Author Topic: 8038 Function Generator Circuit - Sine Wave Glitch  (Read 6061 times)

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Offline icon

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8038 Function Generator Circuit - Sine Wave Glitch
« on: April 05, 2012, 05:30:39 AM »
Hi

I have a function generator based on the venerable (=crap and no longer available) 8038. The square and triangle waves seem pretty clean, but the sine has a funny glitch that appears as the frequency increases. Here it is circa 1KHz:[1]



Would I be way off the mark thinking that decoupling capacitor(s) near the chip might fix the problem? If so, given that the chip uses +/-15V would 2 caps, one from Vcc to ground and one from Vee to ground be needed? It's a ready made PCB so I'm going to have to bodge something up on the solder side.  :-\

If I'm wrong, what might be the cause?

Cheers

John
[1] Courtesy of my shiny new DS1052 DS1102. Winking smiley.

Offline Bored@Work

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Re: 8038 Function Generator Circuit - Sine Wave Glitch
« Reply #1 on: April 05, 2012, 05:57:11 AM »
I am not sure if it was the 8038 or some other common function generator IC, but one of them is known for this nasty habit. If I remember correctly it was typically an issue of the PCB layout, adjustments and keeping the supply power stable.
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Offline david77

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Re: 8038 Function Generator Circuit - Sine Wave Glitch
« Reply #2 on: April 05, 2012, 06:03:17 AM »
I think you're thinkin of the good old XR2206. I remember reading about it in Elektor, they claimed they managed to get around that problem.

Offline amspire

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Re: 8038 Function Generator Circuit - Sine Wave Glitch
« Reply #3 on: April 05, 2012, 08:05:53 AM »
Looks like the 8038  needs a decoupling capacitor near the IC.

Those spikes you are seeing are caused by the crossover current surge as the capacitor is switched between charge and discharge.

You need something like a 1 uF ceramic cap across the power pins.

Richard.

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Re: 8038 Function Generator Circuit - Sine Wave Glitch
« Reply #4 on: April 05, 2012, 09:01:01 AM »
You need something like a 1 uF ceramic cap across the power pins.

Thanks; just one between VCC and VEE (+15V, -15V), or from each pin individually to ground? Sorry if that's a dumb question.

John

Offline amspire

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Re: 8038 Function Generator Circuit - Sine Wave Glitch
« Reply #5 on: April 05, 2012, 09:02:54 AM »
Each pin to ground.

Online mikeselectricstuff

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Re: 8038 Function Generator Circuit - Sine Wave Glitch
« Reply #6 on: April 05, 2012, 10:02:28 AM »
ISTR that sinewave nipples  were a standard quirk of this chip - not sure if there was a solution.
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Re: 8038 Function Generator Circuit - Sine Wave Glitch
« Reply #7 on: April 05, 2012, 10:10:49 AM »
I have never seen a cure for this glitch in the chip.

Offline grenert

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Re: 8038 Function Generator Circuit - Sine Wave Glitch
« Reply #8 on: April 05, 2012, 12:28:46 PM »
If you add a little lowpass filtering (like at 1 MHz) to the output, the glitch is a bit improved.  The chip is good to only 100 kHz or so anyway.

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Re: 8038 Function Generator Circuit - Sine Wave Glitch
« Reply #9 on: April 05, 2012, 08:06:08 PM »
Ah well, thanks for the additional responses. Despite the addition of two 1uF caps, it still does this (channel 1 is the positive supply):



And I know you all like a laugh, so this is my ghetto-engineered PCB. (I learned my craft in a Chinese instrument manufacturing facility)



[Dave voice: "I don't like it; I don't like it at all"] Despite appearances, nothing is touching anything it shouldn't, and yes, I'll clean the flux off. I think I'll add a lowpass filter and then stick it in a box. Possibly under the bench.

Cheers
John
« Last Edit: April 05, 2012, 08:43:50 PM by icon »

Offline icon

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Re: 8038 Function Generator Circuit - Sine Wave Glitch
« Reply #10 on: April 06, 2012, 12:28:42 AM »
About as good as I can get it:



Filtered trace is channel 2, of course.

John

Offline w2aew

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Re: 8038 Function Generator Circuit - Sine Wave Glitch
« Reply #11 on: April 06, 2012, 06:46:46 AM »
Since that chip doesn't get attached to ground when used with a split supply, you may want to also consider a bypass cap across the V+ and V- pins directly (pin 6 to pin 11).  Since it's a high-frequency (narrow) glitch, you may want to tack in a medium value ceramic cap - 0.001-0.01uf or so.
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Re: 8038 Function Generator Circuit - Sine Wave Glitch
« Reply #12 on: April 07, 2012, 08:59:00 AM »
Since that chip doesn't get attached to ground when used with a split supply, you may want to also consider a bypass cap across the V+ and V- pins directly (pin 6 to pin 11).  Since it's a high-frequency (narrow) glitch, you may want to tack in a medium value ceramic cap - 0.001-0.01uf or so.

Thanks for the suggestion; tried it - still glitches. As others have said, this chip seems to be curiously resistant to the usual fixes. Perhaps that's why it's no longer available. The low pass filter at least provides a reasonable sine wave, up to 200KHz or so.

Regards
John

Offline 3n2323

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Re: 8038 Function Generator Circuit - Sine Wave Glitch
« Reply #13 on: April 08, 2012, 12:04:21 PM »
hi icon,

how did you include the pictures within your messages, instead of having them as attachments at the bottom and show up as attachment icons? do you have to keep them somewhere online?

Offline bfritz

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Re: 8038 Function Generator Circuit - Sine Wave Glitch
« Reply #14 on: April 08, 2012, 02:14:14 PM »
hi icon,

how did you include the pictures within your messages, instead of having them as attachments at the bottom and show up as attachment icons? do you have to keep them somewhere online?

If using Internet Explorer and Windows, right click on the picture, and choose the "Properties" item.  This will show you the location for any item on a plain 'ole HTML webpage.  Note the poster used the online photobucket service as the location for the photos.


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