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LMX2492 device specific considerations for stability?

Ben Toby
Prodigy 30 points
Other Parts Discussed in Thread: LMX2492, OPA820

I am a college student working with a team in one of my courses to develop a LF-20 GHz signal generator.  One of the stages I am in charge of is a 5-10 GHz frequency synthesizer, making use of the TI LMX2492 as the phase comparator and RFMD RFVC1801 as the VCO.  Our reference is 100 MHz.  No one on my team has worked with physical PLLs before, so is there anything device-specific for the LMX2492 (or even VCO and other components) we should be aware of that may affect stability?  I plan on using the inverting op-amp integrator topology shown in http://www.analog.com/library/analogdialogue/archives/43-12/hv_pll.html for the loop filter, with the bias voltage set to Vcp/2 as the site suggests.

Thanks in advance!

  • 0
    TI__Mastermind 27581 points

    Ben,

    In terms of stability, no, but a few things:

    1. When you use the inverting polarity, invert the polarity of hte phase detector.   I have done some experiments that suggest that if you increase the PFD_DLY word setting from 860 ps to 1200 ps, you get better phase noise in inverting mode, although the evidence is circumstantial.

    2.  The LMX2492 1/f PLL noise varies with tuning votlage.   In general, you might find better phase noise at lower tuning voltages than 1/2 of the charge pump supply.  So when you choose your bias point, maybe try 1-1.5 volts if your op amp rails let you.

    3.   Consider using an op amp like the OPA820 as one possible suggestion of many.

    4.    The RFVC series of VCOs is being discontinued by RFMD as I was told.  Maybeit doesn't matter to you for your project, but just keeping you aware.  For the RFVC1843, the tuning gain goes very high at lower tuning voltages, so be aware of this as well.

  • 0 in reply to Dean Banerjee
    Prodigy 30 points

    Dean,

    Thank you for the information!  Very helpful.