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What can be a signal sweep used form?

  1. Sep 10, 2009 #1
    So I have access to a signal generator and a spectrum analyzer and one of the SG functions is to generate a sweep. Lets say the frequency starts from 50 MHz and goes up to 100 MHz, the amplitude can also be a sweep from one value to another

    The thing is what applications can that feature have?

    What I am doing basically is getting these 2 devices to work and use them to show signal analysis concepts to Industrial Engineering students who don't have a background in signal analysis

  2. jcsd
  3. Sep 10, 2009 #2


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  4. Sep 10, 2009 #3


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    You would set the spectrum analyser to receive the frequencies of interest.
    Then, pass the signal through the device being tested and observe on the spectrum analyser to see if the device passes all frequencies equally or not.

    Signal generator...................> Device being tested....................>Spectrum analyser.

    For example if you got a piece of coaxial cable about 0.66 meters long and shorted one end of it, it should resonate at about 75 MHz. The open end would be a high impedance at 75 MHz.

    So if you put the signal generator then a 470 ohm resistor then the centre conductor of the coax then the shield of the the coax to ground, at 75 MHz there will be an increase in signal across the coax which you can scan by putting the input of the spectrum analyser across the open end of the coax.

    Try this before you do it in front of students, because the actual resonant frequency depends on the velocity factor of the coax. I have allowed a VF of 0.66 which is fairly typical for RF grade coax cable. Be prepared to optimize the 470 ohm resistor, too.

    Set the spectrum analyser to maximum bandwidth until you find the frequency that gives resonance and then zoom in to view it.

    You can make up various tuned circuits to get different effects. Typical tuned circuits for 75 MHz would be a 10 turn coil 1 cm diameter and a variable capacitor of about 30 pF maximum.
  5. Sep 10, 2009 #4

    You can show them the frequency response of different circuits such as low pass, and bandpass filters, or transmission line resonators. If the signal runs through a diode, then harmonics will be generated easy to see. If the signal generator is capable of square and triangle waveforms, then the spectrum analyzer will decompose the waveform into its frequency components as described mathematically in Fourier analysis.

    Hook up a wire into the spectrum analyzer, and set the reference to minimum. You will be able to pick up spikes from local radio stations, aircraft transmission, and if the SA goes high to 800 or 1900 MHz you will be able to see radio activity coming from your students cells phones.
  6. Sep 10, 2009 #5
    Hi Kiza-
    A shorted cable is a quarter-wave impedance transformer. See thumbnail.
    Bob S

    Attached Files:

  7. Sep 10, 2009 #6


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    Thanks Bob, I was hoping you might do that.

    How long was your quarter wave stub?

    Looks like my 470 ohms could be a bit higher. Maybe 2 K or so to get a sharper peak?

    I think I could find a good home for that spectrum analyser if they were throwing it out. :)
  8. Sep 11, 2009 #7
    The notation on the frequency sweep thumbnail for the transmission line is Td= 8n Z0=50, meaning 8 ns delay and 50 ohms impedance. So for beta = 2/3, the delay is 20 cm per ns, or 160 cm for something like RG-8 or RG-58. So the first peak in the frequency sweep is at 1000 MHz/(4 x 8) = 31.25 MHz.
    Bob S
  9. Sep 11, 2009 #8
    Thanks for all the responses by the way, im gona try some of your suggestions. Of course if i have any doubts i will bug you guys again :tongue:
  10. Sep 11, 2009 #9
    Ok I am working on another activity. In the signal generator I can modulate signals using AM and FM techniques

    What I dont understand is some information given on the screen when I do the modulation, and the manual assumes the user knows what it is...

    Anyway, for example I set the SG to generate a 100 MHz signal at 12 dBm

    Now I go to modulation and i input the following

    Depth/dev 90% (What is this exactly?)
    Rate 2.98 KHz (Frequency of the waveform below?)
    Form sine, square, ramp etc

    Now what I am doing here?

    Am i modulating the 100 MHz signal on the 2.98 KHz one?

  11. Sep 11, 2009 #10


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    Why are you trying to teach things that you don't understand in the first place?

    - Warren
  12. Sep 11, 2009 #11
    If you have AM modulation on an RF signal, you will hear an audible tone when you tune the signal generator across the passband of an AM radio receiver. Same for an FM modulation tuned across the passband of an FM radio receiver. The depth of AM modulation is when the peak AM modulation amplitude is 90% of the RF carrier signal.
  13. Sep 12, 2009 #12
    The thing is like this, I am an Industrial Engineering grad student and we have an Information Systems Engineering division here. Students in that area of concentration take telecommunication concepts and wireless network classes.

    Personally I am working on a project designing a prototype that works gathering data automatically, I also work in RFID

    Now, in the lab I have access to we have the spectrum analyzer and the signal generator which were donated some years ago and no one has ever used them. So my job with those devices is to see how can we use them to show concepts from the classes in a practical way
  14. Sep 12, 2009 #13
    Ok got that, thanks
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