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LMX2592EVM: Reducing Output Frequency Harmonics from LMX2592 and RF Synthesizer Architecture

Anna Wisniowiecki
Prodigy 50 points
Part Number: LMX2592EVM
Other Parts Discussed in Thread: LMX2592,

Similar to Matthew Nickels61 (from E2E Clock & Timing Forum, "LMX2592: Reducing harmonic content of LMX2592 output"), I am using a LMX2592EVM as a tunable RF synthesizer. Specifically, I will tune the output from 150MHz up to 1.6GHz. In testing output frequencies between 100-490MHz, I observed strong odd harmonics of the output frequency. I have listed my configuration below. Is it typical of this type of frequency synthesizer to produce strong odd harmonics or harmonics, in general? If so, is there a way to reduce the harmonic levels significantly without a tunable filter? Also, do you expect that I will consistently see strong odd harmonics over my whole range (150MHz-1.6GHz) using the configuration below? Would you expect I would see even harmonics as well? Is there a different frequency synthesizer architecture that has similar or better performance without producing harmonics?

Configuration:
- Using on-board oscillator with Fpd=50MHz
- Output LO=150MHz to output LO=1.6GHz using channel divider and only output A (output B disabled)
- 4th order MASH
- Single-ended mode
- Fractional-N mode

Appreciate your insight.

  • 0
    TI__Guru** 107230 points

    Hi Anna,

    When the VCO signal get divided down to a lower frequency, the odd harmonics will be higher. It is because the output waveform of the divider is a square wave, especially at low output frequency. 

    There is no way, inside the chip, that we can do to reduce the harmonics level. 

  • 0 in reply to Noel Fung
    Prodigy 50 points

    Hi Noel,

    Thank you for your response. A few more questions. 

    1) Is the frequency synthesizer (PLL/VCO loop) the source of the harmonics and then they are augmented by the divider? Do all divider types used with these frequency synthesizers result in harmonic production or augmentation?

    2) Is there a different frequency synthesizer architecture or divider available that does not produce harmonics (with similar performance specifications to the LMX2592)? I would like to know if I can avoid adding a tunable filter to my product.

    3) Can the divider also result in even harmonics?

    Appreciate your help,

    Anna 

  • 0 in reply to Anna Wisniowiecki
    TI__Guru** 107230 points

    Hi Anna,

    The output waveform from the VCO is sine wave, however, once the signal go through a divider (which is basically a counter), the waveform becomes a square wave. At higher divided down output frequency, because of parasitic and the characteristic of the divider, the output waveform is more or less a clipped sine wave, odd harmonics are not too bad. However, at low output frequency, the waveform is basically a square wave and therefore it contains a lot of harmonic contents. 

    In short, as long as the VCO output gets divided down, bad harmonics is unavoidable because of the square wave nature.