op amp oscillator distortion

[chopcooey]

hello there, i am currently making an analog function generator for a project; i know my design could be improved, but i have a problem from which i have no idea the source is.

Basically, i am using an op amp(opa602) wien bridge oscillator to generate sine waves, and varying frequency with a potentiometer. At this point i have no problem, im obtaining clean sine waves at up to higher than 1.5 MHz, as long as my closed loop gain of 3 is well adjusted.

Then, im using the sine wave output to drive a comparator(tried with and without hysteresis) circuit (with op amp lm7171), and i generate decent square waves of the same frequencies. But my problem is that, when i plug the output of the sinewave wien oscillator to the comparator, my sine wave becomes more and more distorted as frequency increases.




I could probably use an extra switch to stop powering the comparator for when i want a sine wave but that's not really what i wanna do. I could probably also make a square wave oscillator seperatly instead, but i would rather have my frequency control in just one spot. Has anyone any idea as to what creates that distortion and how to deal with it?

 

[1/2dime]

If you would post the schematic....., by your description I would think you need a decoupling circuit between the 2 functions. E

 

[chopcooey]

If you would post the schematic....., by your description I would think you need a decoupling circuit between the 2 functions. E

The capacitor values are something like 100pF, and the diode circuits serve as a gain limiting element (as opposed to the op amp saturation) The first op amp is opa602 and the second is lm7171. The 2 pots must be at the same value (i used digipots)

 

[chopcooey]

oops i forgot a resistor in series with the diodes at the second op amp output in my schematics

 

[Venkadesh_M]

That may due to slew rate problems, try google it..

 

[kam1787]

your 'comparator' is much too crude. use a proper comparator [hi speed] or Schmitt trigger

 

[LvW]

The capacitor values are something like 100pF, and the diode circuits serve as a gain limiting element (as opposed to the op amp saturation) The first op amp is opa602 and the second is lm7171. The 2 pots must be at the same value (i used digipots)

I am a bit surprised that you get oscillations at all - because your circuit is NOT a WIEN type oscillator.
The correct circuit normally does not need any decoupling stage.
More than that, what about a gain factor of 3 - with two equal resistors in the negative feedback path?

 

[chopcooey]

I am a bit surprised that you get oscillations at all - because your circuit is NOT a WIEN type oscillator.
The correct circuit normally does not need any decoupling stage.
More than that, what about a gain factor of 3 - with two equal resistors in the negative feedback path?

omg so much fail sorry i just watched my schematics and it's totally wrong. here's a picture i found of the exact same circuit

Also, i did something that somewhat corrected the problem, i put a capacitor between negative supply rail and ground. I think it has to do with decoupling/coupling and DC/signal ground since the capacitor acts as an open circuit to DC and short circuit to AC if the capacitor is large enough, but still i don't understand exactly why i needed it in my case.

As for the comparator, i used an op amp with a very high slew rate to make sure i had clean square waves, but im not sure what an ideal comparator would be for my purpose (comparing sine waves from 0 to 1000 kHz, and wether i should add hysteresis or not. Any recommandations? Thank you for the help

 

[tgootee]

There are some things that you need to do better, in terms of both your circuit and trying to get help with it.

First, until you learn exactly why and how, ALWAYS, always (always!) put a 0.1 uF X7R ceramic cap from EVERY power pin to ground, for EVERY active device, with no more than 2 mm between cap and pin, and the shortest possible ground connection, to the load or device ground. And ALWAYS put a 10 uF or larger aluminum electrolytic cap in parallel with that 0.1 uF. For higher-power devices, use a larger electrolytic cap.

Second, post YOUR schematic, not one with missing components or values, or no power supply connections, or input or output unlabeled, etc. Always label with your power supply voltages, power supply ratings if possible, and as much other information as possible. It would also be extremely helpful to have a photo of your circuit, since layout and wire length and placement can easily cause problems at these frequencies.

Your schematic doesn't have to be fancy. A hand-drawn sketch that you capture with your cellphone camera would be just fine. The information on it is what matters. And a photo of the actual circuit will help tremendously, sometimes.

In your comparator circuit, you might want a resistor below D3, in series with the ground, for current limiting whenever D3 is forward biased.

 

[LvW]

Chopcooey, as another question:

Earlier you mentioned a potentiometer for frequency adjustment.
Again, I am surprised because it is not possible to change the frequency of a WIEN type oscillator using a single potentiometer only without influencing the oscillation condition (Barkhausen).