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PLL question

Discussion in 'Electronic Design' started by Tp, Jan 13, 2005.

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  1. Tp

    Tp Guest

    Hello!

    I'm building a frequency multiplier using a PLL 4046. I'm thinking to
    use microcontroller as a dividing element in the feedback loop. I'm
    using a phase comparator II. There is a few us delay in dividing
    process due to microcontroller and I'd like to know does is cause any
    problems.

    Tp
     
  2. Andrew Holme

    Andrew Holme Guest

    The delay will cause phase shift. It's not a problem if the delay is
    constant: it just means the VCO edges won't coincide with reference
    edges - they'll be slightly offset.

    A variable delay (e.g. due to interrupts, jitter on the CPU clock or
    concurrent processing) would cause phase modulation (and therefore
    frequency modulation) on the VCO.

    If your microcontroller has a dedicated counter, which is clocked by
    the VCO, and which toggles its output without software intervention,
    then you should be fine.
     
  3. John Smith

    John Smith Guest


    I'm doing the same thing. I am using a 1 MHz reference oscillator
    (ECS-100AC), an MM74HC4046N, and a couple of counters in a Cypress
    microcontroller. I divide the 1 MHz down to 10 kHz. My 4046 VCO is running
    about 1.6 MHz and is divided by 160 for the 4046's phase comparator II. I
    only care about the frequency of the 1.6 MHz signal, not the phase.

    The only problem I've had is phase noise (jitter). I have determined that
    the 1 MHz reference is rock steady. The 4046 VCO is very steady, but only if
    I disable the 1 MHz oscillator and the micro and drive the VCO frequency
    control with a fixed DC. When the loop is closed, the VCO control voltage is
    steady, indicating no loop oscillations. I conclude that there is 1 MHz
    noise or microcontroller noise getting into the 4046 somewhere (Vcc is
    bypassed at each chip).

    It appears that I need to get the circuit off the breadboard (the type where
    one plugs the component into holes with spring contacts) and put it on a
    piece of copper ground plane and bypass the chips with short-leaded caps. I
    have been surprised to learn that even a half inch of component leads can
    cause problems even at this low frequency.

    Anyway, be aware that you can have phase noise problems if you are not
    careful with your layout, bypassing, or filtering.

    Good luck.

    John
     
  4. Joerg

    Joerg Guest

    Hi John,
    That's the ticket. I have never seen any RF stuff work well on these
    plug-in boards. Therefore, we do not have any of these boards in the
    lab. What I found to work really well are little islands of parts on
    copper clad. Wainwright strips are used a lot but cut up pieces of
    copper clad and some glue would also do. It doesn't look pretty and the
    more you touch the copper the uglier it becomes. But those kinds of
    prototypes can operate nicely into the GHz range.

    Regards, Joerg
     
  5. John Smith

    John Smith Guest


    Hi, Joerg -

    Yes, that's the way I normally do it when higher frequencies are involved.
    This time I was led astray by the 1.6 MHz maximum frequency. Stupid me. I
    wasn't thinking about the nanosecond rise and fall times of all three
    devices.

    John
     
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