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looking for a suitable vco

Discussion in 'Electronic Design' started by [email protected], Dec 15, 2006.

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  1. Guest

    Hi,

    In my project, I drive an oscillating device (oscillator) with an VCO
    reference signal. In order to lock the oscillator's frequency, a PLL is
    employed. To keep the device oscillating the input frequency needs to
    be hit at least as precise as 0.005 Hz.

    The problem, though, when the oscillator is changed, its center
    frequency changes, too. It can be in the range between 11.75kHz-12kHz.

    Because of other clocks that need to be synchronized and generated, the
    VCO's output frequency must be in the range 13.865-14.16 MHz. Actually,
    a VCXO would fit my needs best to tune the frequency as accurate as 5.9
    Hz at 14 MHz (0.005 Hz at 12kHz). However, if I pick a VCXO for say
    14.1 MHz that is +/-100ppm pullable, it may work with some oscillators,
    but not with all possible.

    So, I need a VCO whose center freqency is coarse adjustable in a
    relatively wide band (13.8-14.2 MHz) and it should have a fine
    frequency tuning resolution using the input voltage.

    Has anyone an idea what VCO could fit for this?

    Thanks,

    Stefan
     
  2. Guest

    Have a look at a digital phase locked loop controlling an Analog
    Devices direct digital synthesis chip. The original was the AD9850

    http://www.analog.com/UploadedFiles/Data_Sheets/AD9850.pdf

    but the range is now huge. The cheapest, slowest part - the AD9833 - is
    probably too slow, with a clock frequency of 25MHz, to produce a
    respectable looking 14MHz clock. Something with a clock frequency
    closer to 100MHz would make life a lot easier, but you'd still have to
    be pretty careful with the design of the anti-aliasing filter. Happily,
    the Aanlog Devices range goes up to 1GHz, albeit at a similarly
    astronomical price.
     
  3. Guest

    Hi,

    a dds seems to be a good choice. But i need to program the dds's output
    frequency with an microcontroller which means, that I need to transfer
    parts of the PLL into software, right?

    Thanks,

    -Stefan
     
  4. maxfoo

    maxfoo Guest

    You could use the DDS as a reference frequency for a PLL/VCO, or just use a DDS
    alone. try the analog devices 98xx series they have very good phase noise specs.
     
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