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How to measure RF carriers

Discussion in 'Electronic Design' started by Klem, Sep 22, 2006.

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  1. Klem

    Klem Guest

    Hello Everyone,

    An Engineer at my work told me that I should use an oscilloscope to
    measure RF signals (60-400MHz)

    I have never heard of such thing other then back in the days of tubes.
    Our current designs consist of LNA's and Rf amps with any of their
    specs in dB, I have always done troubleshooting/testing with means of
    a Spectrum analyzer and a powered probe.

    What are your ideas on this?

    I appreciate any comments,

  2. Joerg

    Joerg Guest

    Hello Klem,

    A spectrum analyzer is the usual instrument here. Unless you are chasing
    non-linearities in large amplifiers.
  3. John Larkin

    John Larkin Guest

    We're designing some wideband RF power amps, and we plan to do all the
    testing with sampling oscilloscopes. We're concerned with time-domain
    stuff like phase shift versus time, rf envelope amplitude droop over
    long bursts, things that would be hard to measure in the frequency

  4. Joerg

    Joerg Guest

    Hello John,
    But AFAIR you guys have some >10GHz scopes. The stuff with a high drool
    factor among RF engineers and where you have to talk to the bank before
    signing the P.O. Although, sometimes one can luck out. When I bid on an
    older HP 1GHz scope I was totally surprised when they called me to pay
    and pick it up. Turns out there was only one bid ;-)
  5. EE123

    EE123 Guest

    What is the carrier frequemcy that you
    are working at?
    RF can mean a lot of different
    things to different people.

  6. Phil Allison

    Phil Allison Guest

    ** Groper alert !

    ** See the numbers in brackets ??

    ........ Phil
  7. Genome

    Genome Guest

    Yes!!! Radio 2 used to be on the Long Wave at 200KHz so that makes no sense
    at all.

    I would be inclined to ask the Engineer how he does it. Either he will tell
    you or look fairly stupid.

  8. John  Larkin

    John Larkin Guest

    A Tek 11801 or equivalent mainframe is usually under $1K on ebay, less
    if you know a trick. 12 or 20 GHz dual-channel sampling heads are
    typically about $700 or so, although TDR runs more. They are
    phenomenally precise instruments.

    I have 8 or 10 of the mainframes by now, and maybe 20 heads, $600k or
    so at last catalog price!

    A guy could equip a basement lab with everything needed to go into the
    picosecond products business for the price of a used Harley.

  9. Tom Bruhns

    Tom Bruhns Guest

    John Larkin wrote:
    Things like that don't seem difficult at all to measure with modern
    frequency domain instruments like we build here. I don't know that
    you'd find them on ebay quite as cheap as the scopes you mentioned in a
    followup posting, though.

  10. Joerg

    Joerg Guest

    Hello Tom,
    Phase shift versus time can be tough (meaning expensive) in frequency
    domain if you are looking at just a couple degrees or so. But for droop
    you are right, we measure that with spectrum analyzers. That stuff is a
    serious concern when designing pulsed Dopplers for ultrasound. Spectrum
    analyzers just don't like to trigger that well, mostly they provide a
    BNC jack in the back for this and you have to concoct some circuitry to
    drive it (but only once).
  11. vasile

    vasile Guest

    Your engineer knows what it said. If you have no money for a spectrum
    analyzer then a good oscilloscope is the next affordable tool for that.
    Knowing the frequency is not enough if you are his boss and you don't
    know to answer him why you'll not buy one.
    You must also ask him which would be the amplitude levels and if will
    be much happy if the scope will also have a good FFT and an analogic
    bandwidtth twice than 400MHz.

  12. Tom Bruhns

    Tom Bruhns Guest

    Well, like I said, it doesn't seem all that difficult with modern
    frequency domain instruments like we build here. They can trigger on
    all sorts of things, including things that scopes can have lots of
    difficulty with--like triggering on energy above a particular level in
    a particular frequency band. If you calibrate out cable lengths
    properly, you can get phase accuracy to a fraction of a degree. How,
    with a scope, do you detect signals at -90dBc and determine that they
    are or are not harmonically related to the fundamental? How do you
    analyze phase noise versus frequency with a scope? How do you plot a
    color map of amplitude versus frequency versus time with a scope, and
    then display a trace of amplitude versus time for any particular
    frequency? I'm not aware of any scopes that digitize with the required
    spurious free dynamic range to do that as well as modern spectral
    analyzers, and I don't believe they come with software that does as
    good a job as what you get with spectral analyzers for frequency-domain
    applications. On the other hand, if you need really wide instantaneous
    bandwidth (perhaps greater than 100MHz), you probably ARE better off
    with a scope--for now.

    In general, I'd say that if you are analyzing a signal and want to know
    its time-domain behaviour, like rise time, overshoot and the like,
    you're better off with a scope; but if you want frequency domain
    information like phase, harmonic content, accurate frequency
    information, and spurious versus frequency, and even the time
    progression of any of those things, you are better off with the proper
    frequency domain analyzer.

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