Oscilloscope – Cheap Oscilloscope Showing 16 MHz Square Wave


I own a cheap oscilloscope Hantek DSO4102C. It's rated bandwidth is 100 MHz, and sample rate is 1 GSa/s. Some info about the tool can be found here: http://hantek.com/en/ProductDetail_3_4163.html
Now I have an Atmega328P MCU running from an external quartz at 16 MHz, without any code it it (chip erased by usbasp), only CKOUT fuse bit is set. So I supposed to see a square wave at PB0 pin, but my scope shows it quite distorted:

MCU's datasheet doesn't mention a pin rise time, which was a big surprise to me, so I cannot check if measured 9.5 ns is a valid value. But judging by Pk-Pk voltage exceeding 6 volts (and even going below zero for a good 560 mV), I believe there's a problem with the scope. Am I right?

I've assembled everything on a breadboard, rather then using Arduino Uno. I've connected ground clip from the scope to the ATMega's ground pin with a wire through breadboard. I'm measuring directly at the output pin (see photo of my layout below). Now I'm getting better results, also with 20 MHz oscillator.
16 MHz
20 MHz
Layout on the breadboard
Obviously, Pk-Pk values are now more close to reality, as well as signal shape. So thanks everybody for the help!

Best Answer

I believe there's a problem with the scope. Am I right?

Don't think so. Overshoot is a perfectly normal phenomenon when measuring a fast-edge signal with a high-impedance probe. (Also, these signals look about as sharp as I'd expect them to be.)

There's many tutorials on sensing high-speed signals: this is the perfect time to read one!

Oh, and there's Gibb's phenomenon, which says that any band-limited observation of a theoretical perfect (or far less band-limited) edge will have some 9% of overshoot; to understand that, I'd recommend looking at the cosine series representation of the square wave and consider what you'll cut off when you get rid of anything above 5× 16 MHz (=the fundamental frequency of your square wave).