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Techniques for creating a 1GHz signal

Discussion in 'Electronic Design' started by mook johnson, Feb 10, 2013.

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  1. mook johnson

    mook johnson Guest

    Gents,

    I may have toe venture from my cozy home in the sum 10MHz range to the
    high frequency RF stuff on the hundreds of MHz to low GHz range.

    First order of business is a 1GHz signwave generator.

    What are the common techniques for generating something that high?

    DDS? Hartley type oscillator, Crystal with upper harmonic filter to
    isolate the 1GHz band. I'm thinking this will be the input to some
    buffer so the output can hump some power. ~1W range into 50 ohms.

    No detailed specs at this point just trying to get a head start on some
    common techniques before the project kicks off.
     
  2. mook johnson

    mook johnson Guest

    I forgot to mention that this needs to be a circuit on a PCBA not a
    piece of test equipment.
     
  3. Jamie

    Jamie Guest

    Have you looked at Gilbert cells into a high pass filter.


    Jamie
     
  4. Nico Coesel

    Nico Coesel Guest

    A very easy way is an Analog devices ADF4350 and a filter. Not cheap
    though...
     
  5. Guest

    maybe something like this:
    http://www.analog.com/en/rfif-components/rfif-transceivers/adf7012/products/product.html


    -Lasse
     
  6. Tim Williams

    Tim Williams Guest

    Does that mean GHz amplifier design is obvious?

    At most frequencies, amplifiers oscillate and oscillators merely amplify!

    Tim
     
  7. Tim Williams

    Tim Williams Guest

    Just to play?

    First of all, do you have the equipment to see what you're doing? A scope
    with 1GHz bandwidth is an obvious starting point; if you hope to see any
    harmonics, 5G, 10G or more would be handy. Big $$$, but the most
    familiar relative to time-domain slow stuff.

    Specturm analyzer is much cheaper, even out past 10G or so, but doesn't
    show time domain activity as well. To do that, you need to infer things
    from sample period (e.g., 60Hz ripple modulating an amplifier, causing
    amplitude modulation, only noticeable at certain refresh rates), sidebands
    and so on. Everything's sideways-upside-down in the frequency domain.
    But it's quite helpful for radio-ey things, where sidebands and harmonics
    are important and easy to pick out.

    You can plod along with, say, an RF voltage probe, but you don't know
    anything about what's happening, just that it's there or not.

    Second, you need parts to do it. MMICs are one thing. BJTs come in that
    range, as do [Si] JFETs, MOS and the fancier PHEMTs and stuff. Offhand, I
    don't have any stuff that goes that high, other than parts I've salvaged
    that I can't find full datasheets for, if anything at all. Some good
    starting points are BFQxxx's and such, many of which have been mentioned
    here before; others can volunteer their favorite HF jellybeans.

    Finally, you need the understanding. Have you played with radio style
    circuits before, and got a grasp of impedance matching and transformation
    and inductors and capacitors and filter design and...? It's easy to see
    the analogy between an inductor and a thin trace, and a capacitor and a
    thick trace; it's only another step to build a GHz oscillator with 1/4
    wave stubs, or a filter with various segments stacked up. Of course, it's
    another to design one from first principles and have it work correctly
    (desired frequency, bandwidth and ripple, etc.), and it would be a whole
    thing further to actually build one from copper clad with an X-Acto blade!
    But as with lumped filters, there are design tools for that (ranging from
    simple calculators to free programs to kilobuck design suites). So if you
    already have a feeling for this sort of stuff, go for it. If you don't,
    you might plan some more projects in the VHF range, where inductors and
    capacitors are still reasonably inductive and capacitive, respectively,
    keeping in mind your experience is directly applicable.

    Some projects are pretty simple, too, even with whatever considerations.
    A pulsed oscillator and wideband detector can be used for very crude
    radar, and it doesn't really even matter what frequency the oscillator
    runs at. If it's making an RF signal, you can be reasonably assured it's
    doing it at whatever GHz your stubs and stuff are tuned for. Zap it off
    an antenna and watch the return blip. Stuff like this was done back in
    the days when scopes were barely pushing 10MHz, and GHz was generated with
    freaky vacuum tubes, like klystrons, backward-wave oscillators and
    magnetrons.

    Tim
     
  8. Guest

    Stability requirements ? Free running or locked to some reference ?

    Tuning range ?

    In the old days for a narrow frequency range a string of 2x and 3x
    multiplier stages with a resonator between each stage. For a larger
    tuning range push pull frequency doublers, which did not need much
    filtering between stages.

    These days apparently the simplest would be use a VCO in some PLL
    construction. Fixed or variable modulus dividers are available to
    several GHz. Alternatively, use some mixer system to reduce the VCO
    frequency to some lower frequency and use some low frequency divider
    or lock the difference frequency to some lower frequency variable
    signal source.

    Of course, one must be careful with topology and loop filter design,
    in order to keep the phase noise at acceptable levels.

    The problem with ordinary DDS design is that you would need to clock
    it above 3 GHz to generate 1 GHz. This would consume a lot of power
    and most likely would be quite expensive. There are some alternative
    ideas of using a VCO at 1 GHz feeding the _clock_ input of the DDS and
    program the chip to produce 1 MHz and compare this to a 1 MHz crystal
    and use the feedback to control the VCO so that the DDS produces 1
    MHz.
    Sounds like some wave analyzers used decades ago. Those used some
    signal sources that produced a lot of harmonics (such as a step
    recovery diode), mix it down with a VFO to some low crystal filter
    frequency (say 10 MHz) and then mix the filtered signal back to
    original frequency using the same VFO, thus the VFO drift was
    canceled. By tuning the VFO, you could select which harmonic you
    needed.
     
  9. Owen Roberts

    Owen Roberts Guest

    50-2000 Mhz is fairly easy with stock VCOs and TI, Fujitsu or AD
    PLLs.

    AD products often come with the VCO on Chip.

    Depending on what your doing, the most important beginner's tools
    for learning basic PLL are probablty:
    1. A decent frequency counter.
    2. RF power meter, Even the low cost ones.
    3. Spec-An.
    4. Oscilloscope of course, for the loop filter.
    5. Older PLL appnotes such as MC145151, MC145170 etc where they give
    good, simple, examples.

    I'm going to disagree with Tim on the scope bandwidth, for many
    applications 10 or 100 Mhz is fine.

    The rationale being the VCO power level at the prescaler is usually
    a common problem at the design stage. A frequency counter is
    inexpensive and shows lock and programming errors quickly. The Spec
    An is there for the harmonics and RF levels.

    Learning to make microstrip with free tools like APPCAD, and adding
    gain with MMICs helps.

    I like the TI PLLatinum parts for the fact they often bring the
    prescaler count out on a pin.

    LMX2430 for example.

    Motorola's AN535 is widely available on the net.

    Just a few ideas,

    Steve
     
  10. Owen Roberts

    Owen Roberts Guest

  11. Nico Coesel

    Nico Coesel Guest

    But you still need a PLL and other circuitry. The ADF4350 just needs a
    reference clock, a few passive parts and a small controller to set the
    registers and it produces about any frequency between 135MHz and
    4.4GHz. There is an improved version as well which goes as low as
    (IIRC) 38MHz.
     
  12. Nico Coesel

    Nico Coesel Guest

  13. Owen Roberts

    Owen Roberts Guest

    I did get two samples of those. Wonderful part. Very tiny package.

    Steve
     
  14. Mark

    Mark Guest

    Tom, I have three words of advice that may help you....

    "ZERO SPAN MODE"

    Mark
     
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