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1 GHz synthesizer

Discussion in 'Electronic Design' started by Andrew Holme, Feb 24, 2010.

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  1. Andrew Holme

    Andrew Holme Guest

    I'm developing a 1 GHz fractional-N synthesizer. The reference input is a
    50 MHz sine wave which I'm converting to LVDS as follows:

    1. 50 MHz sine wave arrives at board mounted SMA in 50-ohm system
    2. Converted to 100 ohms differential using Mini-Circuits ADT2-1T
    3. Carried 10mm along pair of closely-spaced traces to 100-ohm 0402 resistor
    adjacent to differential input of LMH7324 high-speed comparator.

    I think some 1 GHz from my VCO (3 inches away) is getting into the (3.84
    Gbps rated) LMH7324 comparator input and modulating the zero-crossings.
    Result: integer boundary spurs when the VCO frequency is set very close to
    an integer multiple of the reference.

    Reducing reference drive level increases integer boundary spurs.
    Increasing reference drive level reduces integer boundary spurs (quite
    encouraging reduction is possible).
    Touching one side of the transformer secondary with the metal blade of a
    plastic-handled scalpel increases integer boundary spurs.

    Presumably, the 1 GHz enters the comparator as a common-mode signal. I
    would like to try two small caps from the comparator inputs to ground.
    Unfortunately, there are no convenient grounds to be had in that area. The
    transformer secondary centre-tap is grounded and I could easily put small
    (10pF?) caps across the secondary windings; but that's 10mm away. I will
    just have to try a few different things and maybe drill some holes through
    to the continuous copper ground plane on the bottom. Annoyingly, there are
    some decouplers on the bottom just under the 0402 100-ohm.

    How/why is the 1 GHz leaking all over the board like this?

    Should I have split my ground plane between reference frequency and VCO
    frequency areas of the board?

    Any other suggestions?

  2. Andrew Holme

    Andrew Holme Guest

    Thanks for the suggestions. Using a tuned circuit sounds good. I was
    planning to support 10 or 50 MHz reference inputs; but I could drop that
    requirement and stick to one frequency.

    I wanted it to take the -10 dBm 50 MHz STD output from my spectrum analyzer.
    I'm currently needing to boost this by 20dB to push the spurs down.

    Can you elaborate on why a slower comparator might result in less phase
    noise? I can see how it might be less susceptible to 1 GHz and therefore
    lower the spurs; but wouldn't it have more random jitter?
  3. Joerg

    Joerg Guest

    <ouieeeeeeeeee ...> (sound of alarm siren)

    If there is no ground plane in that area that's you first problem, needs
    to be fixed. If there is one and you just don't have a via to it place
    copper tape to the next via so you get a part of a plane there. If too
    crowed put that on the other side and drill a hole. Say a prayer before
    drilling :)

    If the comparator is well bypassed, which I hope it is, then you cold
    also solder caps from inputs to its VDD and VEE. If not well bypassed
    then that needs to be accomplished first.

    As John said transformers have plenty of leakage inductance. This picks
    up RF and you may have to shield the transformer.
    Add a little snippet of copper tape to bridge the 10mm with a fairly
    wide swath of copper.

    In the RF world that is usually a recipe for desaster. A ground split is
    like roulette. You win on one number only, on black nothing happens, on
    red you lose ;-)

    Yes, can you add make-shift shield cans? Shallow shield tops of a few
    millimeters height can be bought but when in a rush at a client I
    sometimes cut up a butter cookie can. The contents are dutifully eaten
    by the crew :)

    Another more nasty trick that borders on cheating is to solder a 2-3cm
    snippet of wire to one side of the transformer and bend it around. Try
    both sides. Then try to find a minimum in the spurs by bending. It acts
    as an antenna and the goal is to make it feed 180 degrees against the
    1GHz that leaks in through other paths. Typically one would use a longer
    than anticipated snippet and when the reaction upon bending is too gross
    cut a chunk off and see whether it affords more neutralization control.
    Bending needs to be done by plastic tweezers. Or in a pinch wooden chop
    sticks, always a good excuse to have lunch at an Asian place.

    Make sure a black cat crosses the lawn and is seen by others when you do
    this so when it works you will be awarded guru status :)
  4. Fred Bartoli

    Fred Bartoli Guest

    Joerg a écrit :
    Did you really use that one? Might be a bear to find the right position...
    Better to bring one with you. I'll add one to the box next time I'll go
    to a customer...
  5. Andrew Holme

    Andrew Holme Guest

    I have a continuous ground plane on the bottom but no ground vias near the
    comparator because it only has VEE -5V, VCCI 3.3V and VCCO 2.5V. There are
    vias to 100nF decouplers on the bottom and I could easily put caps from
    these vias to the inputs. I didn't do this earlier because I wasn't sure if
    the SRF of the 100nF would be effective against 1 GHz; but I could maybe
    replace them with 10nF.

