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LMX2595: Maximum Reference Input Power

Tony Rohlev
Prodigy 230 points
Part Number: LMX2595
Other Parts Discussed in Thread: TIDA-01346,

Hello,

I am interested in what the maximum power going into the reference input (OSCinP / OSCinM) can be at 100 MHz.

This is a 50 ohm system and the OSCinP and OSCinM pins are each AC coupled through a 0.1 uF capacitor with 50 ohm resistor shunt to ground. My 100 MHz reference source is single ended so only the OSCinP pin is driven.

The data sheet specifies the maximum drive level as 2 Vpp (+10 dBm in a 50 ohm system), but I would like to drive it with something closer to 13 dBm. Will that be a problem? At what power level does it become a problem?

Thank you,

Tony

  • 0
    TI__Mastermind 18070 points

    Hi Tony,

    My coworker will get back to you next Monday.

    Regards,
    Hao

  • 0 in reply to Hao Z
    TI__Guru* 88030 points

    Hi Tony,

    It is not recommended to operate the part outside the specification. On one hand, the part may not perform as expected. On the other hand, it may suffer long term or even immediate damage to the part.

  • 0 in reply to Noel Fung
    Prodigy 230 points

    Hello Hao,

    I understand that TI does not recommend using parts beyond their specified limits. However, it's also true that TI does not specify an Absolute Maximum Rating for the OSC input of the LMX2595. Also, if you remember, in a discussion we had about OSC input power a while back (February 25, 2019) you mentioned that for the TIDA-01346 design Dean had specifically removed the 51 ohm shunt resistors from the OSC input in order to boost the voltage seen at the input.

    So my question is: Is the +10 dBm (1 Vpk) on the OSC inputs a hard limit, an absolute maximum rating? Will the device be damaged if I apply +13 dBm (1.4 Vpk)?

    I don't want to put a 3 dB attenuator between my OCXO and the LMX2595 OSC input because it will degrade the SNR, and hence noise performance, of my outrageously low noise OCXO (the Wenzel Golden Citrine). On the other hand I'd hate to burn out my beautiful LMX's. If you have a limiter that you can recommend, that will not degrade the OCXO noise, especially around the critical 100 Hz offset please let me know.

    If it's possible, could we also get Dean's opinion on this?

    Thank you again,

    Tony

  • 0 in reply to Tony Rohlev
    TI__Guru* 88030 points

    Hi Tony,

    For single-ended input, the max. is 2Vpp. With 13dBm signal, the voltage swing will be 2.8Vpp, I think the tolerance is too big here. 

    I don't know how big is the impact of a 3-dB attenuator to the synthesizer phase noise, I expect it is minimal. Maybe you can consider to use a balun to reduce the power input to the LMX device. This one, ADT2-1T+, looks good and relatively low cost.