XR2206 Sine distortion problem

[Neil Groves]

Why you dont use a DDS Function Generator IC like the AD9833 or AD9834?
This Parts Generate Sine Waves by an Sine Table and a D/A converter.
The Output Filter can be made passive an must be configured for the maximum needed frequency.
The dissortion increase as higher the frequency. Maximum is 12,5MHz for the AD9833, but then the Sine Wave has harmonics. I would end at 4MHz.
The generated frequencies are quite exact ( about 0,3Hz - appends from the Clock Source ) and append only from an external Crystal Clock Generator an a frequency tuning word.

It have 2 Outputs Output 1 Sine/Triangle. Output 2 Rectangular.
Output 1 has 2 Ports with an differential output that offers 180° deg phase shift.

The Chips can be steered by an microcontroller via an SPI Interface.

I already have the xr2206 I.C so would like to examine all possibilities since it cost me $16, if all failes though i will take a look at the chips you use.

thanks

Neil.

 

[Neil Groves]

I know this is an old thread and so don't want to keep it active for too long, just had to let you know that i played around with the circuit this morning and with a 1K resistor across pins 13 & 14 of the xr2206 and a 2K2 pot in parallel with the 1K, i am able to adjust for a perfect sinewave from very small (didn't measure that amplitude) to excess of 3v p-p although i still have that glitch top and bottom of the waveform at the higher frequencies (15 Khz-20.1 Khz), i am using the unit to measure frequency response in audio filters so do you think this glitch is going to be a problem?

All i need to do now is sort out my cap values for my frequency ranges, at the moment i have 1Hz-100Hz, then 100Hz-20.1Khz, i need to adjust so i get 1Hz-10Khz, then 10Khz-20Khz, i think i am nearly ready to box it up and call it a done project, i am sooo excited.

Neil.

 

[crutschow]

The glitch probably won't affect your measurements significantly since the high frequency glitch will like be removed by the filter. It somewhat depends upon the desired accuracy of your measurements and the instruments you use to measure the response.

 

[Neil Groves]

Oh one other thing? Since i am going to go to 0.1 Hz at the lower end, is there a way i can get a ramp waveform out of this device? i have no idea what i could use the triangle waveform for but i could use a ramp at very low frequency for sure. i am aiming for 0.1Hz-20Khz as a final range.

Neil.

 

[ericgibbs]

Oh one other thing? Since i am going to go to 0.1 Hz at the lower end, is there a way i can get a ramp waveform out of this device? i have no idea what i could use the triangle waveform for but i could use a ramp at very low frequency for sure. i am aiming for 0.1Hz-20Khz as a final range.

Neil.

hi Neil,
Ref your other thread about AC coupling from the XR2206.
To Ramp sweep from 0.1Hz to higher frequencies you will need a VERY high value coupling capacitor.!

I would recommend a low cost OPA, powered from dual supplies, after the XR and before your 'audio' on test equipment.

With a single turn pot and this OPA you could set the output wave form to swing thru 0V or apply a negative or positive offset to the signal as required, no coupling cap .

 

[Neil Groves]

Thanks Eric, that makes sense but i'm not sure wether to use an offset or not, i guess not as most audio equipment uses single rail supply's therefore would expect an input signal between ground and supply....sorted!

ok i now have my circuit, i have got a good set of waveforms, square, triangle and sine, at range 1: 0-95Hz range 2: 79Hz-8.8Khz and range 3: 8.5Khz-630Khz, thing is i don't need anything above 20Khz, so how do i limit range 3 without affecting the other ranges? apart from that, the aforementioned distortion becomes apparent at 8Khz and gets progressively worse as frequency rises.

Neil.

edit: my timing cap values are 10uf, 0.1uf and 1nf respectively.