    Thanks for putting me straight on that one!
  6. Joerg

    Joerg Guest

    If it's regular X7R ceramics in 0402, 0603 or even 0805 it'll be ok at
    1GHz. No need to swap those out.
  7. Joerg

    Joerg Guest

    I did. It's not that bad. Basically you bend it downwards a bit and then
    rotate it slowly until you hit close to zero. If the noise shoots up way
    more than 2x after soldering it on snip pieces off. If you can't get
    anywhere near zero it needs to go on the other phase.

    Another trick I learned way back in my early ham radio days is the poor
    man's variable capacitor. If you need just a smidgen of signal injection
    and the source is really close by (less than 1cm) use two wires and
    twist tightly several times. Then snip off a millimeter at a time until

    Trick #2 did not come without pain. Had to twist some more on my first
    one, reached in, forgot about the fact that 300V plate voltage was
    riding on one side, snipped end touched index finger ... *BZZZT*

  8. Fred Bartoli

    Fred Bartoli Guest

    Joerg a écrit :
    Yep, knew that one.
    One related I used/invented? where I needed really super fine cap
    adjustment (to GND) was:
    Have two surface pads located on top side, located half an inch to one
    inch apart. One unconnected, the other being your cap terminal.
    Then solder a small bare wire between the pads, so that it makes a flat
    inverted U, something like 5mm above the GND plane.
    And voilà, you have your really fine trim cap. Just bent the wire up and
    down with a small plastic tool while using the PCB as fulcrum and you
    can easily adjust your cap to the few fF.

    Thought of this while thinking about the way some chineese tuners coils
    are hand adjusted. It made wonders once in prod at tweaking HF CMRR.
  9. Joerg

    Joerg Guest

    This is actually how some RF mixer modules are aligned in production. By
    pushing and bending those wire stubs.

    On coils that's been done since a long time. Pushing the windings
    together or if you want to go in the other direction insert the little
    flat wooden stick and twist it so the turns spread again. But a really
    experienced tuning tech never has to go backwards :)

  10. Joerg

    Joerg Guest

    Or build a filter with two peaks, one at 10MHz and the other at 50MHz.
    They are far enough apart, should work.
  11. Fred Bartoli

    Fred Bartoli Guest

    Joerg a écrit :
    Or maybe, if the reference input is a square signal, let it pick the
    10MHz fifth harmonic...
    With a high enough Q you'll probably have enough signal to almost not
  12. Joerg

    Joerg Guest

    The Zeitgeist pub would have some soothing medicine for that :)
  13. Joerg

    Joerg Guest

    In the end it's like what I have seen at an auto shop in Italy. Alfa
    Romeo, hot edition with four independent carburtors, needed adjusting.
    "O mama mia, only Giuseppe can do those, you'll have to wait until he
    gets back".
  14. Joerg

    Joerg Guest

    Oh yeah, and the design is really slick, too. This one was a Julietta
    but with four carbs, each having as "air filter" nothing but a stretched
    piece of cloth above the throat.

    They made a lot of very sporty cars down there. Except they rusted like
    crazy, at least in the 80's.
  15. Fred Bartoli

    Fred Bartoli Guest

    Jim Thompson a écrit :
    You could have a look at spice opus. It's free and has quite a rich
    optimization algorithms set.

    It's been a while since I used it and I don't know whether they've
    included a GUI or not, so it might bring you back to the good
    editor/netlist old days, but for a few RLCs filters this is pb OK.
    (my spice being a 3F5-Xspice based one I could use it straight with some
    hand written added commands for the spice opus unsupported specific

  16. The Dishal procedure is described in
    "Randall W. Rhea: HF filter design and computer simulation"
    Mc Graw-Hill ISBN 0-07-052055-0
    pages 392-394
    Everyone doing practical filter design should have this book.

    Rhea has also a good oscillator design book and is
    the author of that what evolved to Eagleware / Agilent Genesys.
    (has nothing to do with the Eagle layout program)
    Genesys also has a nice filter design plugin.

    regards, Gerhard
  17. JosephKK

    JosephKK Guest

    Here is a deal, do it for 1 kHz and 5 kHz instead and let others scale it.
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