Function Generator Applications

Discussion in 'General Electronics Chat' started by someonesdad, Jul 9, 2009.

  1. someonesdad

    someonesdad Thread Starter Senior Member

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    I am working on a document that discusses function generators and arbitrary waveform generators. It is aimed roughly at folks in technician and beginning engineering/science positions. It acknowledges the usefulness of function generators for a variety of tasks, looks at the DDS method of waveform generation (probably used in most modern equipment), and shows some typical applications.

    I'd like to ask this group of experts for some suggestions about interesting uses of these instruments. My goal is to make the examples interesting, practical, and accessible to the average reader. Here are the example applications I have either worked on so far or am considering (they generally assume the use of a scope):

    1. Troubleshooting: identifying a bad subsystem in a system by injecting a signal into the first subsystem and checking its output, moving to the next subsystem and repeating.
    2. Same as previous, but starting from the last subsystem and working backwards.
    3. Measuring the ESR of a capacitor in circuit (can use an AC voltmeter instead of the scope).
    4. Time domain reflectometry: I show scope traces of reflected pulses in a network cable.
    5. Response testing by sweeping.
    6. Response testing using square waves, pulses, and noise.
    7. How to use a power supply to change the DC offset of a function generator.
    The examples are practical and, where possible, show the results of building the indicated circuit and making the measurements shown.

    I'm aware of B&K's old function generator guidebook and I've looked at a few Agilent and Tek app notes.

    I look forward to your creative ideas!
  2. Mass

    Mass Member

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    This is more frequency related, but I used to work with a laser that could only read frequencies up to 20kHz. We were tuning oscillators in MHz, so we would need to use a generator to output the nominal frequency of the oscillator and subtract the actual oscillator frequency. We would then take the mixed frequency, now in kHz, and supply it to the laser so it could trim it to its target frequency.
  3. Bill_Marsden

    Bill_Marsden Moderator Staff Member

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    Triangle waves are the basis of linear PWM modulation, especially class D audio amps. This is very similar to the system computers use on their audio output, I believe.

    I have an ongoing thread that has been temporarly shelved, until I try some new experiments...

    http://forum.allaboutcircuits.com/showthread.php?t=12405
  4. someonesdad

    someonesdad Thread Starter Senior Member

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    I've never heard of a laser "reading" a frequency; does this mean you could modify its output by inputting a signal?

    Do I understand you to mean you used the function generator to generate some waveform which you then mixed with another signal, then used the difference between the oscillator's signal and the function generator's signal to tell the laser how it should be tuned? This implies your mixing circuit had some kind of nonlinear behavior to get the difference, right?

    Bill, you'll have to educate me some more. What function generator application are you recommending -- using triangle waves for doing PWM?

    I'm gonna be away from the computer for a few days beginning tomorrow, but I'll be interested in others' inputs.
  5. studiot

    studiot E-book Developer

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    Your applications list is biased heavily towards servicing/fault finding activities.

    Function generators are just the modern development of signal generators and can be (and are) used whereever these may have been used in the past.

    One field you have overlooked is development. Of mechanical systems, electronic sytems, etc.

    So they can provide the driving signal for the development of loudspeakers, radar, navigation systems, vibration analysers and counteracters...........

    They are particulary useful when combined with a gating system so that a train of a fixed sequence of pulses may be produced. This includes one shot mode. You should also mention trigger modes.

    Finally I am sure that AAC would be interested in at least part of your document if you were prepared to publish here as well.
  6. someonesdad

    someonesdad Thread Starter Senior Member

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    I agree; this tends to be a common activity of the target audience.

    Actually, I haven't, although one wouldn't know that from the post I made above. I do point out there are many uses in the development and research worlds. But I find it difficult to give meaningful examples because I have to leave out most of the technically interesting stuff to focus on the application of the function generator.

    The document will eventually be published by the company that contracted with me to write it and it will be freely available to anyone on the web. I'll try to remember to post a link to it when it is published.
  7. KL7AJ

    KL7AJ E-book Developer

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    IN fact, they're useful for checking the linearity AND PHASE SHIFT of ANY amplifier.

    Eric
  8. Bill_Marsden

    Bill_Marsden Moderator Staff Member

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    Yep, square waves in particular can tell you a lot, looking at the specific distortions. It is a quick and dirty method of looking at frequency response, both low and high ends.
  9. Mass

    Mass Member

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    The laser had a frequency card. The actual laser beam would vaporize some of the electrode off of a laser trimmable capacitor to modify the oscillator frequency.

    Correct. But I'm not sure whether the mixer was nonlinear. As far as I know, the mixer simply subtracted the frequency of signal A from signal B.
  10. Bill_Marsden

    Bill_Marsden Moderator Staff Member

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    Here is a super simple PWM using a sawtooth, instead of the classic triangle wave.

    [​IMG]

    As far as I know it has never been built. I'm going to have to try it out someday.

    If you were to feed audio on the + input it would be the basis for a decent class D audio, if the sawtooth distortion wasn't too extreme.
  11. beenthere

    beenthere Retired Moderator

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    Questions. The combo of R3 & R4 is 10.6K (and R6/7). Won't 11K work as well? Or 10K, for that matter?

    Why the .07uF value? - the standard is .068.

