Using a spectrum analyzer to find LC resonance?

  1. Hi everyone,

    I am using a signal generator in series with a RLC circuit, generating an AC signal at 1kHz.
    I have my probes connected across the resistor

    When I change the timescale the Fres bandwidth changes on the spectrum analyzer.
    At 1mS the resonant bandwidth is from 25kHz-50kHz
    At 2mS the resonant bandwidth is 13kHz-24kHz
    At 5mS the resonant bandwidth is 4.9-9kHz

    Can anyone here explain why the resonant bandwidth is changing when I change the timescale on the spectrum analyzer? And how I can find the actual resonant frequency of the circuit?

    Am I doing something wrong here?
  2. jcsd
  3. Integral

    Integral 7,288
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    You need to give us some information about the spectrum analyzer you are using. Brand and model number at a minimum. Then maybe someone here could read the manual for you.
  4. davenn

    davenn 4,356
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    you sure its a spectrum analyser ?? not an oscilloscope ?
    or is it a combo and you are using the O'scope function?

    spectrum analysers have freq on the X axis ( Not Time) and power on the Y axis

    As Intergral said .... what make and model is it ?

  5. It's a propscope usb oscilloscope with a spectrum analyzer.
    Last edited: Apr 9, 2013
  6. NascentOxygen

    Staff: Mentor

    What values of R, L, and C are you using?
  7. vk6kro

    vk6kro 4,058
    Science Advisor

    If you connect the R, L and C in series, with the L connected to ground at one end, resonance is achieved when maximum voltage occurs across the inductor.

    So, you vary the frequency to get maximum output on the spectrum analyser connected across the inductor.. Then read the frequency of the signal generator.

    If you can't find resonance, connect a 50 ohm resistor (or similar) across the signal generator output.
  8. My main question:
    Why is the resonant bandwidth changing when I change the timescale?

    Is there a way to calculate fres based on the what the timescale is set at?
  9. vk6kro

    vk6kro 4,058
    Science Advisor

    It sounds like you are using the device in oscilloscope mode.
    A spectrum analyser has frequency as the horizontal scale and amplitude as the vertical scale.

    Try feeding the signal generator output directly into the spectrum analyser.
    You should see a vertical line moving horizontally depending on the frequency.

    If you can't see this, check the user manual to find out how to use the device.
  10. sophiecentaur

    sophiecentaur 14,715
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    Spectrum analysers need to be read with care.
    When you change the timescale on a spectrum analyser ( the scan rate would be a better term) the bandwidth of the receive filter is often changed automatically (the detector needs to be able to follow changes fast enough as the scan progresses). If the filter you are measuring has a narrow bandwidth, it is quite possible that the width of the peak you are seeing is a product of this plus the receiver bandwidth. If you use a very slow scan rate, the receiver filter will be at its narrowest and the narrow peak you can see is more likely to be representative of the filter under test.
    If you have enough controls on your analyser, you should be able to adjust the receiver bandwidth manually (a warning flag "uncal" may show up) and see the same effect with a constant scan speed. It's worth a try, in any case.

    Having read this, I realise my comments are aimed primarily at an analogue analyser (my main experience is with UHF and above) but there may well be an equivalent problem with sample rates and the resulting FFT .
  11. Ok,

    I connected the spectrum analyzer directly to my signal gen as vk6kro suggested. Now I can clearly see that the spectrum analyzer is working. Thank you vk6kro and others for you comments and help thus far.

    This also helped me realize all the signals I have been seeing are just noise:( which is extremely frustrating.

    In the attached picture you can see the 1kHz peak from the signal generator, and then a ton of noise from 25kHz to nearly 50kHz?

    So, can anyone tell me what might be causing all this noise and how I might get rid of it.
    Also, this noise is what is changing frequency range when I chane the timescale....I'm wonding if my oscope is just screwed up?

