Cross Over Distortion? - Page 1
Forum section: electronics / beginners

author: r00n
author_rank: Contributor
timestamp: October 29, 2024, 02:26:13 am
content: Hi,I am getting what I believe to be cross over distortion on the output of the sawtooth oscillator as in this schematic. You can see this in the scope screenshot attached, and I have no idea what may be causing it. My trouble shooting skills are emerging but need help understanding where this is coming from. It doesn't appear to be that way when I look at the output at pin 8 of the 3340 chip. I changed the opamp and its still there, but this time rather than jump up to a voltage, it goes to 0V and hangs for a few ms at 0v before the saw starts to rise again. Additionally, why could there be so much DC offset in the square wave? As the VCO warms up the DC offset does settle down a bit but never completely goes. I hope I have been clear enough and thanks for taking the time to assist me.with kindness.

author: magic
author_rank: Super Contributor
timestamp: October 29, 2024, 05:56:34 am
content: Any larger version of this schematic?I'm not entirely sure where you are probing these signals?The blue trace is weird indeed, it looks like DC offset suddenly increases after zero crossing and (partly) stays that way.Quote from: r00n on October 29, 2024, 02:26:13 amThat's typical behavior of LM358/LM324. If this happens on the output of an opamp while the input to the opamp looks normally, and if the opamp isn't supposed to be one of those types (not sure, does the schematic say TL074?) then I would suspect a fake part.

author: Kurets
author_rank: Regular Contributor
timestamp: October 29, 2024, 06:07:25 am
content: Post a high resolution version of your schematic. The exact blog post where I believe you got the pixelated one from has a higher resolution version too.W.r.t. your question, as the circuit for buffering the sawtooth is just a voltage follower connected opamp, any distortion will be dependent on the specific chip used. Are you using a genuine TL07x?The same goes for DC offset on square. The circuit shown is just an opamp buffering. If it must swing close to the rails, how close it can get depends on the amplifier specs. In the schematic I found, this was also a TL07x which is not able to swing closer than 2 or so V from either rail. Again, are you using genuine components?

author: r00n
author_rank: Contributor
timestamp: October 29, 2024, 09:50:57 pm
content: sorry 'bout that low resolution picture. The schematic is a link to the one on Eddy's website. I downloaded it and what is attached is a higher rezz one.I think the strange behaviour is happening on all the outputs. Check out the new screenshot of the scope with a square wave at 50% duty cycle. It's going crazy on the crossover point too. I assume the triangle wave is doing the same but it's not visible at the apex.I have everything on a pcbway printed board. I made a few of them and the others are working as expected.I would welcome any and all suggestions at this point.with kindness.

author: r00n
author_rank: Contributor
timestamp: October 29, 2024, 09:53:58 pm
content: sorry forgot.I am using TL074 Texas instruments IC I believe. it should be of reasonable quality as I purchased it from Mouser.

author: BillyO
author_rank: Super Contributor
timestamp: October 29, 2024, 11:17:56 pm
content: Your 2nd scope screen shows a completely different set of problems.Probe pins 4, 8 and 11 of the AS3340. It may be it that is having the issues.

author: r00n
author_rank: Contributor
timestamp: October 31, 2024, 11:57:12 pm
content: hysteresis on the crossover point.for those that have may this issue the solution is in the datasheet, and Eddy Bergmans schematic has omitted it for some reason. The 3340 chip benefits from a 1M resistor across pin 4 and 5 - square wave output and PWM control input pins. Check out the datasheet.