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LMX2594: LMX2594

Des Coghlan
Prodigy 100 points

Part Number: LMX2594

In fig 6, Closed Loop PLL at 15G; please clarify what is the frequency range for integrated phase noise = 301.5 mDEg rms? 

What is 2nd harmonic content from 15 to 26 GHz? I need a simple test source for BIT (Built IN Test). Greater than -30dBm is fine.

Why not allow connecting the VCO input to divider/phase detector to come from external circuit? This would allow mixing the VCO with a comb line to make a better synth, one with lower N multiplier and better phase noise?

What is PLL speed in tuning?  Is a calibration usually needed, and does this slow it down? My goal is < 15usecs to tune to a new frquency after receipt of command on SPI bus.

  • 0
    TI__Mastermind 28031 points

    Des,

    1.     The integration range is the entire range 100 Hz to 100 MHz

    2.     The second harmonic is on the order of -20 dBc   It varies a few dBm vs. VCO frequency, but is fairly constaint.  It is not greater than -30

    3.     15 GHz is very high frequency and adding a MUX in the feedback path does add current and complexity.   Also, N divider has minimum divide ratios that would likely also become a limiting factor, unless the phase detector frequency was lowered.  These things being said, you are not the first to ask for a feature like this.   Thank you for the feedback

    4.     The VCO calibration time can be as fast as 5 us with full assistance, or 50 us witn no assistance.  In the partial assist, you can get close to this.  Or also if the frequency change is fairly small, say on the order of 50 us or less.   This also is dependent on the phase detector frequency and the input reference frequency.  The PLLatinum sim tool simulates this calibration time.

    Regards,

    Dean

  • 0 in reply to Dean Banerjee
    TI__Mastermind 28031 points
    Des,

    I want to correct point 2. For output frequencies less than 11 GHz, the harmonic is greater than -30. For output frequencies higher than 11 GHz, I am measuring less than -30 dBc. At 11 GHz, I measure -30 dBc. So maybe we can meet at higher frequencies.
  • 0 in reply to Dean Banerjee
    Prodigy 40 points
    Hi Dean,
    I have a question.What is the disadvantage of full assistance?
  • 0 in reply to user3639876
    TI__Mastermind 28031 points

    The disadvantage of full-assistance is that it requires more effort from the user.

    To do full assistance, the user has to lock the LMX2594 at a previous time and read back all the calibration settings and store in memory.  This is for every part.

    Then the next time you go there, you force these calibration settings.   This look-up table is different for every single part, it is more overhead up front.   Also, full assistance bypasses the calibration, so you might get better phase noise letting the part calibrate.  For instance, if you lock the part at 25C and read back the settings, then these settings are good for 25C.   If the part goes to +85 and you use the parameters you read back for +25C, the part will lock and work, but you might get better phase noise (say ~1 dB) by running the calibration when the VCO is at 85.

    Regards,
    Dean