    I'm going to try it out.
  12. Bill_Marsden

    Bill_Marsden Moderator Staff Member

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    It was lifted from another thread, where the OP set a range of frequencies. I was going for exact values, but thinking about it it can be simplified. How are you going to use it? I didn't even think about the standard cap value, I was making it fit.

    ****

    I remembered later, I was compensating for tolerance, that design should have PWM over the range of the pot with 5% resistors, as close to rull range as I could get.
    Last edited: Jul 11, 2009
  13. beenthere

    beenthere Retired Moderator

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    Primarily I'm curious about the circuit. I will see what the range is and how easily it can be extended. My old Heathkit signal generator has stability problems, so a replacement could be nice to have.
  14. someonesdad

    someonesdad Thread Starter Senior Member

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    The use of the distortion of square waves to estimate amplitude and phase behavior will be included in the document and patterned after the picture on page 34 of the B&K document on using scopes.

    A couple of months ago, I did some simulations that sum a number of odd harmonics and could get a fairly reasonable square wave if I include enough terms. I was interested in getting some program examples that would duplicate the types of behaviors as seen in the above-indicated figure. It wasn't as straightforward as I expected it to be. If anyone has some practical experience doing this, I'd appreciate a PM, as I have some questions. If I get some time later, I'll start a separate thread about it and show what I've gotten so far.

    One thing I've been curious about is whether a function generator, a scope, and perhaps some simple circuit could be used to do frequency and phase characterization of a two port black box. If the generator is capable of sweeping, then the amplitude vs. frequency is not hard to get on the scope. But how about the phase response? It's of equal importance -- is there a straightforward way to measure it? (Pardon my ignorance; I'm not an EE and I only had to take one EE course; I chose to take network theory, which wasn't terribly practical, not to mention a long time ago.)

    I'd also like to elaborate on the response I gave to studiot's post. He was correct in stating that function generators are used in a variety of R&D efforts. I'd love to have some interesting examples, but I need to strip out the stuff that's not relevant -- and this stuff is usually what makes the application interesting. For example, one project I worked on in the early 80's was studying high current density electromigration in thin film magnetic alloys. I had to construct computer-controlled instrumentation and put the thin film heads in a furnace I had to build with a nitrogen atmosphere. But when I strip the unnecessary technical details out, all I can really say is "a square wave from a function generator was amplified by an RF generator". Pretty mundane.

    I'll bet many of you folks have some interesting things you've used function generators for. I'd appreciate it if you'd share those experiences -- if they don't sound too mundane when the interesting contextual information is stripped out. :)
  15. KL7AJ

    KL7AJ E-book Developer

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    Yes it can! If you use the oscilloscope in the X-Y mode, and a ramp signal, non-linear distortion will show up as a bending of the line, and phase shift shows up as an elliptical trace. This was commonly used in A.M. broadcast facilities to test the linearity of transmitters.

    There's a whole world of neat tests we can get into here.

    Eric
  16. someonesdad

    someonesdad Thread Starter Senior Member

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    Thanks, Eric -- this is the kind of thing I'm looking for. But I need a bit more information -- what exactly is plotted with respect to what? And what was desired to be linear -- the gain?

    The harmonic content of a triangle wave and a square wave are similar and I've read that some folks like to check an amplifier with a triangle wave, since deviations from the triangle shape are easy to spot by eye (as opposed to, say, telling when a sine wave is slightly distorted, which is quite hard to do). Does anyone use this and make quantitative measurements with it? If so, I'd appreciate any details.

    Do any of you folks use a tool similar to the Huntron Tracker for troubleshooting? Personally, I couldn't see spending a bunch of money on such a tool, but maybe a home-brewed solution such as the one here could prove useful. I was wondering if a function generator could be useful as a stimulus device for such kind of testing, possibly through a transformer -- anyone recommend such a test tool? You'd be able to continuously adjust the output voltage and the frequency.
  17. rspuzio

    rspuzio Active Member

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    > But how about the phase response? It's of equal importance -- is there a
    > straightforward way to measure it? (Pardon my ignorance; I'm not an EE
    > and I only had to take one EE course;

    I'm not an EE either but, for what it's worth, here's my idea. I am
    thinking of using it one of these years to build myself a phase meter
    when the round tuitt finally comes in.

    The idea is described by the block diagram below:

    [​IMG]

    The two blocks on the left are comparators which turn the sine
    waves into square waves by outputting, say, 5V when the signal
    is positive, 0V otherwise. Into one of them we will feed the
    original signal from the generator, into the other the phase-shifted
    output from the circuit being examined. These two outputs go
    into a bistable circuit which turns on when the one input goes
    high and turns off when the other input goes high. The output
    of the bistable circuit is a square wave which is on for a time
    proportional to the phase difference of the two original signals,
    as illustrated below:

    [​IMG]
    By averaging with a low-pass filter, we obtain a DC signal
    proportional to the phase difference, which we can then
    measure on a meter or plot on a scope.

    > perhaps some simple circuit could be used to do frequency
    > and phase characterization of a two port black box.

    This circuit shouldn't be too hard to put together with either
    IC or discrete components, so it looks like it'll fit your bill.

    Good luck with your writing project! I look forward to
    seeing the finished product.

    Ray

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