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LMX2592: LMX2592 to LMX2592 connection

Itai Eden1
Intellectual 330 points
Part Number: LMX2592
Other Parts Discussed in Thread: LMK04828

Hi,

We are looking for a clocking solution for a system that requires 2 X 2.7GHz for data converters and 8 X 340MHz clocks for transceiver ref clocks.

Our original solution involved using an LMX2592 that will drive two LMK04828, one LMK will drive that data converters while the other LMK will drive the transceiver clocks.

However, when testing on EVMs we saw that the phase noise of the LMK04828 was not so good at 2.7GHz. Note that in order to acheive 2.7GHz from the LMK, we are not using the PLLs (The LMK is working as a clock distributer). The input signal looks good but the output is very weak and as mentioned above with degraded phase noise performance.

Now, we are thinking of alternatives. One that comes to mind is using the LMX2592 as the clock source for the 2 data converters:

1 LMX will drive the DAC and a 2nd LMX2592. The 2nd LMX will drive the ADC and LKM.

Is it possible to connect LMX2592 to LMX2592?

If so, what is the proper connection?

I was thinking 50ohm pullups at the output of the 1st LMX, AC capacitors and then straight to the 2nd LMX inputs.

OR

Do I need additional AC capacitors and 50 ohms to GND at the 2nd LMX input?

Best regards,

Itai

  • 0
    TI__Mastermind 28560 points
    Hello Itai,

    One thing I've noticed is the LMK0482x EVM has some output matching issues which can result in lower clock amplitude at 2.7ish GHz. I think this is in part due to the impedance where output split apart to SMA and then the thick pad at SMA, and also the emitter resistors being not as close to output as they could be.
    - You may also be able to get improved performance by replacing the SMA which has a large barrel with the SMA with a short tab (Cinch Connectivity Solutions Johnson 142-0701-851) along with cutting down the width of the pad at SMA.

    I've measured the amplitude of an LVPECL 20 clock with 240 ohm emitter resistors (eval board default) at 2.7 GHz at over 800 mVpp using solder in probes. When using 120 ohm emitter resistors and LVPECL 20, at 2.7 GHz I've measured over 1.1 Vpp. One final thing you may try for even higher amplitude is to use the HSDS 10 mA mode with 240 ohm emitter resistors.

    Please consider these for test.
    --
    As for LMX and LMK together. What sort of phase relationship requirements do you have for your data converters? Note the LMX2592 will use an unsychronizable divide to get 2.7 GHz. It is possible to use the LMX2592 VCO and lock it with the LMK0482x PLL. This may not help you with your 2x ouptuts unless you do a splitter. Using cascaded PLLs can also introduce more challenges for +/- 0 device clock placement of SYSREF.

    73,
    Timothy