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How to view f spectrum?

Discussion in 'Electronic Basics' started by DaveC, Dec 22, 2003.

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  1. DaveC

    DaveC Guest

    I want to view a frequency spectrum, displayed such that I can see the center
    f of a resonant circuit.

    What do I need to do this? I have a sweep generator and oscilloscope. Is this
    sufficient to the task? Or does this require a spectrum analyzer?

  2. It requires the spectrum analyzer.

    Kevin Aylward
    SuperSpice, a very affordable Mixed-Mode
    Windows Simulator with Schematic Capture,
    Waveform Display, FFT's and Filter Design.
  3. mike

    mike Guest

    I must have misinterpreted the question.
    If you have a ramp out to drive the X of your
    oscilloscope, you should be able to see the response
    of the resonant circuit on the Y-axis.
    If the frequency is too high for the BW of your scope,
    you'll need a detector.

    Spectrum analyzer has this built in, even better with a
    tracking generator, but shouldn't be necessary for this task.

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  4. Don Taylor

    Don Taylor Guest

    So what do you see if
    your circuit doesn't have any active components,
    (so you should only see an attenuated version of input at the output)
    you hook the output of your sweep generator to the input of your circuit
    you hook the ramp signal output of your sweep generator to your scope X input
    you hook the output of your circuit to your scope Y input
    you set the timebase of the scope to let your ramp complete nicely
    or get it into XY mode, depending on what your scope has.

    Roughly what this is doing is generating a lissajous pattern on the scope,
    the ramp/sweep frequency being far lower than the frequency that you are
    sweeping. If your scope is up to it this might do what you are looking for.

    Start out simple and see if you can make this work.
  5. Ed Price

    Ed Price Guest

    To do the job slickly, yes. But bare-bones, no.

    OK, let's assume the "resonant circuit" you ask about is a simple LC
    parallel circuit, sometimes called a "tank" circuit. You connect the signal
    source, which has a 50 ohm source impedance, to the LC network. Remember
    that, at resonance, a parallel LC circuit will exhibit a high impedance.

    If you have a sweep generator that has a DC output proportional to
    frequency, then just connect the analog voltage to the oscilloscope
    horizontal deflection. Connect a voltage probe across the LC network, and
    connect that to the vertical deflection. As you sweep the frequency range,
    you will see a peak at the resonant frequency.

    No signal gen analog voltage capability? Why not just turn the generator
    frequency control by hand until you see a voltage peak, then just read the
    sig gen dial?

    If you were talking about a more complicated "resonant circuit" like
    possibly an IF strip, you could again monitor the strip output while you
    slew the sig gen frequency, automatically or manually. Max output will be at
    resonance (neglecting such things as stagger-tuned elements).

    Now, if you question is more of an operational than design nature, then you
    may be trying to say that you want to view this segment of spectrum to
    observe signals passing through a resonant filter. Then, I'd agree with
    Kevin; you need an SA.

  6. Bob Masta

    Bob Masta Guest

    If you are working at audio frequencies, you may be able to
    use spectrum analyzer software with your computer's
    sound card.

    Shameless plug: If your system can run real-mode DOS
    (Win9x or earlier) and has an ISA-bus Sound Blaster
    (or one of a few others, including a simple home-brew
    that uses your printer port), then you may want to
    check out my Daqarta package. Win32 version is
    in development.

    Bob Masta

    D A Q A R T A
    Data AcQuisition And Real-Time Analysis
  7. dB

    dB Guest

    If you lightly couple both the generator and the 'scope to the circuit
    through low value capacitors and sync the 'scope from the generator
    you should be able to get useful results.
  8. Depending on the requirements, it can also be done with a
    digitizer and software to do the Forier transform of the captured
  9. DaveC

    DaveC Guest

    Audio frequencies, yes. No active components; LC only.

    Scope (Tek 2465 DMS) has Z-axis input at the rear panel, but no X-axis input.

    Does this kill the possibility of simple display of spectrum?

