fractional N pll

Discussion in 'General Electronics Chat' started by franticAAC, Sep 15, 2012.

  1. franticAAC

    Thread Starter Active Member

    Jun 25, 2012
    I have to realize in VHDL a frequency synthesizer in order to generate frequencies from 5Khz to 20Khz with 1Hz step or more if not possible so fine frequencies.
    This is the scheme:
    The N divider is made in VHDL(others are external components) but it permits only integer division.

    With the help of internet i find fractional pll that has the drawback of spurios injection. The solution proposed is sigma delta fractional pll as in the image below.
    I don't understand how it could be implemented in VHDL, and also what are inputs and output of sigma delta modulator.

    the pld is Actel 42MX

    Could you help me? Is there other solutions?
  2. kubeek


    Sep 20, 2005
    First, think about how are you going to implement the RC filter and VCO in vhdl...

    Much better approach would be a counter which when reaches a certain number toggles an output pin and starts counting again. That is how most microcontrolles do it.
  3. franticAAC

    Thread Starter Active Member

    Jun 25, 2012
    the pll hct4046 and rc filter are external analog components

    This approach is not dual modulus prescaler?

    In this way does not introduce a phase noise every time exchange counter (i.e. from N to N+1)?

    Let me know thanks