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Oscilloscopes and spectrum analyzers

  1. Oct 5, 2007 #1
    I know both can graph waveforms but I'm a bit confused about what spectrum analyzer graphs tell you.

    I read an explanation that says
    http://www.andrew.cmu.edu/user/ebuehl/robosapien-lirc/oscilloscope.jpg [Broken]

    That I understand but the explanation of spectrum analyzers is a bit more complicated


    For the oscilloscope the graph shows 1 frequency, 1 cycle, 1 period. It shows the exact magnitude of the wave at every point in time throughout the cycle so obviously its magnitude over time.

    When they say magnitude over frequency what exactly do they mean? If you were only measuring 1 frequency like you were with the oscilloscope would only 1 line appear? Are spectrum analyzers only used for observing the magnitude of multiple frequencies at once?
    Last edited by a moderator: May 3, 2017
  2. jcsd
  3. Oct 5, 2007 #2
    SA (spectrum analyzers) display frequency components with the power level (usually in dBm). This means if you have a signal thats 10MHz, you would see a 10MHz sine wave on the scope in time domain. But with a SA, you would see a delta function at f = 10MHz along with some power level. I take it you haven't learned Fourier Transforms. Basically the SA will show you what the fourier transform of a signal.
    Last edited: Oct 5, 2007
  4. Oct 5, 2007 #3
    SA shows power(signal strength) vs frequency sweep over time
    (ie: you can see power level for 900MHz, 2.4GHz , 2.45GHz etc)

    oscilloscope shows (magnitude vs just 1 frequency)
    (ie: you can see magnitude for 900MHz, to see 2.4GHz, you need to re-tune to pick up that signal)
  5. Oct 6, 2007 #4
    Thanks for the replies. That explains a good bit but I'm still not sure if I understand SA charts. I saw a diagram on an e-book, I can't copy and paste it but it displayed a single line at 60Hz which reached an amplitude of 6. How does the "amplitude" on this kind of graph correspond to voltage?

    Does this chart tell me that the AC wave reaches extremes of +6V and -6V and repeats itself 60 times per second?

    If I had a more complex chart like this
    is this graph displaying multiple waves at different frequencies and amplitudes or single waves with multiple frequencies? I've only started getting into the details about AC waves and the idea of waves with multiple frequencies is confusing me. If its an AC wave it has to have a definite repeating pattern so wouldn't they just measure it from one end of the repeating pattern to the other and that be its frequency?

    Another thing thats confusing me about AC waves is how frequency is the way to identify a wave. When I started reading about it they made it out that it was the frequency of the wave alone that made it unique but obviously you can have 2 waves that repeat themselves the same amount of times per second but have different amplitudes so the two waves are different. Am I missing something here?
    Last edited: Oct 6, 2007
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