# 'square wave' falls into negative voltage

#### jaygatsby

Joined Nov 23, 2011
182
I am looking at a fast 'square wave' that I've generated with a crystal at 23+ MHz, and divided down with two binary counters. The wave is not very square, but square enough to work as a clock I think. I am curious why, once per period, the wave will fall off into negative voltage (as seen on the oscilloscope) before coming back up. This happens at the falling edge of the waveform. So peak to peak becomes something like 7.6V, when the circuit input voltage is 5V.

Thanks

#### crutschow

Joined Mar 14, 2008
32,847
It's due to the ringing on the fast edges caused by stay capacitance and inductance in the circuit.

#### jaygatsby

Joined Nov 23, 2011
182
It's due to the ringing on the fast edges caused by stay capacitance and inductance in the circuit.
I did wonder if putting a capacitor between the Vcc and ground leads on a chip would lessen the issue, but it didn't.

Thanks

#### Blofeld

Joined Feb 21, 2010
83
This application note shows some oscilloscope screen shots of not very perfect square waves, together with a description of the problem. Maybe you can find some idea there to improve your circuit.

http://cds.linear.com/docs/Application Note/an47fa.pdf

(AN 47 High Speed Amplifier Techniques by Jim Williams, 1991, 5.1 MB)

Joined Apr 30, 2011
1,581
IF you're using a plug in type breadboard you can find information on techniques for minimizing stray capacitance and inductance by googling "stray capacitance breadboard". Usually the biggest issues are component placement and signal routing.

#### bertus

Joined Apr 5, 2008
22,215

Joined Dec 26, 2010
2,148
Sometimes this kind of thing may be seen when using an oscilloscope with a probe, even when there is no overshoot present in the original waveform. There are at several ways in which this can arise:

1. Ground "bounce", particularly if the probe ground lead is not as short as it might be.
2. Probe misalignment (in a compensated attenuating probe, most commonly 10X).
3. Inherent frequency response irregularities in probe or scope, not so common and should only be seen at the limit of the equipment bandwidth.
Ground bounce gets to be more of a problem with wider bandwidth scopes, so that at higher frequencies the ground connection length may need to be reduced by using a short spring contact rather than a wire and a crocodile (alligator) clip.

#### MrChips

Joined Oct 2, 2009
29,198
It also depends on what you call an oscilloscope. If you are using a sound card as an oscilloscope, the sound card is AC coupled and so will not have DC response. Also if your scope or scope probe is set for AC input the same thing will happen. You need a scope and scope probe whose frequency response goes all the way down to DC.

#### jaygatsby

Joined Nov 23, 2011
182
Thanks everyone for the knowledge!

#### SgtWookie

Joined Jul 17, 2007
22,230
The inductance of the wiring and the capacitance of the probe can create a series resonant LC circuit that will ring like the dickens when the output of the IC changes states. Adding some resistance in series will quiet down the ringing, at the cost of increasing the rise/fall times.

See the attached.

#### Attachments

• 132.4 KB Views: 30