Who knows the XR2206 function generator?

[Jidis]

Yeah, I know it's obsolete old junk, but I'd like to see it "go".

I was wondering if any of you guys who have dealt with it might spot anything obviously funky in this schematic before I go through too much headbanging or desoldering. I copied it from a design on the web, but I'm not sure if this schematic is exactly the same as whatever the kit had in it, or if there's any possible flaw. Only difference between mine and theirs is that I've got an NP box cap in that C8 220nF position.

Doesn't seem to output anything but a nice 60Hz sine from the AC and doesn't respond to any switch/pot movements. All power and ground pins on the ICs are getting the correct levels and all cap polarity is correct.

FWIW- I initially built a more barebones XR2206 circuit and was wondering what improvements this more elaborate version might provide, but that also allowed me to test the chip in a known good circuit, so I guess I can rule that out as a culprit.

I'll poke at it some more tonight and post back if I can find anything.

Much Thanks!

PS- I'll stick the example schematic from their datasheet on here too if it helps any.

[Deathwish]

you say all the pins have the right voltage but I dont see any connection or reference to pin 4 VCC on ic1 in your circuit diag

[Jidis]

Hi Deathwish,

They've got the power pins and associated parts for everything but the opamp separated from the rest of the diagram. They're over on the right.

-Thanks!

[mij59]

What kind of power supply are you using ?
What is the voltage of pin 2 with respect to GND ?

[Jidis]

Mij59,

Right now, it's a DC wall wart. I think it outputs something like 17V at no load. Now that you mention it, I may try another one, but I figured I was OK on that, as the 7812 was outputting 11.77 or something and the 7805 is right around 5V. Neither of them seemed hot either.

Are you talking about the XR2206's pin 2 (the main wave output)? I'll get it back up and check that.

Thanks!

[Jidis]

Mij59,

I owe you one! 

Somehow the supply seems to have been it, despite what I thought were normal readings on the regulators. I switched to my bench supply and dropped it down to 15V and suddenly everything worked. Odd thing is, it looks like my regulator output levels are the same as they were before. By the way (with the working supply) from main ground to that 2nd pin of the 2206 is around 5VDC right now (seems to have changed since I've been playing with it). From the floating ground, it's about -0.5, but was a +0.3 at first.

On a wonderful side note, it appears to *not* be plagued with the nasty spike that the basic circuit seems to have on the crest of all the sine waves. I pushed it up to the top (377kHz) at max amplitude and it never showed up. I've got a pair of resistors (200ohms total) on that distortion adjustment loop. I may fine tune that to see what it does too.

Thanks again!

[mij59]

Nice find! 

The 60 Hz obviously the mains frequency, so check the wall wart.
May be its just a transformer with a bridge rectifier.

To check if the output of the voltage regulator is working properly use the ac voltage setting of your multi meter, reading should be (close to) zero.

BTW, Scullcom has a nice video on the XR2206 https://www.youtube.com/channel/UCDqryeq1kMDSEQwltWqASrA

[Jidis]

Much thanks for that link too mij59. Watching it now. Looks like an even more elaborate version of the one I just tried. Surprisingly new too (last month). I'll be waiting on the second half and may think about trying it.

It's interesting to see the range of circuits that the 2206 and that Maxim chip show up in. The smaller one I tried was about 4 or 5 resistors and caps, with no extra ICs, but I'm guessing those sorts may also have something to do with the horrible reputation those chips have these days.

Take Care

[nitro2k01]

I've built the following circuit, which is using the XR2206 as an audio oscillator, and I know this circuit works. You could compare your schematic to this one, or maybe even build this one without modification if it fits your needs.

http://www.birthofasynth.com/Thomas_Henry/Pages/XR-VCO.html

[Jidis]

If anybody knows enough about the how the distortion and symmetry controls are implemented-

I was wondering if they're "set and forget" type of controls specific to each unit, or if there are different conditions where you need to re-tweak them. I went ahead and hung them off this one I built to see what they did, and was wondering if I could just replace them with equivalent fixed resistors once I find the values for minimum distortion. I didn't really design the enclosure to accommodate any extra pots.

Thanks for the link too nitro2k01!

