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LMX2820: Suggestions for Phase noise improvement / External VCO

Pathamuthu K
Prodigy 90 points
Part Number: LMX2820

I simulated LMX2820 (Internal VCO) in PLLATINUM tool with our 100MHz OCXO as reference for the frequency range 8.7 to 14.8GHz and achieved the following phase noise results.

Offset (Hz) Phase noise (dbc/Hz)
100MHz OCXO (Ref) LMX2820 @ 14.8GHz
1 -90 -
10 -123 -
100 -130 -90
1K -160 -100
10K -172 -109
100K -178 -109
250K -178 -109
500K -178 -114
1M -180 -122

At 500KHz offset phase noise is around -114dBc/Hz for frequency 14.8GHz but we need phase noise of <-125 dBc/Hz at 500KHz offset for the frequency range 8.7 to 14.8GHz.

Is there any way to achieve phase noise of <-125 dBc/Hz at 500KHz to 1MHz offset for 8.7 to 14.8GHz frequency with internal VCO.

If it is not possible with internal VCO, please suggest any suitable external VCO configurations for LMX2820 to achieve the mentioned specifications and at the same time phase noise below 100KHz offset must not degrade from the previous results.

Note: Simulated file attached for reference.

LMX2820.zip

  • 0
    TI__Guru* 81490 points

    Hi Pathamuthu,

    At 8GHz output, the PLL noise at 500kHz offset is -120dBc/Hz, that means the loop filter bandwidth has to be smaller than 500kHz in order to knock down the phase noise from PLL.

    However, this also means your VCO phase noise at 500kHz offset should be better than -125dBc/Hz so that VCO + PLL noise will be -125dBc/Hz or better.

    The internal VCO phase noise is not better than -125dBc/Hz, so you have to use external VCO to meet your goal. Your VCO will have to meet your phase noise requirement at 100kHz and 500kHz offset.

  • 0 in reply to Noel Fung
    Prodigy 90 points

    Hi Noel Fung,

    Will it be possible to achieve better phase noise upto 1MHz by increasing the Loop bandwidth around 1MHz ?

     
    1. Also please specify what are all the limitations by increasing the Loop bandwidth upto 1MHz?
     
    2. Is it possible to test phase noise performance and PLL stability for around or above 1MHz Loop bandwidth?
  • 0 in reply to Pathamuthu K
    TI__Guru* 81490 points

    Hi Pathamuthu,

    At 1MHz offset, VCO phase noise is better than PLL noise, we should reduce the loop bandwidth so as to reduce PLL noise. As a result, the phase noise at 1MHz offset will be equal to VCO phase noise. At 14.8GHz output, -124.4dBc/Hz at 1MHz offset is the best we can get, if we have a narrow loop bandwidth.

    Please read my blogs to understand the relationship between loop bandwidth vs PLL noise and VCO noise:

  • 0 in reply to Noel Fung
    Intellectual 420 points

    Hi,

    I am asking this question regarding Mr. Pathmuthu.

    Do you have any Reference design or application note to use external VCO with LMX2820 for better phase noise from 300-500KHz onward.

    Please suggest some application circuit with VCO part if you have.

    Our Final output frequency range is 8.7 to 14.7GHz for your info.

    Regards,

    Sugumar

  • 0 in reply to Sugumar Krishnasamy
    TI__Guru* 81490 points

    Hi Sugumar,

    At the moment, we don't have any collateral for this application.

    in general, when using external VCO, the phase noise of this VCO should meet your phase noise requirement at the interested offset frequencies. In this case, the VCO you pick should has phase noise better than -125dBc/Hz at 500kHz offset. Depending on the Vtune voltage requirement of your VCO, you may need to use active loop filter as the charge pump of LMX2820 is only 2.5V.