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Oscilloscope - Frequency Vs Time Graph

  1. Sep 13, 2011 #1
    Heya Everyone

    Ok so my question may seem a little silly but nevertheless I will ask. I need to know what you experienced engineers think about it.

    I have a series LC circuit (on breadboard) - connected to a function generator ( 2V p-p, 1kHz) - I will connect an oscilloscope across the inductor - Is there a way to see the change in frequency with time on oscilloscope screen [Given that frequency of the inductor WILL change due to some external factor ] ?????

    So on y-axis I would like to see the frequency across the inductor and on x-axis the time. Is it possible to do so? I know I can see the voltage across the inductor and then MANUALLY compute the frequency across it, or some oscilloscope show the frequency in a tiny box on the right side of the screen. But what I want is a real time frequency vs time graph. So if the frequency is the same for 10 sec-> I get a straight line in my graph for 10 sec. Is it possible to do so???

    So far I don't have any equipments in my lab, so all my ideas are in my head. There is no way to test what I feel will work or not.

    Any Ideas/suggestion???
  2. jcsd
  3. Sep 13, 2011 #2


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    Staff: Mentor

    You say that your signal generator is set to 1kHz. That will be the only frequency in the circuit, no?
  4. Sep 13, 2011 #3


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    An oscope shows you how a signal changes with time. If you connect across the inductor you will see a sinusoidal wave form, the time between the peaks of this wave form will be the frequecny you want. Most modern digtial scopes have a meausment menu which will do this for you. On older analog scopes you must measure the time yourself and do the arithmetic yourself.

    All a scope can show you is how voltage changes with time.
  5. Sep 13, 2011 #4
    Good evening

    Perhaps if you were to explain why you think there would be a change with time and what would be the agent we might be able to help more.

    It sounds as if you want to use the scope as an XY plotter with time on the X axis and Frequency on the Y axis.

    To achieve this you must convert the frequency of the sampled signal to a varying voltage.

    Such voltage to frequency :blushing: edit: frequency to voltage circuits do exist but more detail is required.

    go well
    Last edited: Sep 14, 2011
  6. Sep 13, 2011 #5
    Thanks for such quick replies :)

    Ok I will explain in detail the purpose of this experiment. The LC circuit I made consists of a known capacitor ( 1nF) and an inductive loop (made by 3 turns of copper wire). I will make the circuit oscillate at its resonant frequency. Now when the circuit is all set up and oscillating at its resonant frequency - I will bring a conductor ( metal plate) close to the loop.

    From my understanding the loop should induce eddy currents into the surface of the conductor , thereby decreasing its own inductance. This decrease in inductance will cause the resonant frequency of the circuit to change ( increase I believe ). This is what I have to measure . Accurate measurement of frequency change is very critical for me.
  7. Sep 13, 2011 #6

    jim hardy

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    try reading up on this technique

    phase locked loop locked to your tank circuit
    VCO pin of PLL will change with frequency

    thatre's a better article in the old Signetics PLL applications book

    and scores of do-it-yourself metal detector websites
  8. Sep 13, 2011 #7


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    Some metal detectors use high frequency oscillators which change frequency when metal is brought near the coil. Frequencies in the range 100 KHz to 2 MHz are used.

    Since we can't hear such frequencies, there are methods used to allow us to hear the result or see it on a meter.

    One is the BFO technique where a stable oscillator is set close the the same frequency as the variable one and you hear an audio frequency ouput when the two are mixed together.

    Another is to set the variable frequency oscillator to the resonant frequency of a ceramic oscillator. Then the variable oscillator output is applied across the ceramic resonator via a series resistor.
    If the frequency changes, the output across the ceramic resonator changes and this is shown on a meter.

    Here is a circuit of such a metal detector:
  9. Sep 13, 2011 #8
    Thanks guys !

    I am not an electrical engineer so I always choose a way with simpler circuits :tongue: That PLL and that metal detector circuit made me :uhh: But sometimes I have no way but to get my hands dirty with EEE stuff :tongue2:

    I found an example on using matlab to get scope's frequency! Here's the link

    http://www.mathworks.com/help/toolbox/systemtest/bqpmajx.html [Broken]
    Last edited by a moderator: May 5, 2017
  10. Sep 13, 2011 #9


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    Those circuits may make sense if you understand the need for them.

    If you have an oscillator giving 400 KHz and you bring a small piece of metal near it, it may shift in frequency by 500 Hz or so.
    You could watch it on an oscilloscope, but you would not be able to notice the difference in frequency, because it is small compared with the 400 KHz.

    So, if we mix the 400 KHz signal with another 400 KHz signal and then bring the small piece of metal near the first oscillator coil, it will change in frequency but the other oscillator will stay on 400 KHz, so then we will hear the difference between the two signals (500 Hz) as a 500 Hz sound from the mixer in headphones.
    If you bring the metal closer, the sound may change to 1000 Hz, which would be an obvious change if you are listening to it, but on an oscilloscope, you would not notice that a change had happened.
  11. Sep 14, 2011 #10
    :approve: Absolutely Correct ! I looked up on that metal detector circuit and now I understand its working !
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