XR2206 Kit Review

Thread Starter

Wendy

Joined Mar 24, 2008
22,834
So I was looking for inexpensive function generator, this kit can be bought online from many sources under $10 each.after botching my first assembly I bought one preassembled ( I know! cheating right?), I was less than happy with the sine wave now will show images just to inform other folks.
XR2206.png XR2206a.jpg
60Hz:
SDS00002.png SDS00001.png SDS00003.png
........Sin wave....................Triangle Wave...........Square Wave
As you can see the sin waves tips have minor points on them.

1KHz:
SDS00004.png SDS00005.png SDS00006.png
........Sin wave....................Triangle Wave..............Square Wave
 
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Thread Starter

Wendy

Joined Mar 24, 2008
22,834
100KHz
SDS00007.png SDS00008.png SDS00009.png
........Sin wave.....................Triangle Wave...........Square Wave

1Mhz
SDS00010.png SDS00011.png SDS00012.png
........Sin wave....................Triangle Wave.............Square Wave

I was thinking about putting this in a box with switches instead of jumper straps to make a full function generator. I could even add a counter in the box to show the frequency. I may yet do this I don't know.
 
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rsjsouza

Joined Apr 21, 2014
350
The XR2206 is known to have some problems as a function generator, but unfortunately I couldn't find the thread where this is discussed.
At any rate, the waveforms you showed are indeed more distorted than I remember. Perhaps an issue on the output amplifier? Or a prtoblem on the IC itself?
 

DickCappels

Joined Aug 21, 2008
8,704
Lazada sells these prebuilt generators in Thailand. This is what their website shows the 1 MHz square wave to look like.

1642665775240.png
If I read the data sheet correctly the square wave come out of the SYNCO pin, which is an open NPN collector output. It would be easily affected by capacitive loading. Reducing the load (X10 or X100 probe) or using a lower value pull-up resistor might help.
 

AnalogKid

Joined Aug 1, 2013
10,052
And check to make sure your scope probe is correctly compensated. It won't affect the sine waveforms, but it might square up the square wave.

Also, the chip has 4 pins that can be connected to pots to trim the sine distortion. I found the kit online, and the photos doesn't show any trim pots. If you reverse-engineer a schematic around pins 13-16, we probably can come up with mods for this.

ak
 
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Thread Starter

Wendy

Joined Mar 24, 2008
22,834
I was using x10 probe that had been compensated. As I said this kit had been presoldered (welded, I still get a kick out of that oriental phrase, apparently soldered doesn't translate very well). I am not interested in modifying this kit design.
 
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atferrari

Joined Jan 6, 2004
4,593
The XR2206 is known to have some problems as a function generator, but unfortunately I couldn't find the thread where this is discussed.
At any rate, the waveforms you showed are indeed more distorted than I remember. Perhaps an issue on the output amplifier? Or a prtoblem on the IC itself?
The first time I 've read about the flaws of the chip was 20++ years ago in the Electronique Pratique or Radio Plans Loisirs magazine.
Additional adjustments were explained for one or two signals.
 

seanstevens

Joined Sep 22, 2009
121
I didnt know they still produced XR2206. I built a function generator just following the datasheet nearly 40 years ago and the outputs were excellent.
Maybe its a bad design?
 

rsjsouza

Joined Apr 21, 2014
350
I didnt know they still produced XR2206. I built a function generator just following the datasheet nearly 40 years ago and the outputs were excellent.
Maybe its a bad design?
Exar ceased production of this device a few decades ago, but others picked it up. It might as what @dl324 mentioned: perhaps a counterfeit IC plagues more modern kits.
 

Thread Starter

Wendy

Joined Mar 24, 2008
22,834
Interesting, so a pot can be put in series with the triangle / sine wave switch to clean it up? That much I can do. I am still thinking about mounting this in a better box as a function generator for my test equipment shelf.
 

Thread Starter

Wendy

Joined Mar 24, 2008
22,834
So here's a poser for our members, what circuit with a unity gain could handle this kits output through the full frequency range?
 

AnalogKid

Joined Aug 1, 2013
10,052
Are you asking about an output buffer? Freq gen in, low impedance out?

The datasheet says the typical max output freq is 1 MHz, and you need to have some gain available at that freq for negative feedback to stabilize operation and drive down the output impedance. An opamp with a GBW (Gain-BandWidth product) of 10 MHz or more should do it. For an old part, try the NE5532 configured as a voltage follower.

Whatever you use must be unity-gain stable. This rules out the NE5534 and some other fast opamps.

ak
 
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bassbindevil

Joined Jan 23, 2014
419
LM318? An even older part. I used one as a video buffer to add a composite video input to my 12" TV.
I built a function generator using the XR2206 back around 1979; I'm sure I used the trim pots and got a reasonably nice sine wave.
 

sparky 1

Joined Nov 3, 2018
716
The original Jameco electronics XR2206 kit had a good sine wave if you fiddled with the potentiometers.
Jameco kit used an off board selector and no spike. Polyester caps improve some, the data sheet shows
the arrangement for quality sine. The Chinese I believe were interested in the most power appealing to hobbyist
who would use it to power things (therefore spike bleeding into the capacitor gang) rather than the signal quality that Exar intended and seen on datasheet specs. A similar design consideration noted ICL8038 datasheet recommending a design change for the square wave to be separately sourced down. Separate output impedance/filter for high and low range should improve the sine. It's a shame the XR2206 ended up on low end carp.
 
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AnalogKid

Joined Aug 1, 2013
10,052
LM318? An even older part. I used one as a video buffer to add a composite video input to my 12" TV.
Ahh, another old classic. I used some in the video section of a custom-built character generator for an airport.

Faster than the 5532, but noiser (which doesn't matter in this application), and it doesn't have the 5532's extra-beefy outut stage (which might matter).

ak
 

DickCappels

Joined Aug 21, 2008
8,704
Back in about 1981 a recent graduate engineer was appalled at seeing hundreds of transistors in our color broadcase TV camera and decided to show us that it can be done with integrated circuits. He chose the LM318 -that did not turn out well. I was amazed at how many transistors we could have in a channel and still have a great noise figure. (Slightly off topic -sorry).

Many years earlier I used the ICL8038 in combinations with sample and hold circuits to make a Sin-Cos generator with which to rotate rasters. The new LM3900 -the first Quad opamp I saw was used. The thermal drift was terrible and I spent an entire WESCOM show standing next to the raster rotation module with a pot tweaker tool.
 
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