many thanks.

BR,

XL

  • 0
    TI__Mastermind 23791 points

    Hi XL, 
    The behavior you describe I have seen on our EVM. 
    I haven seen the output power decease.
    I am not entirely sure why this is the case, maybe an SI issue from layout? it's really hard to say..

    The output power plot in the DS has the traces de-embedded from the measurement. 

    Regards, 

    Vicente 

  • 0 in reply to vicente flores prado
    Prodigy 70 points

    Hi Vicente, 

    I found a way to compensate this power drop by forcing the VCO_DACISET to a much lower value, for example 0 instead of 225 from the readback and I got 2dB higher from previous output power @13.5GHz which is consistent with power @13GHz and 14GHz. On the contrary, the noise floor is increased in stead...

    To be more clear, the output power curves I uploaded before was obtained with no-assist mode.

    In order to eliminate the issues from VCO, I try another frequency @6.75GHz which uses the same VCO as 13.5GHz and a channel divider by 2 in addition, with no-assist it works well with good output power... 

    I still don't know why it has a behavior like that. I am worrying if this problem will happen in our future products.

    Which inductor did you use when you saw this behavior? How much the power drop you have in your lab? Are you sure that this is not a problem from the chip?

    Thx

    BR,

    XL

  • 0 in reply to XIN LEE
    TI__Guru* 98190 points

    Hi XL,

    That particular power dip is due to VCO core change. There are 7 VCO cores in the chip, at that particular frequency range, the VCO core change from VCO6 to VCO7, that's why the output power is suddenly rebounded. 

    We can force the device to use VCO7 and let it calibrate the capcode and DACISET, this could bring the power up. 

  • 0 in reply to Noel Fung
    Prodigy 70 points

    Hi Fung,

    Thanks for help.

    I tried to force the LMX2594 to VCO7 but it can only go down to 13.78GHz with min capcode=191 and Vtune=2V at its limit (the stability will not be good across temperature), I have no choice but using VCO6 for 13.5GHz and lower.... Are there any others solutions?

    Thanks,

    BR,

    XL

  • 0 in reply to XIN LEE
    TI__Guru* 98190 points

    Hi XL,

    Your observation makes sense, that's why we need calibration to determine the VCO core as each core has boundaries.

    If your application needs VCO7, there is no way to boost the power from the chip, you need to aware of the reduced power at high frequency and design your system to adjust the power with other means.

  • 0 in reply to Noel Fung
    Prodigy 70 points

    Hi Noel,

    One more question about the VCO core changes:  why by using VCO6 at 6.75GHz for example (same VCO as 13.5GHz in my case) I didn't get the power dip ? The only difference is that a channel divider by 2 is added (same setting for VCO_SEL, VCO_DACISET and CapCode)... Does the channel divider impact the output power ?

    Thanks.

    XL

  • +1 in reply to XIN LEE
    TI__Guru* 98190 points

    Hi XL,

    My data shown I need to use VCO6 starting from 13.05GHz. output power is -1.8dBm. No assist calibration keeps using VCO6 till 13.75GHz. At this frequency, output power is -5dBm. At 13.8GHz, calibration uses VCO7 and the output power jump back to -0.45dBm. These are the output power resulting directly from the VCO. If you put a divider after the VCO, then the output power is actually coming from the divider, we will get a different output power characteristic.