[Alex Eisenhut]

Can't beat this IMO.

https://archive.org/details/radio_electronics_1988-05

The interface of that site is awful, but the content is there.

[Jidis]

Can't beat this IMO.

https://archive.org/details/radio_electronics_1988-05

Thanks! I hadn't been there before. They've probably got a bunch of articles I'd like. That version of the generator makes me wish I could justify building one. I've even got a couple of those enclosures on hand, but when I started playing with these things, it was mainly just to get sines and squares. I'd be wasting much of the functionality.

The reason I got into the distortion & symmetry tweaks was that my XR2206 circuit has some weird habit of turning its sines into some half wave thing where it lops off the negative side. It would usually happen when I'd connect it to my distortion meter, so I couldn't actually see why the distortion was so high. I'm going to try to figure out what it is when I get a chance.

Speaking of undistorted sines though, this Twin-T thing surprised the heck out of me. It shows as being all the way down in the hundredths of a percent, though it only outputs a single frequency. I think component tolerance may be critical though. The through hole version I built has much higher distortion than the SMD and doesn't use the exact specified values.

Take Care

[Deathwish]

https://ia802704.us.archive.org/12/items/ExarFunctionGeneratorDataBook/ExarFunctionGeneratorDataBook.pdf

[Jidis]

Sorry to drag this ancient thread back up, but I had put that generator aside due to the wacky half sines I mentioned in post eleven and I have other stuff which serves the same purpose.

I got curious about it the other day and breadboarded a clone to try to see what was causing it. Comparing differences in the schematic I used and Exar's datasheet, it seems to be in the parts tied to that amplitude pot on the 2206's pin3. Everyone other than that schematic I used seems to include that pair of 5.1k resistors between the pot and Vcc/ground. Adding them seems to straighten things up here, but I was wondering what the ideal way to implement that would be, considering mine is using this opamp based floating ground. Are the earthbound 5.1k resistor and the cap in the datasheet now taken to AGND? In the schematic I had, there is a 1uF cap in a similar location, but it's tied to the regular circuit ground. If they do float, are they, and the ground for the wave output jack the only things which should be using AGND?

Thanks    (in the picture, the faulty schematic is at the top, Datasheet version is bottom)

[bd139]

Exar's datasheet is shit. I couldn't get the XR2206 to do it's thing with the two 5.1K's in place (I used two 10k in parallel). Seems to be a crap attempt at a virtual ground. Have a look at the Exar Applications Handbook here, particularly AN-14 though - this one works!:

https://archive.org/details/Exar-ApplicationsDataBookOCR

You need two rails for this. The amplitude is actually ground referenced.

Really though, if you're going to do this, use the XR2206 to generate the waveform (say 1v p-p each mode) and control the DC offset and amplitude after the generator with a summing op amp and pot. It doesn't like getting close to the rails and malfunctions heavily.

[Lee Leduc]

Exar TAN-005 application note for the XR2206.
Also the instructions are available for the Jameco XR2206 kit at https://www.jameco.com/Jameco/Products/ProdDS/20685KitInstructions.pdf

[Jidis]

Thanks for the replies! I wasn't aware Exar had a bunch of other books online. I'm going to look over that stuff when I get back and try to keep the breadboarded circuit together until I work something out.

bd139- To be clear, the grounding scheme I used on that one I etched is the top one in the picture, which uses a 741 opamp, but obviously it didn't work as it was supposed to. If those two resistors are serving a similar purpose, maybe that's why they weren't in mine, but they seemed to be the one thing that cleaned up the wave.

Lee- Looks like I had that 005 ap note in my folder already. I'm still not getting what they claim on distortion though. I bought two of the chips, and the other is on a simpler circuit with no add-ons, and I haven't seen either version get lower than 2.5% (assuming my meter is accurate). The more elaborate one has the two trim pots, and 2.5 seems to be what it gets after tweaking. I may be expecting too much of this chip from the way some of you guys talk about it though. That supply issue bd139 mentions doesn't sound promising, and I'm also running eBay chips, so who knows if they're even up to spec.

Thanks!

