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Need VCO for 14 MHz PLL

Discussion in 'Electronic Design' started by W Letendre, Jul 17, 2005.

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  1. W Letendre

    W Letendre Guest

    Need to generate in phase and quadrature digital signals, locked to
    laser detector output at ~ 3.6 MHz (quite stable). Need reasonably high
    precision 90deg shift and 50% duty fator +/- a percent or so. Thought
    of building 4X multiplying PLL, followed by 2 bit quadrature divider.
    So, looking for VCO that will run at 14.4 MHz.

    Seem to find lots of chips for cell phone and other VHF/UHF/uWave apps,
    and old standbys from 556 and 4046 family, but nothing that would work
    at 14.4 MHz. Suppose I could make a Colpitts or Hartley circuit using
    varactor diode for variable capacitance, but, hoping there is a chip
    that will do this. Checked Maxim, National, Motorola, Linear
    Technologies, Burr Brown, with no joy. Any other suggestions?

    W Letendre
  2. Joerg

    Joerg Guest

    Hello W Letendre,

    Look at digitally controlled oscillators (DCO). Analog Devices has lots
    of these. You basically follow with a simple filter that cuts off above
    your desired range, send it the frequency you want and that's it. I know
    that this sounds like buying a pre-cooked pizza (which we'd never do...)
    but if you want it done fast that may be the way to go.

    Regards, Joerg
  3. John Larkin

    John Larkin Guest

    You could just buy a cheap commercial RF VCO from MiniCircuits or
    Emhiser or somebody, and divide it down as necessary. The AD9901 is a
    nice, very fast phase detector.

    The 4x pll + divider is good, but you will get phase ambiguity that
    you'll have to repair somehow.

  4. Jim Thompson

    Jim Thompson Guest

    Naaah! It's trivial to make a quadrature divider that has no
    ambiguity ;-)

    ...Jim Thompson
  5. W Letendre

    W Letendre Guest

    Fortunately, do not need to lock output signals "in phase" to input
    reference; suffices for application that output in phase and quad
    signals have exact 90deg phase match to each other.

    Will check out MinCircuits; should have thought of those guys. They
    make a ton of solder-in RF "gadgets" as I recall....

    W Letendre
  6. W Letendre

    W Letendre Guest

    Fear that DCO would be too expensive for application. Will check
    prices, though; they may have come down since last time I looked at

    W Letendre
  7. John Fields

    John Fields Guest

  8. Tim Wescott

    Tim Wescott Guest

    If "quite stable" means "crystal stable", and you don't need a large
    range, and you have the time for things to settle down, then take a look
    at VXCO's -- they're available from many, if not most, of the crystal
    oscillator folks. And they're nearly crystal stable themselves.
  9. Joerg

    Joerg Guest

    Hello W Letendre,
    That is the downside of DDS chips. They are expensive and that is why I
    dealt with them only once so far. You need one where the master clock is
    about 3 times your frequency or higher. Maybe you could scrape by with
    the AD9835 but that is still over $5:

    However, if you want 14MHz quad you'd need one with 200MHz clock or so,
    to be able to divide down. Those will be more in the $10 range.

    If it's very cost sensitive you'll have to do your own PLL.

    Regards, Joerg
  10. Joerg

    Joerg Guest

    Hello W Letendre,
    I don't want to spoil the broth here but since you mentioned cost: These
    are often well above the cost of a DDS chip.

    Regards, Joerg
  11. maxfoo

    maxfoo Guest

    The DDS chip itself is cheap but you still need a reference frequency source
    that adds to the overall cost.

    The minicircuit vco is about $20, if that's still too expensive look into
    designing your own vco.

    Synergy Microwave has a few good references on vco design, if you want to take
    that route...
  12. Joerg

    Joerg Guest

    Hello John,
    15MHz out of a 4046 is indeed pushing it. For the 4013 I'd use some
    faster HC stuff for better 0-90 tolerance.

    Regards, Joerg
  13. Joerg

    Joerg Guest

    Hello Max,
    Yes, but that boils down to a crystal and a fast unbuffered inverter.
    Plus you can still rent out the remaining inverters from the six-pack.
    Ok, if you need a really quiet clock source you'd have to throw in a
    BFS17 but that won't set you back more than a dime.
    It's not me who needs this VCO. But if it was me and it had to be low
    cost as the OP said I would certainly roll my own.
    Thanks. Ulrich Rhode really knows his stuff. Got his Communication
    Receivers book.

    Regards, Joerg
  14. john jardine

    john jardine Guest

    Had 15megs from a 4046 but it was pushing it. The X4 loop multiplier is
    easily made by using (for example) 2 off, 4013 style, dual flip flops, wired
    as a 'walking ring counter'. All four 0-90-180-270 phases, appear on the 'Q'
    output pins.
  15. Joerg

    Joerg Guest

    Hello John,
    If you could run the logic just on the +6V side maybe HC would work.

    Regards, Joerg
  16. john jardine

    john jardine Guest

    Yes, indeed.
    74HC4046. F=15meg. Vcc=12V. IC=hot!. I've a 100kc-400kc setup using
    CD4046/CD4013's running on the bench at the moment. Even at this relatively
    low speed I've needed a small lead adding to back-off the loop delays.
    (CDxxxx's, as I'm stuck with balanced 6V supplies and signals)
  17. Jim Thompson

    Jim Thompson Guest

    HC will actually stand off about +7V, but the manufacturers won't tell
    you that ;-)

    ...Jim Thompson
  18. Joerg

    Joerg Guest

    Hello Jim,
    Yes, I have read that somewhere. Some Asian mfgs have also allowed
    operation down to 1.5V. But I work mostly in med and there everything
    has to be cast in stone. IOW if it ain't in the data sheet you can't do it.

    Did you get the kitchen drawers done?

    Regards, Joerg
  19. Jim Thompson

    Jim Thompson Guest

    Ordinary 74HC... is optimized at +5V, will work reasonably well down
    to +2.5V, but becomes like molasses at +1.5V

    The I/C stuff I'm working on right now has 5V, 3.3V, 2.5V and 1.8V
    devices on a single chip.
    Not yet. I'm installing wire baskets on ball-bearing glides in the
    cabinets right now :-(

    But it's giving me ideas for the drawers. These commercially-made
    baskets have a cross piece in the back to maintain alignment of the
    ball-bearing glides.

    ...Jim Thompson
  20. Jim Thompson wrote...
    These days the multiplicity of supply-voltages is getting to be a
    real pain for the designer. For example, modern 2.5 and 1.8V cPLDs
    have a rather substantial power draw, and generally their supply is
    not well-made using a linear regulator from +5V. One ends up with
    multiple supply sources, often with special sequencing rules, and
    perhaps with surprise interactions, like the difference frequency
    between two switching regulators. A design I'm finishing now, a
    five-channel DDS frequency generator, has seven supply voltages...
    And it's meant to be well-behaved to the 10ppm level. What a pain!
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