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Original question:

LMX2592: Wideband Spectrum Spurious

LMX2592: AM Noise Modulation

Pradeep Aggarwal
Prodigy 50 points

Part Number: LMX2592
Other Parts Discussed in Thread: LMX2594

This is a continuation of the previous posts which are now closed.

I'm seeing some modulation in the noise at the output of the LMX2592, on a spectrum analyser.

The modulation seems to be fixed between 3.5GHz and 5GHz. The amplitude is not fixed - as I drop the resolution bandwidth, the noise and modulation drops as well. The noise and modulation is visible down to -100dBm.

I measured the above by setting the output frequency to 3GHz and removing the carrier using a notch filter so that I can measure down to -100dBm. Input Reference frequency = 30.72MHz and PFD = 10.72MHz.

Would appreciate if you could have another try to see if you can observe this modulation. The modulation appears like an AM modulation on the noise. The modulation peaks are around 320MHz apart.

Many thanks

  • 0
    TI__Mastermind 28031 points
    Pradeep,

    This souds like this is related tot the output buffer. We have seen issues with the LMX2594, a sister part to this, where putting an external filter can cause some buffer stability issues. In other words, try putting a resistive pad before your filter for better impedance control and I think that there is a good chance that this will make your problem go away.

    Regards,
    Dean