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LMX2594EVM: Verification of Phase Noise Performance

PLL101
Prodigy 30 points
Part Number: LMX2594EVM
Other Parts Discussed in Thread: LMX2594

We recently purchased 2 units of the LMX2594EVM. I went through the user's guide and was trying to replicate the results shown in figure 9 (phase noise at 14 GHz) to ensure that the units were operating within specification. I downloaded TICsPro and was able to communicate with the eval board without issue. I loaded the default configuration (but changed the output MUX setting from "channel divider" to VCO in order to view the 14 GHz signal at the output.
I am using a 100 MHz external oscillator which meets the specifications listed in appendix D at offsets >= 1kHz. I am noticing that the phase noise in the "flat region" i.e offset of 100 kHz that I am measuring is about 2-3 dB higher than what is shown in figure 9 ( -105 to - 106 vs -108 dBc/Hz in datasheet) and about 3 dB higher( -113 vs -116 dBc/Hz in datsheet) at 10 MHz offset. Is this within the normal variability between units? I am measuring the phase noise with a Spectrum Analyzer.
I am also seeing a lot of spurious signals especially between 10 kHz and 20 kHz. I noted that cupping my hands over the eval board minimized the level of these spurious signals. Is it expected that the eval board would be so sensitive?
In figure 9 of the user's guide the output power is listed as ~ 0 dBm. In my setup my output power at the RFOutA connector is about - 1 dBm.
Is what I am seeing to be expected for these units or can you make any suggestions?
Thanks for your help.

  • 0
    TI__Guru* 81580 points

    Hi There,

    I think 1dB variation is reasonable. Can you try increasing the reference clock amplitude? The goal is to increase the slew rate which may be a reason of having lower phase noise. 

    The spurs between 10k and 20kHz, I don't know, could you send me over a screen shot?

    Again, 1 or 2dB variation in output power is reasonable, in fact, this could be simply a test equipment issue or because the device gets heat up after running for a long time.

  • 0 in reply to Noel Fung
    Prodigy 30 points

    Hi,

    Thanks for the reply.  I have limited access right now but as soon as I can I will get a screenshot.  Could you recommend a reasonable power level for the reference?  I believe I was using a sinusoidal signal at 0 dBm.  I didn't see a recommended value in the User's guide for the LMX2594 EVM.    From the datasheet for the PLL chip itself, the max ref voltage is listed as 2Vpp.  I assumed this was differential into 100 ohms.  I am connecting single-ended to the board so I assumed the max voltage would be 0.5 Vp or 4 dBm into 50 ohms.  Is this correct?

  • 0 in reply to PLL101
    TI__Guru* 81580 points

    Hi There,

    0dBm --> 0.63Vpp single-ended. Datasheet is asking for 0.4Vpp to 2Vpp. If possible, you can make it 10dBm. 

  • 0 in reply to Noel Fung
    Prodigy 30 points

    Hi Noel,  sorry for the late reply.  I tried increasing the reference power level from 0 dbm up to 8 dBm.  I did not notice any significant change in the phase noise characteristic.  I am still seeing values of ~-105 dBc/Hz to -106 dBc/Hz at 100kHz offset while the user's guide shows -108 dBc/Hz.  Is this within the normal variation of the units or does this indicate a problem?

    Note that I performed a measurement at 7 GHz, by using the divide by 2 instead of the vco out.  When I do so, I find that the phase noise at 100 kHz offset is -113.5 dBc/Hz.  Since this is the divided output of the vco output it seems to make sense since it is approximately 6 dB below what is shown in the user's guide at 14 GHz, ie-108- 6 = -114 dBc/Hz.   The divider could add a small amount to the phase noise.

  • 0 in reply to PLL101
    TI__Guru* 81580 points

    Hi There,

    The divider will add some little noise but at 100kHz offset, you may not be able to see the difference. 

    How good is your spec An at 14GHz? So far, we measure phase noise with the E5052, not sure if decent spec An is compatible with it in phase noise measurement.