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Original question:

DAC5688: PLL CLOCK MODE

DAC5688: Frequency Modulation with DAC5688 — Avoiding NCO Accumulator Reset on Frequency Change

Muhammad Sobeh
Prodigy 105 points
Part Number: DAC5688

Tool/software:

Hi everyone,

I'm currently working on implementing frequency modulation using the DAC5688. I understand that in order to continuously change the output frequency, I need to update the Frequency Tuning Word (FTW) and issue a sync (either software or hardware).

However, from what I've observed and confirmed in the datasheet, issuing a sync causes the NCO (Numerically Controlled Oscillator) to reset its accumulator. This results in a noticeable glitch in the output signal whenever the frequency is updated.

Is this the intended behavior?
Am I missing a configuration that allows frequency updates without resetting the accumulator?
Or is there an alternative method to perform continuous frequency modulation using the DAC5688 without causing output glitches?

Any guidance or shared experience would be greatly appreciated. Thanks in advance!

  • 0
    TI__Guru* 78584 points

    Hello,

    Everytime you need to change the NCO frequency, you will have to reset the accumulator. You will need to reset the NCO accumulator in order to update the NCO counting logic correctly.

    We recommend you can first send zero data on the CMOS data input, set TXENABLE = low, and then change the NCO frequency.

    You can then set TXENABLE = high, and then send the data again.

  • 0 in reply to Kang Hsia
    Prodigy 105 points
    Kang Hsia said:

    We recommend you can first send zero data on the CMOS data input, set TXENABLE = low, and then change the NCO frequency.

    You can then set TXENABLE = high, and then send the data again.

    What is the expected behavior of doing this?

  • 0 in reply to Muhammad Sobeh
    TI__Guru* 78584 points

    Hi

    since you have temporarily disabled the TX during frequency change, you will not have signal transmissions to avoid glitches

    Kang

  • 0 in reply to Kang Hsia
    Prodigy 105 points

    Hi Kang,

    Thank you again for your response.

    I’d like to clarify that my application involves real-time frequency modulation (FM), not just occasional frequency changes.

    The method of toggling TXENABLE during every frequency update doesn't work for this use case. It interrupts the signal completely and causes noticeable gaps or glitches or in the output. Additionally, issuing a sync resets the NCO's phase accumulator, which introduces discontinuities in the waveform — something that breaks the continuity required for proper FM generation.

    To move forward, I’d appreciate your insight on a few specific points:

    1. Is there any supported way to update the NCO frequency without resetting the phase accumulator?

    2. Can the sync behavior be configured to avoid resetting the accumulator during frequency changes?

    3. Is there an alternative way to achieve smooth, real-time FM — for example, by modulating through the input data path or using an external modulation mode?

    Looking forward to your guidance on whether this kind of FM is achievable with the DAC5688 or if another approach or device might be more suitable.

    Best regards

  • 0 in reply to Muhammad Sobeh
    TI__Guru* 78584 points

    Muhammad,

    You can send CMOS data for the FM modulation. The DAC5688 NCO cannot be changed without sync to the accumulator.

    -Kang