Where can I buy a good cheap signal/function generator?

Discussion in 'General Electronics Chat' started by zero_coke, May 31, 2010.

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  1. zero_coke

    Thread Starter Active Member

    Apr 22, 2009
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    I am a university student and my budget is about $200 so its very limited. I'm sure you guys know where to find a nice signal generator for a nice price. Anyone with any good places to order from or buy from? Is it even possible to find a signal generator for this budget? Oh, by the way, I need something that offers 1 MHz to 20 MHz signal modification.

    Thanks!
     
  2. loosewire

    AAC Fanatic!

    Apr 25, 2008
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    Army surplus catalog.Google Internet.

    Found u.s. army surplus on google It has search.

    For less.com.....Did you copy.
     
    Last edited by a moderator: May 31, 2010
  3. zero_coke

    Thread Starter Active Member

    Apr 22, 2009
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    Yea i check out the army one, but there was none there. I'll try the forless one now and see if they have any. thanks for the input!
     
  4. Wendy

    Moderator

    Mar 24, 2008
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    Just keep your eyes open, something will pop up. HAM events, Craigs list, flea markets, estate sales, it's out there.
     
  5. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
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    Are you sure you need 20 MHz? Several years ago, I built a kit based on the Max038 chip (now obsolete). I believe the store was http://kitsrus.com/kits.html

    If 2 MHz will do, you might consider the VC2002, such as this:

    http://cgi.ebay.com/NEW-0-2-Hz-2-MH...Item&pt=BI_Signal_Sources&hash=item335d75e768

    E-bay is flooded with signal generators. For the price, I am quite pleased with the VC2002 . The digital readout is pretty close.

    I notice there is a newer unit, the VC2003 that is a little more expensive, but might be worth considering.

    http://cgi.ebay.com/1-VC2003-Digita...lectrical_Equipment_Tools&hash=item2a0769aced

    Finally, if you are into boat anchors, there is a plethora of options, such as:

    http://cgi.ebay.com/HP-3325A-Synthe...Item&pt=BI_Signal_Sources&hash=item1c12c83b56

    Suggestions:
    1) Try searching e-bay, and
    2) Try Google search, particularly for DIY electronics.

    John
     
  6. someonesdad

    Senior Member

    Jul 7, 2009
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    If you refine your requirements a bit more, we could advise you better.

    When someone says signal generator, I interpret that to mean an oscillator that provides only sine waves. And that often means RF frequencies and levels -- crudely, somewhere in the 10 to 1000 MHz upper frequency region, with levels in the 0.1 to 1 volt region. Of course, these terms aren't standard, so others will interpret them differently.

    If you said function generator, then I think of a more general-purpose device that has sine, square, triangle, and maybe pulse capabilities. It might be able to modulate and sweep. It can usually output 20 Vpp into 50 ohms and has an upper frequency output in the 1 to 100 MHz range.

    I'm a poor hobbyist (i.e., the dark overlord has emasculated my wallet), so I understand about the need for cheap. :p I have slowly built up my set of measuring equipment up over the last 10 years by buying used stuff. If you are patient, you can find great deals on ebay. A few weeks ago I got an essentially brand-new DC-2 GHz device for $19 (I was the only bidder) -- and when I contacted the distributor for these devices, they said they go for over $2700 new.

    Don't shun an older analog-type function generator -- they can do nice things at low cost. I got an old Wavetek 144 that works perfectly and would be fine for anyone's home lab. The analog models can be tuned using their frequency dials, which is often useful for quick checking, looking for resonances, etc. The modern digital models based on DDS can't really duplicate this functionality except by setting up a sweep. However, the DDS units have the advantage that you can (usually) set the frequency and amplitude to digitally-set values, which is useful for quantitative work. And of course the computer can talk to them.

    RSR Electronics sells a little box that outputs 10 MHz sine and square waves: http://www.elexp.com/tst_sg10.htm. It's small and convenient. However, you can't quickly tune the frequency over a decent range like you can with the analog models with a frequency dial (you press the little push buttons to change the frequency). There's a less expensive unit that does fixed frequencies: http://www.elexp.com/tst_105.htm. B&K sells the same units for about twice the money. Take a look at RSR's other stuff: http://www.elexp.com/am_rsr.htm under test equipment. I know these units don't go to your stated 20 MHz, but do you really need to go that high?

    The other option is to build your own equipment. Personally, I didn't have the time for this when I was working and now that I do have the time, I'm not really interested in spending my time on such a task. But it is an eminently good way to get something that works for you and you know it was built right and how it works. The downside is that you'd probably want an oscilloscope to help you build it and, if you don't have that scope yet, that might be a hurdle.

