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LMX2541: Phase Noise Spec and Best Strategy to Minimise Noise

David Kearney
Intellectual 590 points
Part Number: LMX2541
Other Parts Discussed in Thread: LMX2572LP, , LMX2485E

Hi, am looking to implement a LO with pretty strict phase noise requirements in the range 140 - 160 MHz. I have had a look at the LMX2541SQ2060E and it looks like the VCO output divider can actually improve PN performance by running the VCO at a higher frequency and then dividing down at the output - is this correct? Are there any synthesizer ICs which would have better phase noise performance for this frequency range? Or would using a PLL with external VCO or external varactor diode solution be more suitable?

Many Thanks in advance,

Dave

  • 0
    TI__Guru* 94680 points

    Hi David,

    Correct, a divide-by-2 improves phase noise by 6dB. 

    What is your phase noise requirement; number of output; output format; current consumption?

  • 0 in reply to Noel Fung
    Intellectual 590 points

    Thanks for the reply. My operating frequency range is ~155 - 165 MHz. Our channel BW is 25 kHz, and we have a requirement for -70dBc at adjacent channels, which equates to -70-(20log[25])=-114 dBc/Hz phase noise spec. Current consumption is not an issue. We only need one output. My questions are:

    1. Is there a good choice of PLL synth for the above application?

    2. Do you recommend using a divide by 2/4/8 output divider - would this theoretically reduce phase noise by 3/6/9 dB?

    3. Is there anything to be gained in using an external varactor in the VCO, since the integrated varactor contributes noise to the VCO?

    Many Thanks,

    Dave

  • 0 in reply to David Kearney
    TI__Guru* 94680 points

    Hi David,

    I would recommend LMX2572LP, it has better phase noise than LMX2541. We can get <-130dBc/Hz phase noise at 25kHz offset.

    All synthesizer (PLL with built-in VCO) use high frequency VCO (in GHz) and divider to support "any frequency" output. So if you decide to use synthesizer, you will have to use the divider.

    Other recommendation is use standalone PLL, such as LMX2485E. You need to pick a VCO (or design your own) that could meet the phase noise requirement.

  • 0 in reply to Noel Fung
    Intellectual 590 points

    Thanks Noel, where would be the best place to get the best pricing for these options? Am hoping to find a part in the $3-$5 range for 1k pcs. Many Thanks, Dave

  • 0 in reply to David Kearney
    TI__Guru* 94680 points

    Hi David,

    I am not the right person to discuss price issue, please try to contact a local TI representative to settle the price.