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Thread: Hi Don

  1. #1
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    Lightbulb Hi Don

    Would you be able to furnish the basic distinctions between signal analyzers, dynamic signal analyzers, spectrum analyzers, dynamic spectrum analyzers, network analyzers, and vector analyzers?

    I expect some of the difference pertains to proprietary design and marketing. Thanks for your consideration, Don
    Vbr,
    Sam
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  2. #2
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    I haven't really used "dynamic" analyzers in the audio sense as my work is much higher speed. However, I can define what some of the basic instruments do... Let me think on this a bit. For now, I need to toddle off and practice.
    Don Herman
    "After silence, that which best expresses the inexpressible, is music" - Aldous Huxley

  3. #3
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    Quote Originally Posted by DonH50 View Post
    I haven't really used "dynamic" analyzers in the audio sense as my work is much higher speed. However, I can define what some of the basic instruments do... Let me think on this a bit. For now, I need to toddle off and practice.
    Thanks, Don. I'm gonna mess around with a precision digital caliper...going to measure reel flange thickness -- no, REALLY! Hehe ;-)
    Vbr,
    Sam
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  4. #4
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    Forgot about this one, sorry... Here's a quick off-the-cuff guess:

    Network analyzers are used to measure the frequency response of networks. They do not intrinsically operate in the time domain, but many include an inverse-FFT function that provides a calculated time response to a step or impulse input. Think of a box with two ports, but you can't see inside. You apply a signal to one port and measure at the other, and repeat in the other direction (most do the switching automatically to measure both ports). The network analyzer measures the frequency response, the transfer function, from one port to the other by sweeping the signal frequency at one port and measuring the resulting signal at the other. From that you may infer some of the properties of the box. Thus, a network analyzer includes a source (sweep generator) and receiver that are synchronized to provide the response at each frequency point in the sweep. Simple ones measure the magnitude only; more complex models measure magnitude and phase and thus provide vector information.

    A spectrum analyzer measures the frequency spectrum of an input signal and is what one uses to determine discrete frequency components in a signal. A spectrum analyzer does not include a source. It can be used to measure the signal-to-noise ratio (SNR) by summing all the frequency components except the fundamental (test tone) and comparing the magnitude of the sum to the magnitude of the test tone. Similarly, it is used to provide the distortion (THD, IMD) of a test tone by comparing the magnitude of the fundamental to the sum of all the distortion components. (Not that the “sum” is actually a root-sum-square, the square root of the sum of all tones squared.) You could make a network analyzer by synchronizing a sweep generator and a spectrum analyzer.

    A signal analyzer is generally a multi-function device that measures frequency response, SNR, THD, IMD, etc. Most audio test instruments are multi-purpose devices, including a signal source to provide fixed or swept tones and a sensitive receiver that can serve in a network or spectrum analyzer role.

    Dynamic analyzers allow you to measure signals over time, allowing you to perform for example a plot of the decay in frequencies over time in a room, or plots of distortion vs. power over multiple sweeps.

    The output of a network analyzer is a frequency sweep like this:
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    The output of a spectrum analyzer looks like this:
    Name:  spectrum..JPG
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    Don Herman
    "After silence, that which best expresses the inexpressible, is music" - Aldous Huxley

  5. #5
    Site Founder And Administrator amirm's Avatar
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    Nicely done as always Don.
    Amir
    Founder, Madrona Digital Audio, Video, Home Automation
    Contributing Editor, Widescreen Review Magazine

  6. #6
    Member Sponsor microstrip's Avatar
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    Don,

    One aspect that has always bothered me is the technique used to implement most signal analyzers. We can find either instruments based in sweep, pink noise or maximum-lengthsequence (MLS) techniques. As expected, each developer claims better results using the technique he implements. Do you have a feeling on which would be more appropriate for room measurements? I am not considering loudspeaker development, that has different requirements.

  7. #7
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    Quote Originally Posted by microstrip View Post
    Do you have a feeling on which would be more appropriate for room measurements?
    I'm not Don, but for me the overwhelming advantage of sweep tones over noise is the software can apply a tracking filter to increase the s/n ratio. This is why most modern software like Room EQ Wizard and ETF etc use a sweep.

    --Ethan

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    +1 (Good to see you chime in, Ethan!) Also, swept tones are well-defined in frequency and amplitude, unlike pink noise or MLS that is statisical. Meaning you need to takes lots of measurements over time to get the details. Pink noise is great for averaging over time to get a general idea of the response and is often used with a spectrum analyzer to provide a look at the overall FR of a system. However, you cannot (ideally) predict the power (voltage, phase, etc.) of any given frequency at any given time, making it a poor choice compared to the precision of a network analyzer or other swept system. I have almost no experience with MLS systems for audio.

    For a quick look at the overall response pink noise is great. For detailed measurements and fine-tuning I use a sweeper (frequency domain) and/or (im)pulse functions (time domain).
    Don Herman
    "After silence, that which best expresses the inexpressible, is music" - Aldous Huxley

  9. #9
    Member Sponsor microstrip's Avatar
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    Thanks to both of you.

    Just a few hours ago I found in the "A Sound Engineers Guide to Audio Test and Measurement " the following sentence "The advantage of the MLS is its excellent noise immunity and fast measurement time, making it a favorite of loudspeaker designers. A disadvantage is that the noise-like stimulus can be annoying, sometimes requiring that measurements be done after hours. The use of MLS has waned in recent years to log-swept sine measurements made on dual-channel FFT analyzers" . So, thanks also to google books!

  10. #10
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    This is a great thread so far!

    Thanx to you folks.

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