    Attached Files:

  12. davenn

    davenn 4,356
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    and what is worse is that all that noise is substantially stronger in level that your 1kHz signal
    which should be the strongest signal there

    OK you need to determine the source of the noise, either where its being generated or getting into the circuit from an external source.

    disconnect the sig gen and LC circuit from the scope/analyser ... is the noise still there ?
    If it is, it could be artifacts generated by the scope hardware or software OR it could be noise generated by the computer getting into the scope hardware ( computers are VERY noisey electronics)
    if there is no noise spikes visible, then proceed ....

    Connect the LC circuit only, does the noise reappear ?
    if so, maybe the wiring is picking up radiated noise from something around your workshop
    if there is no noise spikes visible, then proceed ....

    finally test the signal generator output, connect the sig gen directly to the scope.
    are there noise spikes or a clean 1kHz spike ?

    go through that for a start and let us know the results :)

  13. Dave,

    When I turn off the signal generator the noise is still there. ...also, when I do not have the probes connected to anything the noise is still there.

    I think your right about it likely being the computer. I will try to move the computer away as far as the usb cord will allow (about 2 ft) and see if the noise is still there. If it is I can work around it, but it is very frustrating.

    Is there anything I could do to remove the noise if it's from the computer?
  14. davenn

    davenn 4,356
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    Hopefully this is the unit you have ....

    Possibly ...
    You could try a clamp-on ferrite choke on the USB lead right where it enters the computer
    that may emphasis on the word MAY :) cut the noise level down.
    depending on the length of that USB lead you may be able to wrap a couple of turns through the ferrite choke.

    clamp-on ferrite choke ....



    Attached Files:

  15. vk6kro

    vk6kro 4,058
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    It might be computer noise coming down the USB cable or pickup from a noisy broadband modem, but this should be showing on the sine wave shown as part of the screen grab.

    This looks quite clean.

    So, it may be a faulty device, perhaps becoming unstable.

    Could you take it back to where you bought it and get the supplier to demonstrate that it is not faulty?

    If this isn't possible, it is still a useful device and you could just live with the noise problem. Real Spectrum Analysers cost hundreds or thousands of dollars, so you couldn't expect the same results with a USB device.
  16. OK, Thanks everyone for the help and support.

    I will try the ferrite choke.

    BTW, my wireless router is only 2 feet away from my setup so I'll also try moving further away from it.

    If those things don't do the trick I'm not to worried. I'm still pretty happy with the oscope.
  17. sophiecentaur

    sophiecentaur 14,715
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    If I understand correctly, the 1kHz signal is on the top of your graph and the result of the FFT is on the bottom. This is very suspicious because the FFT is just some maths, done on the original time-domain signal. If that signal is clean (and it appears to be), then a simple FFT will not generate anything spurious like the other picture shows. If 'noise' is visible on the frequency plot then it should also be very visible on the time plot.

    But I have another problem with what you say. You seem to imply that putting a 1kHz signal into your filter will show its frequency characteristic. This is not true. If there is only energy at 1kHz then there will be no information about the filter response elsewhere. You need, either a swept frequency (tracking) input or an impulse if you want to see the response of a filter.

    I agree with vk6kro and suggest that you read the manual to make sure you are doing what you think you are doing with the instrument.
  18. AlephZero

    AlephZero 7,248
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    FWIW, two of the USB devices I use came with their own cables, which have something that looks similar to a choke built into the cable. (And it's at the device end, not at the computer end - the connectors at each end of the cable are different so you can't reverse it).

    Maybe your device is supposed to use a similar cable, but it has got "lost" among the rest of the lab equipment?
  19. jim hardy

    jim hardy 5,460
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    The truth about computer produced electrical noise is leaking out !
  20. sophiecentaur

    sophiecentaur 14,715
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    They are a real nuisance for lots of receiving equipment. You need an antenna up in the attic or on the roof; a hand held receiver gets all sorts of old rubbish from the computers.

    But is still don't see why the spectrum of that nice looking sine wave has so much apparent HF shash on it, according to the 'analyser mode'. That scope trace should, by rights, be a mush unless there's some extensive LP filtering of the signal when in 'scope mode'. We need to know more details about what those display pictures actually represent.
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