LMX2594EVM: Verification of Phase Noise Performance

Prodigy 20 points

Replies: 3

Views: 57

Part Number: LMX2594EVM

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.

3 Replies

  • 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.

  • In reply to Noel Fung:


    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?

  • In reply to PLL101:

    Hi There,

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