[Jidis]

Am I doing something wrong with my test connections to this thing? The sine looks clean if I use the main circuit ground, but if I clip my scope probe's ground clip to the floating AGND (which is where the wave out is supposed to get ground), I get the attached picture. Same holds true if anything else gets grounded to AGND (other than the amplitude trimmer) or it somehow makes its way back to that main ground.

AFAIK, the test gear, including the regulated supply that's feeding the XR2206, is all getting AC/Ground from the same strip. I thought the whole point of a floating ground was to avoid these sort of conflicts. Seems anything you use this output signal for is going to have some path back to it from its power source. I'm also figuring if it were some sort of ground loop issue, it would just be noisier or something and wouldn't instantly jump into this weird single pole shape. I'm feeding it a clean 12V, but it stays roughly the same shape at lower voltages. Am I missing something?

Thanks Again!

[MickM]

Hi;
   I watched this a couple of days ago:

"Fixing the issues with the XR2206 Function-Generator from EBay ".

[bd139]

Am I doing something wrong with my test connections to this thing? The sine looks clean if I use the main circuit ground, but if I clip my scope probe's ground clip to the floating AGND (which is where the wave out is supposed to get ground), I get the attached picture. Same holds true if anything else gets grounded to AGND (other than the amplitude trimmer) or it somehow makes its way back to that main ground.

AFAIK, the test gear, including the regulated supply that's feeding the XR2206, is all getting AC/Ground from the same strip. I thought the whole point of a floating ground was to avoid these sort of conflicts. Seems anything you use this output signal for is going to have some path back to it from its power source. I'm also figuring if it were some sort of ground loop issue, it would just be noisier or something and wouldn't instantly jump into this weird single pole shape. I'm feeding it a clean 12V, but it stays roughly the same shape at lower voltages. Am I missing something?

Thanks Again!

I haven't tried it with a floating ground. I used two rails.

Check the voltage between your XR2206 floating ground and your scope's ground. If there are more than zero volts there then when you clip the ground probe of the scope to the output it is not going to work as it's shorting the bottom half of the supply out.

You can AC couple the output however! Connect your scope ground to the real ground (not the virtual one) and stick a 10uF or so capacitor (negative side to the scope probe) between the probe and the XR2206. That will block the DC component. That should let you know if it's an issue with the virtual ground or the generator itself.

[Jidis]

Thanks Mick, hadn't seen that yet. I guess on the bright side my chip doesn't go haywire on higher voltages like his does, so maybe that's a good sign as to its authenticity. It didn't start bouncing until up past 18V here, but that's on this messy breadboarded one. I'm going to finish that video tomorrow and try to see that other one he mentions as well, though these circuits seem to vary pretty widely, so I'm not sure how much will apply to this one.

Still baffled as to how this floating ground is supposed to act if anybody has any ideas.

Take Care

[Jidis]

bd139-

There is indeed a significant difference between the two grounds (5.98V). Is there any solution to that, or something else to check? Using the real ground to test doesn't really tell me much in this case even with the probe filtered through that cap, as it doesn't malfunction until that floating ground is in use. It looks OK even without the cap.

Do you see anything inherently wrong with the way they did that opamp ground section?

FWIW- I double checked the bench supply and scope's power source. They're plugged right next to each other on the same power strip and the ground continuity measures fine between them.

Thanks!

[bd139]

I'm wondering if that's a dodgy XR2206. Where did you get it from?

[Jidis]

I'm wondering if that's a dodgy XR2206. Where did you get it from?

eBay and probably China (...hangs head in shame  )

Like I say though, it doesn't screw up at higher voltages like some, but who knows if they all have the same stuff wrong with them. I can't remember if the auction listed it as authentic or not.

Take Care
<EDIT> - To add- I just attached one of the Twin-T sine wave oscillators to the same opamp ground circuit, and the wave from that stays clean using the floating ground or the regular one. The XR2206 does however have that additional pot using the floating ground though.

[bd139]

Ahh that might be it.  They are out of production now so it might be a hooky one.

I found a source here: http://www.taydaelectronics.com/xr-2206-xr2206cp-xr2206-monolithic-function-generator-ic.html

They actually have mid 90s date codes on them which is a good sign and they work properly. I've had 3-4 of them from there.