freq generator that can set exact frequencies?

Discussion in 'The Projects Forum' started by fawcetteng, Dec 13, 2010.

  1. fawcetteng

    Thread Starter New Member

    Dec 11, 2010
    Looking to build a simple sine or square wave generator that i can know what frequency it is outputing. That is, either have a way to set the frequency "exactly", or have a built in frequency counter to display to me what the frequency is. I want to be able to accurately calibrate older automotive tachometers. Would really like the frequency accuracy to be less then 2hz (this equals 50rpm on a 4 cylinder auto). So the ranges I need are below.

    If not easy to build, can someone point me to something on fleabay that would work, for no more then maybe $100?

    square or sine pulses, +/- 5v or +/-12V, or 0-5 or 0-12V DC.
    frequencies in the range of 133Hz to 600Hz.
  2. SgtWookie


    Jul 17, 2007
    Are you in the USA or Canada? If so, the AC is 60Hz. In most other places, it's 50Hz.

    Once rectified, it's 120Hz rippled DC and 100Hz rippled DC, respectively. It's usually very accurate.

    Would a single frequency like that be sufficient, or do you need a range of frequencies?
  3. t06afre

    AAC Fanatic!

    May 11, 2009
    A sound card waveform generator may create a sinewave in that frequency range with a about 1 Hz accuracy I think. I guess you can find such tool free on the net. The output voltage range of a sound card will not be in your range. But a simple amplifier may take care of that.
    I might be able to whip something up for you(windows os) if you do not find anything
  4. bertus


    Apr 5, 2008

    You can make an oscillator with an cd4060 and a 32768 watch crystal.
    On the following outputs you will find the following frequencies:
    Code ( (Unknown Language)):
    2. Pin  7 -> 2048 Hz
    3. Pin  5 -> 1024 Hz
    4. Pin  4 ->  512 Hz
    5. Pin  6 ->  256 Hz
    6. Pin 14 ->  128 Hz
    7. Pin 13 ->   64 Hz
    8. Pin 15 ->   32 Hz
    See the attached PDF how to connect the crystal to the chip.

  5. debe

    Senior Member

    Sep 21, 2010
    Hi I find where i live the mains is 50Hz, using a small transformer steping the mains down to 6V ac gives 1500Rpm for 4Cyl & 1000Rpm for 6Cyl tachos. Found this acurate for calibrating tachos.
  6. Wendy


    Mar 24, 2008
    You could also build a simple freq counter (it has been done with PICs for lower frequencies) to set a simple oscillator. Depends on your application.
  7. fawcetteng

    Thread Starter New Member

    Dec 11, 2010
    All great ideas - thanks for the help. I've found these older tachs, even with a good cleaning, are no longer linear, so setting them at say 1500rpm they can be as much as 500rpm off up at redline.

    Using the sound card intrigues me. I assume a "simple amp" is some sort of op amp, a power supply, and a few resistors? Can anyone point me in the right direction to get amp up to 9 or 12v peak output from the sound card's output? I can write the software to output the sound wave.

    Bill - I like your idea (and the watch crystal osscilator one), but dont know much about what you are talking about. PICS and osscilator. As a grease monkey/ME , dabbling in electronics I come here for specifics.
  8. t06afre

    AAC Fanatic!

    May 11, 2009
    Components will be victim to ageing problems. Capacitors and resistors values will drift. Some capacitors may also dry out. Solder joints are prone to crack. So cleaning will not be enough in order to restore old electronics in most cases.
    A typical opamp may deliver around 15 to 20 mA without problem. Will this be good enough for your setup. Or is more current needed in order to drive your tachs in your test bench
  9. fawcetteng

    Thread Starter New Member

    Dec 11, 2010
    yes, 20mA will be enough to trigger the tach. I can power it seperately. Thanks!
  10. bassplayer142

    Active Member

    Jan 2, 2007
    If you have experience with microcontrollers you could use a DDS (direct digital synthesis) IC. The one I'm using AD9834, has a enormous frequency range of up to ~37Mhz and 0.004 Hz accuracy. It can output sine, triangle, and square with the built in comparator.

    This is probably overkill if you need a simple oscillator but if I finish my project sometime in the near future I can add it to this site to take the design stage out of the question. I'm trying to make a full out function generator out of it.