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Help with 4046 PLL

Discussion in 'Electronic Basics' started by Ed, Apr 10, 2005.

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  1. Ed

    Ed Guest

    Hi,

    I'm having trouble understanding the 4046 PLL data sheet. I want to lock on
    to a frequency of 200KHz and remain locked between 150KHz and 250KHz. From
    the data sheet I worked out I needed:

    C1 = 100pF
    R2 = 100K
    R1 = 66K

    But when I try these components the frequencies are much higher.

    Can anyone tell me what I'm doing wrong? The data sheet can be seen here:

    http://home.planet.nl/~midde639/specs/HEF4046.PDF

    I'm using 2.b on page 9 with:

    Fmin = 150KHz
    Fmax = 250KHz

    Cheers,
     
  2. Andrew Holme

    Andrew Holme Guest

    Are you using a HEF4046 or is it a 74HC4046?
     
  3. Ed

    Ed Guest

    Sorry I should have mentioned that in the post. I'm using the HEF4046 with
    a 12V regulated supply.
     
  4. Your choices don't look far off, but I would read the graphs this way.
    With a 12 volt supply (interpolating between the 10 volt and 15 volt
    supply lines), 100k for R1 and 100pF gives a minimum frequency from
    Fig 8 of about 200kHz.

    So R1 should be increased to about (200kHz/150Hz)*100kOhms=133kOhms.
    I would increase that a bit to cover any capacitance tolerance and
    stray pin capacitance. Say, 140kOhms.

    To get the maximum frequency guaranteed to go above 250kHz, the
    parallel combination of R1 and R2 need to be less than
    (150kHz/250kHz)*R1=0.6*R1. I would go 10% lower, in case the cap has
    the opposite worrisome tolerance. So .9*.6=.54

    ..54*R1=1/(1/R1+1/R2)
    75,600=1/(1/140K+1/R2)
    R2=164kOhms

    See if something nearer to to these values work better.
     
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