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LMX2820EVM: The mesured Phase Noise @10kHz~100kHz offset at Fpfd=100MHz does not match the simulated values

Masashi Wada
Prodigy 15 points
Part Number: LMX2820EVM
Other Parts Discussed in Thread: LMX2820

Hi TI TEAM

I am using LMX2820EVM.

Could you tell me the reason why the phase noise @10kHz~100kHz offset at Fpfd=100MHz does not match the simulated values?

I am usign LMX2820EVM at the settings below.

I tried using the doubler in LMX2820 to increase the frequency Fpfd to 200MHz.
Then the measuremen data matched the simulated values within 1dB.

Is it necessary to increase the phase comparison frequency to achieve phase noise performance?
Please let me know how high I should increase the phase comparison frequency to get the good phase noise performance?


//Settings No.1
OSC in=100MHz
Fpfd=100MHz
Fvco=9700MHz
output Freq =4850MHz

The simulated value@30kHz:-118dBc/Hz
The Measured value@30kHz:-112dBc/Hz

There is the 5dB difference betwee the simulatede value and the measured value.

//Settings No.2
OSC in=100MHz
Fpfd=200MHz
Fvco=9700MHz
output Freq =4850MHz

The simulated value@30kHz:-118dBc/Hz
The Measured value@30kHz:-117dBc/Hz

There is the 1dB difference betwee the simulatede value and the measured value.

Thanks.

  • 0
    TI__Guru** 107230 points

    Hi Wada-san,

    I believe this is the error in the EVM, please change C31 to 10µF, this should improve the phase noise at these offset. The phase noise can be further optimized if we also change C29 to 680nF or 1µF. 

  • 0 in reply to Noel Fung
    Prodigy 15 points

    Hi Noel-san

    Thank you so much.
    I will try what you told me.

    By the way  why could you find that C29,30,31 for "VCO" is not suitable ?

    I thought the reason for the phase noise degration was something about PFD.

  • 0 in reply to Masashi Wada
    TI__Guru** 107230 points

    Hi Wada-san,

    We found out similar phase noise issue with the incorrect capacitor values on the EVM. Please give it a try to see if you are actually having the same issue. If there is no improvement after the change, we can come back to investigate what may be the root cause. General speaking, with fpd = 100MHz, loop bandwidth is smaller than with fpd = 200MHz. As a result, in-band VCO noise will increase. Since the incorrect capacitor values will lead to higher VCO phase noise, so I guess you are having the same problem with these capacitors.

  • 0 in reply to Noel Fung
    Prodigy 15 points

    Hi Noel-san

    Thank you for your reply.

    I tried to change the capacitors to what you told me.
    The phase noise about 10kHz~100kHz was improved by that changes.

    On the Settings No.1 by changing C31 to 1uF the phasenoise at 30kHz offset was improved to -116dBc/Hz.

    Then on the Settings No.1 by changing C31 to 1uF and C29 to 0.68uF or 1uF the phasenoise at 30kHz offset was improved to -118dBc/Hz.

    Also on the Setting No.2  the phase noise was improved.

    I have recognized this problem has to do with the pushing of the VCO.

    Do you think my recognition is right?

  • +1 in reply to Masashi Wada
    TI__Guru** 107230 points

    Hi Wada-san,

    I think the noises could be coming from the internal LDOs. These LDOs need a bigger capacitor at their outputs in order to filter low frequency noises (<100kHz). So, yes, we could say that it is a frequency pushing issue.

  • 0 in reply to Noel Fung
    Prodigy 15 points

    Hi Noel-san,

    Thank you very much for your reply.

    I understand what makes the noises worse.

    Thank you very much.