  10. Don Taylor

    Don Taylor Guest

    Excellent, just about what I was guessing from your original posting.
    1: You don't need the Z-axis input on this. That is for driving the
    brightness in relation to your input signals, don't need that.
    What you want is an external X input, usually the scope generates
    this by itself, with the time base controlling how fast that goes.
    Sometimes it isn't obvious how to use one of your signals to drive X.
    2: You have a couple of input channels in your 2465, that will do fine.

    See if you can grope around the net or find an example or some kind
    individual to get you a button-by-button description of how to draw
    lissajous patterns on your scope. THese are the "loopy" patterns
    where if you have a 3 Hz sine wave and a 2 Hz sine wave it will make
    up a pattern with 3 lobes in one direction and 2 lobes in the other.
    Asking for specific instructions for your scope and this display should
    get you at least one good helpful answer somewhere. It has been too
    long for me to be able to tell you how to do this step by step, and
    I wasn't on the 2465's.

    For your nice audio no-active-components want-to-see-frequency-response
    you are looking for a ramp or sawtooth output from your sweep generator.
    That is going to drive the horizontal part of your display. And your
    substantially higher audio signal that is sweeping upwards is going to
    drive your vertical part of the display. Actually it is going to be
    bouncing up and down wildly but it will be scribbling on top of itself.
    The amplitude of the frequency coming out of your circuit is going to
    look like a bright green vertical line, the height of that line is the
    frequency response you are wanting to see as you sweep across your
    filter range.

    I hope this helps. Toss me mail if it works out or if you can't figure
    out how to do this and I'll try to help.

    Email address is valid, been "dont" on the net since before there was spam!
  11. DaveC

    DaveC Guest

    OK. Found it in the User's Guide:
    "Full counterclockwise rotation of the SEC/DIV switch selects the X-Y display
    feature. In X-Y, the "CH2 OR X" input drives the horizontal deflection
    That is plainly labelled on the sig gen.
    Thanks. I look forward to the phosphorous pyrotechnics!
  12. Ed Price

    Ed Price Guest


    I think you should look a bit more closely at the time-base knob of your
    2465. Mine sure has an X-axis front-panel capability.

  13. CWatters

    CWatters Guest

    There are some programs you can get that might do this using a PC sound
  14. Spajky®

    Spajky® Guest

    Is there another "name" also for serial resonant LC circuit or is just
    called serial "tank" circ.?

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  15. ddwyer

    ddwyer Guest

    A spectrum analyser may be an overkill.
    As this is an audio application the simplest approach would be to drive
    a variable frequency into the network and manually sweep the frequency,
    to observe the amplitude at the other side of the network by means of an
    oscilloscope or ac voltmeter.
    Use the scope to estimate the frequency and amplitude.
    If the measurement is to be repetitive then use the X output from the
    baxk of the scope to drive the frequency .
  16. DaveC

    DaveC Guest

    I got it all hooked up: ramp output from sig gen connected to X (Ch 2) scope
    input. Sig sweep output drives RC circuit and is connected to Y (Ch 1) scope

    Not clear that this is the waveform I was expecting; no clear peaks, quite
    broad spectrum with high amplitude.

    One thing is weird; the ramp frequency is only 15 hz. Shouldn't this be the
    same frequency as the sweep?

  17. Ed Price

    Ed Price Guest


    Your terminology is a bit fuzzy; I don't know what you are describing when
    you talk about a "broad spectrum."

    Not all signal sweepers work alike. Some will scan from frequency A to
    frequency B, and then halt. Some will stop at B, then reset to A. You appear
    to have the sweep rate set to "continuous", so the sweeper starts it's sweep
    and ramp at the same time. At the end of the sweep, the sweeper resets to
    frequency A, resets the ramp voltage accordingly, and begins a new sweep.
    Unfortunately, there's often no definition of the time needed to reset.
    Also, some sweeper have a little pause or hesitation as they begin their
    sweep. The sweep frequency may be quite a bit slower than the sweep rate
    during the sweep.

    Finally, the sharpness of the peak (at resonance) will be affected greatly
    by the Q of the circuit. You may be slipping past the peak, without noticing
    it, because it's so narrow. Try a slower sweep rate, or a narrower frequency
    span width.

  18. DaveC

    DaveC Guest

    Thanks, Ed. I'll try your suggestions.

    Happy Holidays,
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