Problems with a XR-2206 function generator

Discussion in 'General Electronics Chat' started by adam555, Sep 11, 2014.

  1. adam555

    Thread Starter Active Member

    Aug 17, 2013
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    I'm having a bit of a problem making a XR-2206 work: it won't work above 12V (or +6 / -6V split), when it should do it up to 26V (or +13V / -13V).

    I've been trying the test circuits from the datasheet, and they all seem to work fine, but only as long as I don't exceed +6 and -6 volts (or 12V in the single supply examples).

    This is a picture of what I get in an oscilloscope when I pass that voltage:

    WP_20140911_001.jpg

    I also noticed that if I increase the frequency with a pot through the Timing Resistor, it will work with higher voltages -up to the 23 V limit at the maximum frequency- but again, the results get all messed up as soon as I lower the frequency back; so it seems that it needs Vcc to increase along the output frequency.

    Is anyone familiar with this IC, or has any idea why I'm getting these results?

    Thanks in advance...
     
  2. bance

    Member

    Aug 11, 2012
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    Could you be a little clearer about exactly what is happening?

    On the one hand you say the chip doesn't work if you supply it with more than 12V and on the other you say it works OK up to the 23V limit.......

    In the scope picture it looks like the chip is oscillating, or the scope isn't triggering correctly.
     
  3. crutschow

    Expert

    Mar 14, 2008
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    Post a schematic of your circuit.
     
  4. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    Scope looks like your ground clip is not attached or synch is set to channel 2 or one of any possible rookie errors.

    What is your supply voltage and expected output voltage?
     
  5. adam555

    Thread Starter Active Member

    Aug 17, 2013
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    The circuits I tried are the ones in the datasheet (I attached both to this post), plus other simple circuits I found on the internet, like this one:

    [​IMG]

    They all produce the same result; both in single and split supply voltage.

    The voltage ranges at which the chip will work as it should are 8.5V to 12V, or +-4.25Vto +-6V with a split supply voltage. This range, according to the datasheet, should be up to 26V or +-13V.

    This was tested using a 100nF timing capacitor for C, and a 1Mohm timing resistor for R1; which outputs a frequency of around 20Hz. I left R2 and pin 9 open for the test; as these are for the Frequency Shift Keying. Pins 13 and 14 are also open.

    Now, I also noticed that the IC will take higher voltage if I increase the frequency. I'll try to explain it again so it doesn't sound confusing like in the first post. If I change the value of R1, which as I said above is at 1Mohm through a potentiometer, and lower the value to the minimum allowed (which is 1Kohm according to the datasheet), the chip will take up to 15V (or +-7.5V). Furthermore, if I change the timing capacitor C from 100nF to 1nF (which according to the datasheet is the minimum value), now I can increase the voltage all the way to the maximum voltage without any problems. This shouldn't be the case; I should be able to use any voltage rage with any frequency, instead of only low voltages for low frequencies and high voltages for high frequencies.

    Regarding whether or not the picture of the oscilloscope posted above shows an incorrect trigger: I'm absolutely sure that's not the case. It's the first thing I checked, and there's no way to sync the output with any trigger level. Besides, when I increase the voltage above +-6V there is not frequency change or voltage change in the output; the trace just becomes increasingly blurred and starts to shake until you get that output.
     
    Last edited: Sep 12, 2014
  6. bance

    Member

    Aug 11, 2012
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    You have got bypass caps on the supply rail haven't you?
    Make sure there is a pull up on pin 11, use the output of pin 11 to trigger your scope.
     
  7. adam555

    Thread Starter Active Member

    Aug 17, 2013
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    I tried with various values for the bypass caps (both combined and individual for the split supply) with values from 1nF to 100uF and it makes no difference; in fact, it does the exact same with or without them.

    I also have a pullup resister on pin 11, though I was using pin 2 for the output (sine / triangular wave). I also get the same result with the square wave on pin 11 with pullup: a distorted output in low frequencies for voltages over +-6V.
     
  8. adam555

    Thread Starter Active Member

    Aug 17, 2013
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    I tried everything I could think of, and at this point I'm pretty sure I'm not doing anything wrong. I got this chip on ebay from China, so I rather think they sent a dodgy one or even a fake.
     
  9. alfacliff

    Well-Known Member

    Dec 13, 2013
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    the waveforms look normal, but the synch is off. the chip can output a sawtooth wave like that shown. a bad or disconnected ground would be more of a sinewave. the 2206 is a "function generator" and can produce sine, triangular, and square waves.
     
  10. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    When you want to use the FSK function, the scope screen could show something like yours.
    You must test the two frequencies seperate by givin a high or low signal to the FSK input.
    Then the scopescreen should be stable.
    Also check the sync settings of the scope.

    Bertus
     
  11. adam555

    Thread Starter Active Member

    Aug 17, 2013
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    I tried all three waveforms, and all three begin to distort above +-6V on low frequencies.

    I assure you the trigger is not out of sync. This is a picture with the trigger out of sync:

    WP_20140912_003.jpg

    And this is a picture of what happens if I lower the voltage just half a volt. Without touching the oscilloscope or the circuit the trace simply becomes clear and stable:

    WP_20140912_002.jpg
     
    Last edited: Sep 12, 2014
  12. adam555

    Thread Starter Active Member

    Aug 17, 2013
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    Also tried FSK and both frequencies, combined and separately: same result.
     
  13. alfacliff

    Well-Known Member

    Dec 13, 2013
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    your scope should be able to synch on a triangular wave, are you nsure you have it set correctly? do not use tvv, tvh, or line synch, synch to dc level should be best.
     
  14. crutschow

    Expert

    Mar 14, 2008
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    It may not be the problem, but just to make sure you should have a 0.1uF ceramic cap directly between each of the supply pins of the IC and the ground pin.
     
  15. adam555

    Thread Starter Active Member

    Aug 17, 2013
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    I posted a picture of a triangular wave in sync above. And just to make sure I know how to sync the trigger of my oscilloscope I double checked with another function generator. :p

    No change... I replaced the 10uF electrolitic caps for 1uF ceramic directly from each supply pin to ground and the result is the same. Yesterday I also tried with values from 1nF to 100uF and it makes no difference.
     
  16. crutschow

    Expert

    Mar 14, 2008
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    I meant across the pins on the IC, not the power supply. The supply doesn't need decoupling (which likely already has its own capacitor), it's the IC that does.
     
  17. MrChips

    Moderator

    Oct 2, 2009
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    Try triggering the scope off the square wave output.
     
  18. adam555

    Thread Starter Active Member

    Aug 17, 2013
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    I understand it looks a lot like the oscilloscope is out of sync, so I took a series of pictures showing step by step what happens when you increase the voltage from +-6V to +-7V. Nothing else was changed, on the oscilloscope -set at .2 s/div and .5 V/div- nor the circuit; just the voltage is increased -as you can see in the voltmeter.

    WP_20140912_004.jpg
    WP_20140912_005.jpg
    WP_20140912_006.jpg
     
  19. adam555

    Thread Starter Active Member

    Aug 17, 2013
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    Yes, that's what I meant: I put the caps on the IC supply pins (from pin 4 to ground and pin 12 to ground), not on the circuit supply. Yesterday I also tried decoupling both pins together with a cap of different values directly over the IC from pin 4 to 12; same result.
     
  20. adam555

    Thread Starter Active Member

    Aug 17, 2013
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    The same happens with the square wave, you also see it as if it was out of sync... but it's not, they are what I assume random pulses.
     
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