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LMX2615EVM-CVAL: Discrepancy in Phase Noise Plots

Deepak Attri
Prodigy 30 points
Part Number: LMX2615EVM-CVAL

Tool/software:

I am using 10MHz input reference for LMX2615EVM-CVAL board. I generated output frequency around 6.84 GHz and observed the output phase noise. My main concern regards to phase noise around 100 Hz-1KHz

I could observe that Phase noise plots varied significantly even for nearby integer output frequencies i.e. 6.82GHz, 6.83GHz and 6.84GHz (plots attached). Also, phase noise of Fractional  output frequencies tends to be on the better side than integer output frequencies, i.e. 6.8246GHz and 6.8346GHz (Plot attached).

Why phase noise of fractional output frequencies are better than integer output frequencies?

Why phase noise varies so much for nearby integer frequencies, even though there is not much of a change in multiplier integer?

Even best of phase noise plots are far away from Phase noise specifications of LMX2615. Is it because we are using 10MHz input reference and LMX2615EVM-CAL board is tuned according to 100MHz? Does tuning loop filters help in improving phase noise? What options do I have to improve phase noise with 10Mhz input reference.

Regards

Deepak

  • 0
    TI__Guru* 93670 points

    Hi Deepak,

    How did you interface the reference clock to the synthesizer? Is the 10MHz clock output sine wave?

  • 0 in reply to Noel Fung
    Prodigy 30 points

    Yes, reference clock is 10MHz sine wave

  • 0 in reply to Deepak Attri
    TI__Guru* 93670 points

    Hi Deepak,

    Can you change the OSCin coupling capacitor from 0.1µF to 1µF?

    Is the configuration same as the other post you placed in below?

    e2e.ti.com/.../lmx2615evm-cval-spurious-observed-in-spectrum-output

  • 0 in reply to Noel Fung
    Prodigy 30 points

    Hi Noel,

                 Yes, we can give it a try. Will update you on the same.

    Yes, configuration is same.

  • 0 in reply to Deepak Attri
    TI__Guru* 93670 points

    Hi Deepak,

    Here is my data, fpd = 10MHz.

    6820MHz

    6830MHz

    6840MHz

    6824.6MHz

    6834.6MHz

  • 0 in reply to Noel Fung
    Prodigy 30 points

    Hi Neol,

    Thank you for your inputs. With your input and further optimization related to input voltage and OUTA_PWR setting, we could improve the performance. We went ahead with our measurements and have further queries:

    • We were able to achieve around -89 dBc (b/w 100-1KHz) at 6.84GHz. However it is to be noted that OUTPUT MUX of OUTB is also channel divider and we have not enabled SysRef.
    • Activating SysRef (Master Mode, continuous output) degrades phase noise by 8 dB (from -89dBc to -81dBc). Please, see the attached plot. Can you share any recommendation to minimize this degradation? To achieve our target specification, we require a phase noise of better than -85 dBc/Hz between 100 to 1KHz. Is there a way to improve this?
    • This SysRef is very essential to us, to drive certain digital circuits requiring clock more than 23MHz.
    • We also observed that if SysRef setting is activated and even when the OUTB_PD is disabled, the phase noise is still degraded.

    Thanks

  • 0 in reply to Deepak Attri
    Prodigy 30 points

    Hi Neol,

              We are still awaiting a response for our query from your side . Your valuable inputs will help us further improve and meet our target specifications.

  • 0 in reply to Deepak Attri
    TI__Guru* 93670 points

    Hi Deepak,

    SYSREF frequency is 23.75MHz, the spurs should be multiple of 23.75Hz. The spurs or noise in your plot is between 100Hz and 1kHz, they are not due to the SYSREF generator. See below the plots, there is no difference between with SYSREF (SysRefReq pin = HIGH) and no SYSREF (SysRefReq pin = LOW).

    with Sysref

    no Sysref

    SYSREF spurs is at 23.75MHz.

    with Sysref.

    no Sysref.

    Reduce RFoutB output power can reduce the SYSREF spurs. 

  • 0 in reply to Noel Fung
    Prodigy 30 points

    Hi Noel,

             Thanks for your input, we repeated our measurements with reference input frequency of 90MHz sine wave. We observed similar phase noise plots for 6.84GHz output "with & without SYSREF". No degradation observed. Plots are attached for your reference:

    SYSREF off

    SYSREF on

    However, as soon as we shift back to our targeted reference input frequency of 10MHz sine wave, we again observed the degradation in phase noise with SYSREF on. As mentioned previously, Phase noise degraded from -89dBc (with SYSREF off) to -81dBc (with SYSREF on).

    In your opinion, what could be the reason for degradation observed in phase noise upon turning on SYSREF, when reference input frequency is 10MHz sine wave while there is no such observation with 90MHz sine wave reference input frequency? 

    As our final targeted reference input frequency is 10MHz sine wave along with SYSREF output, can you suggest some ways to improve the observed degradation in phase noise?

  • 0 in reply to Deepak Attri
    TI__Genius 17400 points

    Hello,

    The team is out for the holidays. Please expect a response by Jan 2 at the earliest.

    Thanks,

    Kadeem