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Original question:

SN74LV4046A: Loop bandwidth design

SN74LV4046A: Frequency fluctuation

Liu Yang
Expert 6295 points
Part Number: SN74LV4046A
Other Parts Discussed in Thread: CD74HC4046A,

Hi Team,

Please help me answer the follow question, thank you so much.

The output frequency changes momentarily (about 20 μs) with the SN74LV4046ANS. A constant 1.6V voltage is applied from the external power supply to VCOIN. 3.3k for R1 and 33pF between C1A and C1B. The VCC 5V waveform was fine. Under this condition, the output frequency changes momentarily. Are there any problems with the conditions of use?
An image and a circuit diagram of frequency fluctuation converted to voltage are attached.

Best Regards,
Tom Liu

  • 0
    TI__Genius 16725 points

    Hello Tom,

    The only thing I see is that the datasheet mentions min for C1 should be 40 pF and you are just below that. This could be causing the problem so I would increase your capacitor there.

    How are you converting the frequency to voltage and where are you measuring this? And what is the ratio of frequency change to voltage change?

    Thanks!

    -Karan

  • 0 in reply to Karan Kotadia
    TI__Expert 6295 points

    Hello Karan,

    It changed to 40pF, but the situation did not change.

    The frequency change was converted to voltage as observed with the FM measurement function of the 8901B MODULATION ANALYZER made by HP. For the ratio, input the function generator output with Deviation set to 25 KHz to the 8901B, and attach the image obtained by observing the MODULATION OUTPUT with a digital oscilloscope.

    There was one more thing that after I replaced the SN74LV4046ANS with CD74HC4046AME4, the symptoms have disappeared. (Lot? 11AXSDM) The symptom of SN74LV4046ANS lot is 76H7EVK.

    Best Regards,
    Tom Liu

  • 0 in reply to Liu Yang
    TI__Genius 16725 points

    Hi Tom,

    I am not sure on if that fluctuation is unexpected or if its just the behavior of that device. I also don't know anything about this HP modulation analyzer to know how much frequency fluctuation is occurring from your voltage plot.

    If the CD74HC4046 works for you then I recommend just using that part instead of the LV4046.

    Thanks!

    -Karan

  • 0 in reply to Karan Kotadia
    TI__Expert 6295 points

    Hi,

    What is the difference between SN74LV4046A and CD74HC4046A, such in performance and etc.

    Best Regards,
    Tom Liu

  • 0 in reply to Liu Yang
    TI__Genius 16725 points

    Hi Tom,

    The CD74HC4046A is a Harris acquired part. It is a much older part so it has higher ICC current ratings, this could be due to the test conditions at the time or that the device does in fact consume a bit more power. All the important specs like frequency and delays are around the same as the SN74LV4046A. You should compare any key parameters you require for your use case but I believe they will be similar.

    Thanks!
    -Karan