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LMX2571 RFoutTx - Square wave or sine output?

Braddon Van Slyke
Prodigy 130 points
Other Parts Discussed in Thread: LMX2571, LMX2581

Hi,

I'm currently looking into both the LMX2571 and LMX2581 as a local oscillator source for a I Q baseband modulator and demodulator.  Both the modulator and demodulator need sine wave inputs (the datasheet specs the -dBc requirements on harmonics).

I was looking at the LMX2571 spreadsheet, and section 8.1.8 states "Push-pull is easy to use; all that is required is a DC-blocking capacitor at the output. The output waveform is square wave and therefore, harmonics rich."  It also refers to the open-drain output option where harmonics can be reduced.

I was under the impression that the 2571 generated a sine output, but perhaps I am wrong?  How about the 2581?  I don't see any similar notes regarding the output of the 2581, so I'm hoping its output is closer to a sine?

Of course, I could lowpass filter the 2571 output to reduce the harmonics, but I would like to be able to select various frequencies on-the-fly, and would like to avoid the fixed filter - fixed freq. solution.

Thanks,

Brad

  • 0
    TI__Mastermind 28021 points
    Brad,
    Although the internal VCO of the LMX2571 generates a sine wave, it is always sent through a divider that it goes through a divider and it gets squared up. At these lower frequencies, it looks more like a sine wavve. If you use the pull-up components, there may be more opportunity to reduce the harmonics.

    Now if you are talking about using the external VCO, this output of the direct VCO will be a sine wave. If you divide it, it would be closer to a square wave. If you send it direct through our output buffer, I am not sure.


    Regards,
    Dean
  • 0 in reply to Dean Banerjee
    Prodigy 130 points
    Thank you for your quick answer regarding the 2571. I would assume then, that the same can be said for the LMX2581, that is, the 2581 output goes through a divider, resulting in a square wave output as well?
  • 0 in reply to Braddon Van Slyke
    Prodigy 130 points
    I would assume then, that the same can be said for the LMX2581, that is, the 2581 output goes through a divider, resulting in a square wave output as well?
  • 0 in reply to Braddon Van Slyke
    TI__Mastermind 28021 points

    Brad,

    These devices are similar, but not the same output.

    The LMX2571 output offers a push-pull and open collector programmable options, while the LMX2581 is always open collector.

    At lower frequencies, both of these look more like a square wave, but the LMX2571 targets lower frequency applications, so the frequency matters.

    Regards,
    Dean