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CD74HC4046A: PLL IC Not Oscillating at correct frequency

Jackson Kuehler
Prodigy 10 points
Part Number: CD74HC4046A

Tool/software:

I  am new to PLL design, and I am designing a local oscillator using the CD74HC4046A PLL IC. I have a schematic setup and a reference signal of 15.8 MHz from a waveform generator, but I cannot get the correct value out of the loop filter. How do I determine the right values for R1 and C1, and what frequency does my loop filter need to be designed for?

  • 0
    TI__Expert 8810 points

    Hi Jackson, 

    I see that you mention you're not seeing the correct value out of your loop filter, can you clarify what you're measuring compared to what you expect? The loop filter will affect the capture range and lock time of the PLL. If you're inputting a very clean 15.8MHz reference then you can probably use a relatively narrow bandwidth (maybe in the range of a few kHz to 10s of kHz). If your input is noisy and the PLL is having a hard time locking then you could try using a wider loop bandwidth. 

    Regards, 

    Connor