need help with metering the output of a tone gen.

Discussion in 'The Projects Forum' started by loosecannon, Aug 13, 2013.

  1. loosecannon

    Thread Starter New Member

    Aug 13, 2013
    12
    0
    hi all,

    first post here, and im hoping to get a little help with a project im contemplating.

    basically, im going to buy a ramsey kit 1khz tone generator, and i would like to connect a millivolt meter to it so that i can somewhat accurately set the output level.

    here is the kit im buying:
    http://www.ebay.com/itm/150916291174?ssPageName=STRK:MEWAX:IT&_trksid=p3984.m1438.l2649



    im thinking that i need some sort of diode to convert the AC sinewave to a DC voltage and connect the meter to that somehow.



    so, first question is, what are the important meter specs to look for?
    does it have to be an AC meter? a DC meter?

    second question is can someone lead me towards a diagram of the circuit ill have to build to connect the meter to the output of the tone gen?


    thanks for any and all advice,
    loosecannon
     
  2. Shagas

    Active Member

    May 13, 2013
    802
    74
    You can go with a simple solution and build a peak detector circuit like this one :

    http://www.falstad.com/circuit/e-peak-detect.html

    And then measure the output with a simple DMM ( digital multimeter ) which you can get for pretty cheap although it's not advisable to spend less than 20-30 dollars/euros for one . More if you are serious about electronics . You can spend 5-10 dollars for a pocket one which will measure your basics which in this case is enough.
    DMM's will typically measure ac waveforms of a greater voltage ie. 200 volts etc , not good for small signal stuff.
    In your case the output is 0-100 mV .

    If you want to measure ( and see ) the ac waveform then you need an oscilloscope.
    You can get pocket ones like I have
    http://www.gme.cz/osciloskop-jednokanalovy-10mhz-velleman-hps140i-p720-090
    for less than 100 bucks . This isn't a very serious tool , but it's very handy and gets the job done.
     
  3. MrChips

    Moderator

    Oct 2, 2009
    12,421
    3,356
    You can connect a DMM set to AC range and measure the output directly.
     
  4. Shagas

    Active Member

    May 13, 2013
    802
    74
    It would have to be a more expensive DMM no ?
    Mine measures AC voltage in 100mv steps . Got a 40 dollar DMM
    Op's generator outputs 0-100mV . So 1mv steps would be required at least
     
  5. wayneh

    Expert

    Sep 9, 2010
    12,093
    3,030
    I'd use a sound-card based, software oscilloscope. This will allow you to see the waveform. I suppose it would not be so useful if you really want accurate measurement the peak height.

    What are yo really hoping to do? If I need a tone generator these days, I just use my iPod and an app like FreqGen, which can generate a square, triangle or sine wave at any audio frequency. I believe the peak height is controlled but you'd have to research that aspect of it.
     
  6. bountyhunter

    Well-Known Member

    Sep 7, 2009
    2,498
    507
    A lot of meters don't do 1kHz bandwidth on AC. Even if they do, the lowest range on most meters is 200 mV range so it won't give the accuracy he wants.

    I did exactly this on a sine wave generator I built some years back: I used a high quality peak detector to get peak value followed by a scaling amplifier which ended up giving me the DC value of the peak-to-peak voltage.
     
  7. MrChips

    Moderator

    Oct 2, 2009
    12,421
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    As a quick test I took a B+K Precision Model 391A DMM.
    Set the range to 2VAC. Input 2V peak-to-peak 1kHz sinewave.
    Reading was 0.685V. Expected reading is 0.707V.

    The specs says +/- 2% reading over 500Hz to 2kHz.
    Thus the reading falls within specs.

    There is also a 200mV range available on this meter.
     
  8. MrChips

    Moderator

    Oct 2, 2009
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    Did the same thing with 200mV peak-to-peak on 200mVAC range.
    Reading is 70.48mV. Well within acceptable result.
     
  9. bountyhunter

    Well-Known Member

    Sep 7, 2009
    2,498
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    That +/- 2% spec always applies to the value of FULL SCALE reading.

    BK 391A is a pretty good meter, measures true RMS and sells for about $200. The cheaper ones are only accurate to a couple hundred Hz to measure AC power voltage.
     