    If you get copies of the old HP manuals from the 1980s, you can find older instruments that do what you want. These things cost thousands to tens of thousands of dollars new; you can get them for $100 or a few hundred dollars today on ebay. As John mentioned, they are boat anchors, but they are also fabulous pieces of equipment if they're working. A few of years ago a friend sent me an HP3326 in perfect operating condition for free (he got a pallet of 15 of them government surplus for a song). It's a work of art and I love it -- but it weighs 60 pounds and takes up space on the bench that I can fit four typical half-rack width instruments.
     
  7. zero_coke

    Thread Starter Active Member

    Apr 22, 2009
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    Thanks for all the great replies! As to the use of this function generator, what I am planning on doing with the function generator is that I am going to hook it up to my LC circuit and have it oscillate at 9.9 MHz, and my electrical knowledge is limited because I am still a student in learning process. I need a function generator to basically take input from the outlet (60 Hz, 120 Volts), and up this frequency of 60 Hz to approx. 9900 Hz. Can anyone suggest anything? Maybe I don't need a function generator after all?
     
  8. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
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    Do you mean 9.9 MHz or 9.9 KHz? There is a thousand-fold difference. Why do you need 9.9 MHz (or 9.9 KHz)? That is, how did you come up with that number?

    John
     
  9. zero_coke

    Thread Starter Active Member

    Apr 22, 2009
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    I meant 9.9 MHz. I came up with that number because it is the frequency at which my primary LC circuit will be at resonance with a second identical secondary coil that I am trying to feed electricity to, wirelessly, from my primary coil. Equation used was: f = 1 / 2*PI*root(L*C)
     
  10. SgtWookie

    Expert

    Jul 17, 2007
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    What are your values of L and C?

    Depending on how much power you were trying to send, you might use a video amplifier to drive your tank circuit.
     
  11. zero_coke

    Thread Starter Active Member

    Apr 22, 2009
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    Okay, SgtWookie, here are the details:

    I am trying to build a wireless charger for my iPod. According to Apple, the iPod charger is a:

    input: 100-240V ~ 50-60Hz 0.2A
    output: 5.0V 500mA

    My primary LC circuit will need to be able to transfer that much current to the secondary coil.

    I am planning on using:

    -147.7 KHz frequency (from the function generator)
    -Multi-layer single row coils of 19 turns, wrapped around a 2x4 piece of wood with nails at corners, so diameter is approx. 4 inches (coil is more like square)
    -Each coil (primary and secondary) has a capacitor of 0.02 uF rating attached in parallel. I'm not sure of the voltage, but I think I will have to get film capacitors of 5-10 Volt rating with 0.02 uF capacitance I guess...

    [​IMG]
    So the function generator will be providing an AC signal of 147.7 KHz and 110 Volts, and I need to wireless couple this to the secondary coil, convert the AC signal to DC, and then feed it into the iPod. Oh, and one question: Does 110 volt affect this circuit? I mean, do I need a transformer in my secondary coil to bring the 110 volt down to 5 volts? In other words, does voltage too get transferred? Isn't it just current that is induced into the secondary coil? If so, how much current will be transferred? Would it be the same current as what primary circuit has? If that's the case, then the signal generator is taking in 0.5 amps because its connected to mains, which is 60 hertz 0.5 amps 110 volts. If that is the case, then how can I bring down the current to 0.02 amps in the secondary coil since my iPod only is rated to take 0.02 amps?
     
  12. SgtWookie

    Expert

    Jul 17, 2007
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    What kind of "range" do you expect to get out of this hypothetical "wireless charger?"

    You DO know that the strength of a magnetic field decreases in intensity as a function of the square of the distance, right? And so if the magnetic field is sufficient at 1" separation, it will be 1/4 that much just 2" away, right?
     
  13. zero_coke

    Thread Starter Active Member

    Apr 22, 2009
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    Yes sir, that is correct. I am planning on wrapping copper wire around my desk and then tape it to the desk so it does not move. This way, by placing the iPod on the desk I should be able to charge it.

    I did some more research here and there so I am expecting 1-2 feet range with this new setup, but who knows, I may not be able to since I haven't experimented it yet. I have modified my primary and receiving coils to yield that range:

    Capacitor: 0.012 uF
    Inductor: 20 cm radius, 15 turns, 5 cm length = 72 uH
    Frequency: ~170 KHz

    I am aware that the iPod's coil will not pick up any power if it is perpendicular to the primary coil because the magnetic flux will not be going through it and so at 90 degrees it will not be able to pick up any power. However, since I will be wrapping around the desk, this should be feasible since this inductive coupling is only affected by orientation (which is fine since it will be wrapped around the desk and the iPod coil will be parallel to it on the desk) and by metal/ferrous objects but my desk is wood, so I guess I should be ok.

