Slight ringing

electric550

I have attached an image of the out put of an oscope. one channel has one prop on it the other has another prop on it. both are toggling a pin. As you can see one pin has some noise on it...the board I made, and the other pin has no noise on it...the prop dev board. I was wondering if there are any suggestions for making the output of my board less noise, caps somewher...or such. Another funny thing to note was that the signals slowly moved passed each other as time went by. The props were running the same exact code at the time with the same crystals, I guess the tolerance of the crystal provides a slight difference, anyways i thought it was somewhat interesting...the oscope is not incredibly accurate but it shows something is going on.
Any suggestions are appreciated. Thanks.

Post Edited (electric550) : 8/13/2009 8:54:41 PM GMT

Peter Jakacki

Bear in mind that you are observing what your test instrument sees, and the test instrument becomes part of the circuit it probes. You have two different boards with two different grounds but which one did you hook the probe common to? Hint: Test your board completely independently of the other and vice versa and compare that with the results you have already taken.

Also, if it is indeed your board we can then only guess as to what it may be and proceed with conjecture while we are without the vital information that you possess and are yet withholding.

[noparse]:)[/noparse]

*Peter*

Leon

Your board might have some problems with the PCB layout.

Leon

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Nick Mueller

Where were your ground clips? On just one board?

Nick

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electric550

The plots are the same whether I use different grounds or the same...If I measure them both speparte they scope shows the same info.

Tubular

To me that looks more like a sawtooth waveform on Vcc, rather than sinusoidal 'ringing'. What kind of power supply is feeding your prop circuit? Swithching reg?

Fixing it may be as simple as some extra caps, or looking at the layout of your incoming power traces

tubular

localroger

That sawtooth is pretty close to 15 KHz, the TV scan rate ... do you have any pins on that board generating video that might be getting coupled to it?

Peter Jakacki

Good pick tubular, and the switching frequency looks like it's around 50KHz too, so it must be one of those ancient "simple" switchers. But that's a lot of switching ripple which is normally hard to see on the scope. So yes, it could be as simple as too little filter capacitance or even poor capacitors as low ESR and inductance is required.

If this doesn't fix it then it probably is poor pcb layout, but once again an opportunity to give us the information required to avoid conjecture has passed.

At least try more filter capacitance with "fresh" electrolytics or add a 100uF or so tantalum across the output.

I normally wouldn't convert directly down to 3.3V myself as I prefer the efficiency of switching combined with local linear regulators so I use 5V switching and small 3.3V LDO regulators. Even with a poor and noisy switcher this would still result in a clean 3.3V.

*Peter*

electric550

oh yep it is the regulator thanks...this is crazy take a look at the output of the 3.3 and 5V regulators. Thanks for the help.

electric550

I put some caps on it and it is much cleaner

Peter Jakacki

I don't know what you mean by much cleaner because those waveforms aren't even bad, they are shocking. Much cleaner doesn't sound encouraging.

Though none of us are under obligation to respond or for the OP to elaborate further it is considerate to provide some details to those who helped you find a solution. Why do we bother? We are helpful and curious to know and others can learn too. I suspect you still have a problem though. But so be it.

*Peter*

electric550

well here are the regulators with 22uF caps across the outputs. I tried a bunch of different values it seemed these provided enough filtering plus they were small enough to fit onto the board alright. I ran the switching program and the outputs have very little noise. So thanks for the help.

Peter Jakacki

electric, if I didn't laugh then I'd go mad, and if I were mad I'd still laugh. Why didn't you tell us which regulator chip you were using, the inductor value, etc etc and then proceed to tell us that you picked the value of the filter caps simply because they looked like they would do and that they were small enough to fit!!!

A typical LM2575 switcher which is what you may be using (see I have to guess) switches at around 52KHz and typically requires a 330uH inductor and 330uF of filter capacitance, not 22uF caps.

You've got me scratching my head over may things, no wonder my hair is thinning.

*Peter*

Toby Seckshund

Peter, are you sure that it is all the confused scratching that is wearing out your hair ?

Or is it your rotating hat ?

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electric550

I am using the LM1084. I have one fixed 3.3V version, and I have another fixed 5V version. I would not recommend this device. They clame to operate up to 5 amps, but let me tell you my scenario.

1. If input voltage to 3.3V regulator is above 5V and pulling 100mA regulator begins to get hot
2. If input voltage to 3.3V regulator is above 12v and pulling 100mA regulator is almost too hot to touch.

Similar characteristics occur with the 5V regulator if the differential between the input voltage and the regulated voltage become large the regulator acts poorly...this is in the data sheet but I did not expect the problem to be so bad.

On another note I am thinking about trying to come up with a design that is more efficient. ie use different regulators.
I would also like to run the device off of usb power, then have the device switch to input power when enough voltage is applied. I have been looking for an example circuit but I am having trouble finding it. If anybody has any ideas that would be good.

localroger

@electric550, if you're using an un heat-sinked LM1084 (or one sinked only to the PCB) then that's pretty normal. It's actually worse because the 1084 is a low-dropout device designed to work correctly with less voltage differential than cheaper regs can. At 12V, the 1084 is dissipating (100 mA * (12 - 3.3)V) or just under a watt. An un heat-sinked case that size will get very hot, though it probably won't fry (most semiconductors don't mind temperatures well above the boiling point of water, which will cause 3rd degree burns if you touch them, but that's why we can solder them). If it operates continuously and doesn't get too hot to touch without burning you, it's doing fine -- but it is wasting all that power. Switchers are noisy but they don't do that.