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[ LMX2582 ] Input Amplitude, Valid Termination for Single-End input and Single-End Output

Kenichiro
Mastermind 9165 points
Other Parts Discussed in Thread: LMX2582

[ LMX2582 ] Input Amplitude, Valid Termination for Single-End input and Single-End Output

Hi,

Can you help me to clarify the definition of input amplitude requirement?

If above is correct, it seems that the oscillator which suggested in the typical application drawing has larger amplitude than specification.



Also, if we use single-ended input to OSCinP, what is the valid termination of OSCinN?
As my understanding, OSCinN should be terminated via Cin(1uF, same as OSCinP) and 50ohm resistor to GND. It's not clear for me that having R22(18ohm) and R21(68ohm).

On top of that, if customer use only RFoutAP, do you suggest to terminate RFoutAM with L3, R32(49.9ohm), C17(100pF) and R33(49.9ohm) as illustrated?



Thank you for your support in advance.
Regards,
Ken

  • 0
    TI__Expert 4085 points
    Hi Ken,

    Thank you for your interest in our products. We received your question and will be answering it as soon as possible. We appreciate your patience.

    Regards,
    Julian
  • 0 in reply to Julian Di Matteo
    Mastermind 9165 points
    Hi Julian,

    Did you get any feedback from your team?
    Please let me know, if you need further information from my side.

    Thanks,
    Ken
  • 0 in reply to Kenichiro
    Mastermind 9165 points
    Julian,

    Can you please support this query from my customer?

    Thanks,
    Ken
  • 0
    TI__Mastermind 27911 points
    Ken,
    Let's start with OSCin. For single-ended operation, you want the impedance as seen from the OSCinN pin looking out to be similar to that of the OSCinP. For instance, if OSCinP has a signal source with 50 ohm output impedance, then OSCinP sees (18+50)||68+18 = 52 ohm, so you would put a AC couping cap with about a 50 ohm resistor. It's not totally critical, but the concept is that if the input is balanced, then any spur energy on the ground plane cancels out. We have sometimes seen this help.

    As for RFout single-ended, we just put a 50 ohm sma termination on the unused side. If you populate R33 with 49.9 ohm, you get pretty much the same effect.

    Regards,
    Dean
  • 0 in reply to Dean Banerjee
    Mastermind 9165 points

    Hi Dean,

    Can you help to clarify the definition of input amplitude? If my understanding is correct, the oscillator that you suggested in typical application has larger output swing than specified by LMX2582 datasheet.

    Thank you for your support!
    Regards,
    Ken

  • 0 in reply to Kenichiro
    Mastermind 9165 points

    Hi Dean,

    Can you help to clarify following queries from my customer?

    <Q1: Input Amplitude>:

    Just want to make sure, it seems that the output amplitude of CWX813 is larger than what you specified in the data sheet.




    <Q2: Unused Output Buffer>:

    I may use either Output Buffer A or B by disabling this with OUTx_PD register.
    In this configuration, can I leave unused outputs open? or should I use 50ohm termination?

    <Q3: Unused Output Pin for Single-Ended Output>:

    Quote:
    As for RFout single-ended, we just put a 50 ohm sma termination on the unused side. If you populate R33 with 49.9 ohm, you get pretty much the same effect.

    As you mentioned, unused output pin can be terminated with 50ohm. However what about output pull-up (either resistor or inductor)? From the perspective of keeping symmetry between positive and negative, this pull-up still needed.


    Thank you for your support in advance.
    Regards,
    Ken



  • 0 in reply to Kenichiro
    TI__Guru* 98190 points
    Hi Kenichiro,

    CMOS type XO output Vpp is 3.3V when the load is high impedance (kohms). However, Vpp drops if the load is very light (<1k ohms). On our EVM, we have R19 (68 ohms) to ground, which will effectively reduce the output voltage of the XO to less than 2Vpp.

    For the powerdowned unused output buffer, you can simply leave the pins floating without any pull-up or termination.

    Your schematic is correct for the unused pin. Both pins should have the same type and value of pull-up even thought pin M is not used. If pin P goes to a 50ohms load, then pin M should also be terminated with 50ohms resistor.
  • 0 in reply to Noel Fung
    Mastermind 9165 points
    Thank you for your help, Noel!