    I need to find a cheap square wave (to get maximum peak current due to the long peak line of the square) function generator that can provide the necessary frequency (1 - 200 KHz should be fine).

    However, I have few questions regarding the function generator:

    What is that output cable called? It has one lead on one side of it which goes into the function generator, and other side has a red and black lead.

    What is the output power of the function generator? Is it the same as the mains (110 volts, X frequency (adjustable from the knob), 0.2 A) ? Or does it change the current and all?
     
  14. retched

    AAC Fanatic!

    Dec 5, 2009
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    Any thoughts on what the magnetic flux will have on the IPod itself?

    Its great if you can charge it, but if you wipe the thing, no good.
     
  15. SgtWookie

    Expert

    Jul 17, 2007
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    You don't see anything a bit klugey in this idea? You do realize that anything that contains ferrous metal (such as a pen, a mechanical pencil, a paper clip, stapler, iPod, etc.) that gets near the inductor will change it's characteristics considerably, right?

    If you get n Watts of power transfer at 1 foot vertical displacement, you will get n/4 at 2 feet vertical displacement

    I don't know how you calculated the 72uH for the inductor, but for your given dimensions I get far different numbers, which will change the resonant frequency quite a bit.

    Didn't occur to you that the iPod has ferrous metal in it, right?

    Did you know that a square wave is composed of the fundamental frequency, plus the sum of all of the odd harmonics of the fundamental frequency? An LC tank circuit won't be happy resonating at anything other than the fundamental frequency.

    They usually have BNC connectors. You can usually select 50 or 75 Ohm impedance. Impedance mismatches will cause reflected energy at higher frequencies.
    There are various test leads available that can connect to BNC fittings.
    Pomona Electronics stocks an assortment. They are not cheap.

    Generally, not much. They aren't designed to supply high power. They are designed to output accurate waveforms over a wide range of frequencies.

    No.
     
  16. zero_coke

    Thread Starter Active Member

    Apr 22, 2009
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    Ah, thank you for the replies. Regarding the metallic objects and such, this project's whole purpose is to make the coil's resonate together at a particular resonant frequency such that no other objects can get affected by the magnetic field unless the random metallic object coincidentally has the same resonant frequency as the primary coils'.

    The only effect a ferrous/metal object has on the experiment is it blocks the flux from getting to the secondary coil for some reason. It might be able to wipe out the hard drive of the iPod, however, I think you need a really strong magnetic field to accomplish that, so it is safe I am assuming.

    Regarding the Inductance value of 72 uH, I got that by using the formula:

    L = radius^2 * turns^2 / 9*radius + 10*length

    This formula is for the coil that has a single layer coil so it looks like:
    OOO
    OOO
    (from the side view)

    However, I made a mistake in the calculations of it, so now I got 154 uH, and a resonant frequency of 117 KHz. What did you get SgtWookie?
     
  17. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
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    Your formula is for an air core inductor. Even the four nails you plan to use will change the inductance, not to mention whatever your desk is made of. John
     
  18. lmartinez

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    Mar 8, 2009
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  19. zero_coke

    Thread Starter Active Member

    Apr 22, 2009
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    Thanks John and Imartinez.

    John: I plan on removing the coil from the 2x4 piece of wood once I've wrapped it to make it look like a coil. It is only used to mould my coil into a coil-like shape. Can wood have an inductance???

    Imartinez: I have read EVERYTHING that has to do with wireless electricity. Well not to exaggerate I guess, but I did a lot of research on this topic. I appreciate your link. The problem with this whole project boils down to this question (for me):

    What is the difference between inductive coupling, evanescent coupling, RF coupling? Which one did MIT do? I think they did some "tunneling" effect where they were able to transfer power over 2 meters by doing this. However, they say there is no electromagnetic radiation; just a magnetic field / flux pulses back and forth and does not affect surrounding objects at all. So I don't understand how they did that despite having read their article. Could it be the same way I'm trying to do this experiment, except they used 10 MHz instead of my 154 KHz frequency? Might it be that this 65x higher frequency they used be the reason why they were able to light up a lightbulb 2 meters away?
     
  20. SgtWookie

    Expert

    Jul 17, 2007
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    If you try to use inductive coupling to charge your iPod, and your iPod has its' operating system on magnetic storage media, it will become an iBrick.

    If your iPod uses magnetic media to store files with, they will be erased very, very completely - and so will anything else that comes within the magnetic field of the coil that you wish to build.

    It will be a rather effective degaussing coil.
     
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