Sine Wave Output on the IC8038 Circuit

[sbdwag]

I built the Function Generator listed in the Projects using the ICL8038 Chip and have some problems with distortion at the High end of the frequency range 1Mhz. Below are a couple of pictures of the completed project and also a screenshot of the Scope image showing the ringing at the voltage peaks.

Does anybody know what causes this.

Regards
Dennis Waggoner

 

[audioguru2]

Hi Dennis,
Welcome to our forum. ;D
Didn't you look at the datasheet for the old ICL8038?
I used it in 1975 and its lousy performance was pretty good in those days.
Maxim has their newer MAX038 which is an improvement.

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[audioguru2]

BTW, the old LF351 opamp is also contributing to your problem. I've never seen one.

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[sbdwag]

BTW, the old LF351 opamp is also contributing to your problem. I've never seen one.

Actually I replaced it with the TL071 ?

I did look at the datasheets and knew distortion increased over 200khz or so. I just wondered if the ringing would be considered part of the THD or something else.

Thanks for the welcome. I do plan on a project using the max038 chip along with the buf634 amp. Just putting together the project now doing more research on shielding the circuit and checking components (mainly caps) requirements. I also plan on using the circuit shown in the diy kit101 found readily available as a pdf on the web.

One thing I did find after breadboarding the circuit was that all of the pots in the circuit needed to be current limiting except the output to the opamp which needs to be a voltage divider.

Im basically a neophyte at Electronics with on a few hours toward my degree. I have difficulity understanding some circuts and when the pot should be connected as Voltage dividers and when they should be connected a series variable resistors.

Regards
Dennis Waggoner

 

[audioguru2]

Hi Dennis,
A TL071 is similar to an LF351 opamp since they both have difficulty with signals over 100kHz.
1MHz is a radio frequency so long wires can ruin the performance of your project.
The parts on your perf board are very far apart causing series inductance in the long wires between parts. Capacitance between the long wires upsets the match of low-value capacitors and couples high frequency distortion products from wire to wire.

Problems caused by long wires can be minimised if critical matched capacitors and resistors are mounted directly on the range switch.

A resistive voltage divider usually sets a voltage for a high impedance input of a device.
A series variable resistance usually needs another current-limiting resistor in series with it to set the maximum current from a source or to its load.

 

[sbdwag]

A TL071 is similar to an LF351 opamp since they both have difficulty with signals over 100kHz.
1MHz is a radio frequency so long wires can ruin the performance of your project.
The parts on your perf board are very far apart causing series inductance in the long wires between parts. Capacitance between the long wires upsets the match of low-value capacitors and couples high frequency distortion products from wire to wire.

Problems caused by long wires can be minimised if critical matched capacitors and resistors are mounted directly on the range switch.

Thank you Thank you

That kind of information I cannot get out of class. 

I will use that information to my advantage when I make the Max038 project. I will try to design and place the pots and switches directly on the pref boards and elimate all of the twisted wire leads. That should eliminate the inductance and also minimize capitance and thus reduce harmful resonance in the circuit. Tell me more tricks Please


A resistive voltage divider usually sets a voltage for a high impedance input of a device.
A series variable resistance usually needs another current-limiting resistor in series with it to set the maximum current from a source or to its load.

Make perfect since now. You would not normally see a resistor in series in a voltage divider circuit. And you would  have a resistor in series with a current limiting device because of the  input current requirements of the device. Beside there is no signal present on pins 1 and 12 of the 8038 so why would you need voltage (or signal) that a voltage divider would give you.

AH. I am only a grasshopper (50 year old grasshopper)  but I am learning to hop and spit.

Regards
Dennis Waggoner

 

[audioguru2]

Hi Dennis,
Intersil have application notes about their ICL8038 and Maxim have them about theirs.
Here is part of a note from Intersil:
http://www.datasheetarchive.com/semiconductors/pdfdatasheet.php?Datasheet=933731

I didn't know that you are a fairly old grasshopper. I thought you are a young whipper-snapper.
I am a retired 60 year old geezer. I still hop very well but I never learned how to spit. ;D

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