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VCXO in LMX2541 PLL

fernando henrique cardoso
Prodigy 80 points
Other Parts Discussed in Thread: CDCE72010, LMX2541, LMX2485E

Hello guys...

We are design high performance data acquisition system with 4 channels and we need to provide a ultra low jitter signal these 4 125 MSPS ADC´s that will do undersampling in a signal of 500 MHz. We need to use a PLL to lock the  reference signal of 0.625 MHz and the output signal to ADC´s will be 117.5 MHz. Besides generate a fixed frequency clock of 117.5 MHz  we also need sometimes to change this frequency +- 50 Hz steps in a reange of 1 or 2 KHz. we know that is possible to tune a VCXO of  +-20ppm Pulling range. 

Many high performance clock distribution like CDCE72010 and LMK family have no possibility to generate this fine frequency tunning... I am considering to use a High performance PLL, for ex. LMX2541 but I guess the performance is poor compared with CDCE and LMK family, right?

So,my questions are:

How to compare the jitter performance or phase noise performance of LMX2541 and IC´s like CDCE and LMK? is the LMX2541 worst for this application?

It´s possible to do fine frequency tunning using CDCE72010 or LMK family (dozens of  Hz step in few KHz range)?

VCXO is really the best option to provide best jitter performance to PLL or ADC clock?

It´s possible to use a External VCXO companyng LMX2541? in the datasheet they only mention possibility to use external VCO...

thanks in advance

  • 0
    TI__Mastermind 28001 points

    Fernando,

    The LMX2541 PLL performance is on par with the LMK series and better than the CDCE performance.   As for the VCO, you are bypassing anyways.  It has many features you want, like external VCXO support through ExtVCOin pin.

    However, this might not be the best part because the frequencies you are choosing.   The ExtVCOin Pin minimum frequency is 400 MHz, which your 117.5 MHz violates.  The OSCin pin minimum specification is 5 MHz, which your 625 kHz violates.  For the ExtVCOin Pin, you could use a higher frequency and use our internal dividers, but there is an issue at the OSCin pin

    The LMX2485E PLL is closer and much cheaper.  It can go down to 50 MHz for the VCXO and the fractional denominator can go from 1 to 4194303-- more than sufficient for what you need for your application.  The only problem is the minimum input frequency is 5 MHz, which your 625 kHz violates.

    There as a large selection of LMX and LMK products that can generate your frequencies, but your 625 kHz input is problematic for most of them.  The reason for this is that we typically specify this for a sine wave and at lower frequencies, there is an issue with slew rate.  In truth, if the input is a square wave, many of these deivces that do not have integrated VCOs (low input frequencies also cause issues with integrated VCOs), such as the LMX2485E, could typically work.   However, we do not test this in production and do NOT GUARANTEE this performance.