Neewbie question. Please help

Discussion in 'The Projects Forum' started by Johnirua, Jun 14, 2006.

  1. Johnirua

    Thread Starter New Member

    Jun 14, 2006
    1
    0
    Hello i am trying to build a Square wave generator for a step motor and i use a xr-2206 chip to set the frequency. The problem i have is that when i set it higher then 100kHZ the signal gets useless (looks like a sharkfin) and i can not use it. The scheme i use is figure 11 on this page and i use a 1000ρF timing capacitor.

    http://www.exar.com/products/XR2206v103.pdf

    If anyone knows what i am doing wrong, please help cause i have been stuck with this for 2 days now and i have absolutely no clue how to fix it :(

    Allso please ingnore my bad english :)

    /John
     
  2. mozikluv

    AAC Fanatic!

    Jan 22, 2004
    1,437
    1
    hi john,

    for starters, 1. what freq. are you trying to use?
    2. what type of timing cap did you use, is it the ceramic type?
    3. how about your foil trace layout, lots of things could go awry due to poor trace layout.
     
  3. aac

    Active Member

    Jun 13, 2005
    35
    0
    When you say it looks like a shark fin, I'm guessing the waveform goes slowly positive then quickly in the negative direction. Of course without knowing what you are loading the square wave output with, I can't say for sure. You are using pin 11 as the output, right? You will notice that the 2206 has an active device, the transistor int eh 2206, to pull the voltage down. You are supplying the resistor to pull the voltage up. If the RC time constant, of the circuit you put on the output, is long, you will get a waveform like you have described as a sharks fin. You need to reduce the capacitance on output pin 11, the pullup resistance on to pin 11 or both. The transistor in the 2206 will discharge whatever capacitance you have put on pin 11 more quickly. Don't forget even your scope probe will have capacitance. Do you know the capacitance of your scope probe? Did you use the recomended 10KΩ resistor for the pullup? I think your problem lies with the components on pin 11.

    Your english is fine. I hope you get your circuit working soon.
     
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