    Last edited: Aug 13, 2013
  10. bountyhunter

    Well-Known Member

    Sep 7, 2009
    2,498
    507
    How are you getting the peak to peak level? If you are eyeballing it on a scope, there is a couple percent error just in the viewing. Only way to check the accuracy of a meter is to compare the reading to a better (more accurate) meter.
     
  11. MrChips

    Moderator

    Oct 2, 2009
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    Yes. The 2V signal was done by eyeballing the sine wave peak-to-peak on the scope.

    The second measurement done with the 200mV peak-to-peak was done by using the MEASURE feature of the TEKTRONIX TDS1002 scope.

    That could explain why the second measurement was in better agreement.

    I could go back and repeat the first measurement. But you get the point.
     
  12. loosecannon

    Thread Starter New Member

    Aug 13, 2013
    12
    0
    thanks for the replies guys.

    what im really wanting to do is make a little portable unit that gives me some sort of indication of the output level.

    the kit has a variable output level, and id like to know whether im putting out 20mV or 80mV.

    doesnt have to be dead nuts accurate, just an indicator.

    my plan is to buy the ramsey kit, buy a mV meter movement, and mount them in a small project box.

    i just need to know what circuit to build to connect the meter to the kits output.

    thanks again,
    loosecannon
     
  13. bance

    Member

    Aug 11, 2012
    315
    34
    That kit looks the same as the vellleman kit, I built it but I'm afraid it's not very good at all....
    You could build your own from op-amps or a VCO such as ICL8038 (a deprecated chip but still available on e-bay)
    The XR2206 is still available I believe and would give a greater frequency range, slightly OT I know.

    HTH Steve
     
    Last edited: Aug 16, 2013
  14. loosecannon

    Thread Starter New Member

    Aug 13, 2013
    12
    0
    thanks for the reply.

    im only interested in using it as a 1khz sinewave.

    i know that it will not be exactly 1khz, but im pretty sure i can tweak a couple of component values, while measuring with my freq counter and get it right on.

    i just need to know how to hook an analog meter movement to it so i can monitor the output level.

    loosecannon
     
  15. #12

    Expert

    Nov 30, 2010
    16,257
    6,757
    That kit only puts out 100 mv RMS. You could just turn the knob down to 80% to get about 80 millivolts.
     
  16. loosecannon

    Thread Starter New Member

    Aug 13, 2013
    12
    0
    well i found another kit that might work better, seeing as the initial kit i linked to sounds like it doesnt work very well.

    http://www.ebay.com/itm/CanaKits-CK...625?pt=LH_DefaultDomain_0&hash=item53fb8ee951

    i think ill try to find another source for it as i dont really want to pay 8 bucks to ship a pc board. LOL

    this one says its output is variable from 0-6 volts p to p so ill have to look at some different meters, but im wondering if thats not some sort of typo.

    6 volts peak to peak seems like a lot of output, but who knows.

    im surprised no one has led me to a simple diode detector circuit so i can drive the meter yet.

    im not quite sure how to build one, but i think it would be pretty simple for someone who is versed in electronics.

    thanks for all the help so far,
    loosecannon
     
  17. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    this one will work:
     
  18. loosecannon

    Thread Starter New Member

    Aug 13, 2013
    12
    0
    so, i would attach V+ and V- to the output of the tone gen in parallel?

    and the output of the circuit would go to the meter leads?

    also, in the diagram it looks like there is a cap going from pin 3 to ground, but i cant see the value of it.
    if someone else can see it, please post the cap value.

    thanks,
    loosecannon
     
  19. shortbus

    AAC Fanatic!

    Sep 30, 2009
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    Loosecannon, This is a kind of "insider" AAC question, and I hope you will stick around here long enough to understand it. That said, are you related to another of our FAMOUS members, Loosewire? Read some in the "off topic" section to get the joke.:)

    Welcome to AAC !
     
    GopherT likes this.
  20. bountyhunter

    Well-Known Member

    Sep 7, 2009
    2,498
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    Pin 3 is the input signal, pin 7 is the peak output signal to the meter. V+ and V- are the positive and negative power rails. You can use a resistive divider to ground off pin 7 to "adjust the calibration".
     
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