Problems getting Triangle from XR2206

[Slipnfall]

Hi,
I built a PCB around a design example in the Exar 2206 technical
document. I followed closely to the 'test' circuit, but am unable to
get a triangle wave output. I have of course checked connections and
swapped ICs. My SYNC and Sine out work properly. However when I open
the connection from pin 13 to 14, I should get a triangle(13 and 14
are the THD adj pins). When I open the pin, I get the *same* output,
no jitter, no lower Vpk-pk, nothing different. Any suggestions? When
jumpered or unjumpered, I can view a ramp signal on those pins.

Oh and one quick note - why can't one adjust the square/sync output
for duty cycle?? If this is indeed a Sq/Sin/Tri generator, all three
should be adjustable for d/s. I have used the MAX038 before and it
seemed more feature robust.

Thanks in advance,
-Jamie

 

[Roger Johansson]

I built a PCB around a design example in the Exar 2206 technical
document. I followed closely to the 'test' circuit, but am unable to
get a triangle wave output. I have of course checked connections and
swapped ICs. My SYNC and Sine out work properly. However when I open
the connection from pin 13 to 14, I should get a triangle(13 and 14
are the THD adj pins). When I open the pin, I get the *same* output,
no jitter, no lower Vpk-pk, nothing different. Any suggestions? When
jumpered or unjumpered, I can view a ramp signal on those pins.

Are you really sure none of the pins 13 and 14 are connected to anything
at all, including long copper tracks or wires?

Then the chip is faulty, but it is more likely that you have made a
mistake in your wiring with these connections or somewhere else in the
circuit.

These pins are very sensitive for capacitance and long leads, any
connection to them should be kept as short as possible, something like
10 mm is okay, more can lead to problems with the wave form for sinus.

If you study the sinus wave output in an oscilloscope you may find that
there is a fast pulse at the top of each sinus cycle, that is a sure
sign that you have too long leads connected to these pins.

You could try to find other schematics for 2206 and compare, and maybe
change something in your circuit if there are other solutions.

Do a google search for
2206 circuit
and you will get some documents, like this:
http://www.google.com/url?sa=U&start=1&q=http://www.nitehawk.com/rasmit/s
emifaq.html&e=912

 

[Jon Yaeger]

Are you really sure none of the pins 13 and 14 are connected to anything
at all, including long copper tracks or wires?

Then the chip is faulty, but it is more likely that you have made a
mistake in your wiring with these connections or somewhere else in the
circuit.

These pins are very sensitive for capacitance and long leads, any
connection to them should be kept as short as possible, something like
10 mm is okay, more can lead to problems with the wave form for sinus.

If you study the sinus wave output in an oscilloscope you may find that
there is a fast pulse at the top of each sinus cycle, that is a sure
sign that you have too long leads connected to these pins.

You could try to find other schematics for 2206 and compare, and maybe
change something in your circuit if there are other solutions.

Do a google search for
2206 circuit
and you will get some documents, like this:
http://www.google.com/url?sa=U&start=1&q=http://www.nitehawk.com/rasmit/s
emifaq.html&e=912

Popular Electronics had a project for a crystal-controlled digitally
programmable oscillator based around the Exar 2206. I've got a copy of the
article and built a pair of them. It works pretty well, but the Exar seems
a bit flakey and IIRC, I had to replace then because of faults. Simplest
thing might be to try another.

Jon