Are these oscilloscope readings right?

[ardilla]

I just bought myself a new oscilloscope in order to take my hobby to the next level. I wrote this simple Arduino square wave sketch and stuck a probe in pin 6.

void setup() {
  pinMode(6, OUTPUT);
}

void loop() {
  digitalWrite(6, HIGH);
  delay(100);
  digitalWrite(6, LOW);
  delay(100);
}

In the attached screenshots you will see the square wave and two levels of zoom into its rising edge. My questions are:

1. Why am I getting a max 4.6V reading? When I disconnect the probe and measure using my multimeter it reads 4.98V. I made sure I calibrated the probes following the manual instructions.

2. At the top of the wave there is about 1µs of "wobble". Is this normal? I tried sticking a 10µF capacitor on the power rails and the wobble smoothed out about 20-30% but was still there.

[Grumpy_Mike]

1. Why am I getting a max 4.6V reading? When I disconnect the probe and measure using my multimeter it reads 4.98V. I made sure I calibrated the probes following the manual instructions.

Are you sure the calibration is correct, what to do get when you measure a DC voltage with both the meter and the scope at the same time.

2. At the top of the wave there is about 1µs of "wobble". Is this normal?

This is called ringing, it is due to the inductance in the scope lead and the miss match of impedance between the lead and the scope input. It is not real but an artifact. Part of the skill in using a scope is to recognise the difference between the two. There should be a compensation screw on the scope probe you can adjust to get it better. However, you can pay several thousand dollars on a good scope probe alone so don't be too hard on it or expect too much of it.

[polymorph]

Placing the ground clip of a scope probe is a science, and a bit of an art. Put it in the wrong spot and ground currents show up as if they were on the probe tip. If you lengthen the ground wire or just loop it near the wrong part of the circuit, the ground wire itself has currents induced in it that again show up as if they were signals at the probe tip.

Use a scope that is connected to the ground connection on the power plug to connect to a piece of electronics that is also connected to that ground, or to a separate ground, and your readings can be upset by currents flowing through those grounds.

I grew up using analog scopes, and was taught (and observed) that a scope is not an accurate voltage measuring device. Obviously that has changed with digital scopes. I would also like to know how you measured voltage of a square wave with a DMM. Did you mean that you set the output of the Arduino to a High and then measured it?

Have you connected both the DMM and scope to a known 5V source at the same time to compare and calibrate?

[MarkT]

To get accurate ring-free signals into a scope is hard - you need a low impedance
or active probe and the signal and ground connections to your board need to be
as close together as possible to avoid picking up induced noise. Standard scope
probes are really meant for analog circuitry, not logic. Most of the ringing is likely to
be due to the big loop formed by the ground lead and croc-clip - that loop is both
a large stray inductance and a magnetic loop antenna picking up nearby signals.

Where you have stray inductance and capacitance, but only high resistances (1M
for scope input), you have resonant LC circuits - its the resonances in the system
that cause ringing whether artifacts of the probes or really in the circuit
(for instance MOSFET H-bridge output).

[ardilla]

Thank you all for your answers.

As it turns out, I tried reproducing the same setup this weekend and couldn't; this time the voltage was 5.08V and the rippling was almost nothing compared to what I measured initially.

Based on the comments, I played around with the probe's cable, alligator clip positioning, and cable curl. Indeed they have a lot of influence on what the oscilloscope picks up. I also tried positioning the ground curl over different parts of my circuit and sometimes the readings were unusable. Didn't know the damned cable could have such a drastic effect!

If any other newbie like me is reading this and considering buying an oscilloscope, don't think about it twice and go ahead. Yes, they can a little expensive but what you can learn from hooking up even the simplest components is invaluable. Just this weekend I tried a few experiments with capacitors/diodes and being able to measure and SEE capacitor discharge times and see how diode rectification works just blew my mind!

[cmiyc]

ardilla:
If any other newbie like me is reading this and considering buying an oscilloscope,

If I could only have one piece of test equipment in a lab, it would be a two channel scope. If you get desperate enough, you can even measure current with it (using a small shunt resistor.)

[polymorph]

And unlike the older analog 'scopes, the inexpensive ones anyway, even an inexpensive DSO like a Rigol can do accurate voltage measurements.

[ardilla]

[quote author=James C4S link=topic=223951.msg1630370#msg1630370 date=1394597460]

ardilla:
If any other newbie like me is reading this and considering buying an oscilloscope,

If I could only have one piece of test equipment in a lab, it would be a two channel scope.[/quote]
Yeah, I've had it for a week or so and don't know what I'd do without it. And I probably know how to use 1% of its full feature set. I kind of went overboard and bought an OWON DS6062. This device has more than enough to last me my whole "hobbyist" life.

[polymorph]

That's good! Better too much scope than too little.

Cheap tools are expensive.