buying a function generator

Discussion in 'General Electronics Chat' started by samjesse, Aug 25, 2009.

  1. samjesse

    Thread Starter Active Member

    Sep 14, 2008
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    0
    Hi

    I am considering buying a function generator to use it simulate input signals on automotive electronic components and test their output on a scope.

    what do I look for in buying a function generator?

    thx
     
  2. Wendy

    Moderator

    Mar 24, 2008
    20,765
    2,536
    Most basic function generators have a few types of signals, usually switch selectable. Things like square, triangle, and sine waves. It sounds like you are after something outside the norm for most electronics type of uses. You may want think about what kind of signals you want to duplicate.
     
  3. samjesse

    Thread Starter Active Member

    Sep 14, 2008
    202
    0
    square wave, with/with-out DC coupling, frequency and duty cycle independent controls. I would like to get more than what I just stated in case I find a need for a wave I do not have.
     
  4. someonesdad

    Senior Member

    Jul 7, 2009
    1,585
    141
    Probably the first thing to define is how much money you're willing to spend. This tends to drive everything else.

    Consider buying used equipment from ebay and other auction sites. If you're patient and know what you're looking for, you can get some good deals. A friend bought 10 or 15 used HP 3326's in an auction and cherry picked one and gave it to me. It's a quintessential boat anchor, but it lets you control most everything you'd want to control on some sine, square, or pulse waveforms (except, sadly, rise and fall times). It's basically two HP 3325 generators in one box (the 3325 was a popular HP function generator from the early 1980's) except it has a knob which the 3325 should have had.

    Personally, I'd look for a modern DDS (direct digital synthesis) generator that would give me sines, square waves, and triangle waves. Ramps (sawtooth waves) can be handy too. Be careful of the specs saying that the symmetry control can get you ramps. You usually want the falling part of the ramp to be as fast as possible and the symmetry control never gets you to that point.

    If you can afford one, the arbitrary waveform generator can be very powerful and useful. But they can go for $1000 to $2000 new or more, so are often not within the reach of the hobbyist.

    I have an older Wavetek 144 sweep generator that works well. It's an analog device, but I only paid $65 for it delivered from a guy who I trust who guarantees his work. If you think you might want to do frequency characterization of circuits, the sweeping generator would be a nice choice. You can also do more limited sweeping with a generator that allows FM modulation or a VCO (voltage controlled oscillator) input.

    Realize that when the manufacturer states that a DDS generator is e.g. 25 MHz, they mean that it will put out sine waves of that frequency. The frequency of other waveforms is often less, sometimes much less (this is especially true of DDS generators). For example, I have a particular generator on loan that is rated to put out 80 MHz sine waves. The square wave output is half that frequency. The other waveforms only go up to 100 kHz.

    If you're interested in audio measurements, DDS generators may not be appropriate, as they don't have significantly low distortion (due to the way the waveforms are synthesized).

    Decide what amplitude capabilities you want. A typical modern generator may put out 10-20 volts peak-to-peak into 50 ohms. You'll also likely want it to have DC offset capabilities. If it doesn't, you can also get there if you can put the output in series with a DC supply -- but both BNC output connections need to be floating (or you need to float the instrument with an isolation transformer) AND you have to know you're staying within the manufacturer's recommendations.

    Personally, I consider the DC offset ability to be mandatory. It lets you turn square waves into unipolar pulses or put a small ripple voltage on a larger DC voltage.
     
  5. samjesse

    Thread Starter Active Member

    Sep 14, 2008
    202
    0
    I am looking for a function generator which can do the following;

    when connected to an LC parallel circuit, the user can set it to automatically zero on "self tune" to achieve the resonance frequency of the circuit.

    if not, then is it possible to built a circuit that will do that? say, by sensing the current and using that as an input in order to change the frequency up or down until the minimum current is obtained?

    thx
     
  6. bountyhunter

    Well-Known Member

    Sep 7, 2009
    2,498
    507
    You need to know the frequency range you must cover and how precise the waves have to be. For audio testing, you need sine waves with super low distortion. For general purpose use, you don't really care about harmonic distortion.
     
  7. samjesse

    Thread Starter Active Member

    Sep 14, 2008
    202
    0
    I mean, given a frequency range is known and a square wave is needed. what is the general solution plan? i.e. is there a function generator which does this? is that an add on off the shelf device? do I have to build my own circuit? .. if so, where can I read to do it? thx
     
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