Electronic – What can reduce overshoot and ringing on a simple square wave pulse generator

impedance-matchingringing

I've built a simple RC and Schmitt-trigger-based square wave pulse generator. On the breadboard, it has some obvious unwanted qualities due to jumper length, the breadboard itself, etc.

Schematic and breadboard version:

Fast Edge Pulse Gen Schematic!
Fast Edge Pulse Gen Breadboarded

And the waveform output:

Overshoot and Ringing Output

In particular, the rising edge of the square wave has a substantial amount of overshoot (about 200mV over 500mV peak) and ringing. It is easy to make it worse, by physically touching R1. See edits for correct info.

In looking for solutions I've ran into terms like snubbers and dampening for RF circuits and things beyond my hobbyist pay grade.

Anindo suggests in an answer to a related question that one should use a 50Ω resistor for a load. I am measuring the output from the first Schmitt trigger (IC1D, at pin 2). The remaining triggers are used with 220Ω resistors to create an approximately 50Ω impedance, but I get almost identical results measuring at the output node.

This fast-edge pulse generator is purely for my own experimenting/education, so there is nothing critical about it. If I decide to make a soldered board of it, what sort of things can I do to ensure it's better than its breadboard cousin?

Edit:

I mistakenly was in AC coupled mode for the previous screenshots and measurements. Here are some more screens showing the signal at pin 1 and 2 of the IC (input triangle wave on 1, output square on 2). They are now DC coupled. The probes were always in X10 but the scope itself was in X1 (brand new scope, oops!). The overshoot however is still significant: on the output which is 0-5V, the overshoot (shown by the dashed white cursor lines) is 2.36V. Note that the overshoot on the input is only about 500mV. Is the input ripple due to the proximity of pins 1 and 2 on the breadboard?

Input (ch. 2/blue) on pin 1, and output (ch. 1/yellow) on pin 2:

FEP Input on Pin 1 and Output on Pin 2, 100us Time Base

Overshoot measured w/ DC Coupling:

FEP Overshoots, DC Coupled, 50ns Time Base

Removing resistor R2 and measuring at pin 4 (IC1E output) did not yield any noticeable difference from the signal at pin 2.

I should mention that the original tutorial/video by W2AEW from where I got the information for this circuit also has some overshoot, but not to the degree I have. His circuit is soldered on a board which probably helps a lot.

Original author's (W2AEW) waveform (at node OUT) with maybe 500mV over 5V:

Original Author W2AEW Scope Pic

Original author's soldered version:

Original Author W2AEW Soldered Circuit

Edit 2:

Here's a picture of the overall setup including lead lengths to the PSU and scope:

Overview

Edit 3:

And finally, VCC (yellow) and the OUT node (blue) on the scope to show the coinciding ripple:

VCC and OUT, coinciding ripple

Best Answer

From the looks of the new scope traces added to the question, specifically the Vcc trace, it appears that the ringing is originating in poor regulation of the supply at the point of use - most likely not at the bench supply output. While shorter leads from the bench power supply will certainly help by reducing lead inductance, that won't be enough when the transition is as sharp as you're looking for.

  • Add a hefty capacitor on the breadboard across the supply rails, closest to the IC: Start with 100 uF.
  • In parallel with the 0.1 uF decoupling capacitor shown in your schematic, and touching the Schmitt Trigger supply pins, add a 10 uF electrolytic capacitor.
  • Trim the leads of all 3 capacitors above to the bare minimum which will still make positive contact with the breadboard contacts. Those leads are adding inductance you don't want.
  • Add a load from the output you are reading to the ground pin, as close to the output pin as possible - 220 Ohms should be fine, and again you want leads trimmed to minimum.

  • If you absolutely must avoid overshoot / undershoot beyond a few hundred milliVolts, add small signal Schottky diodes from the output pin to both supply and ground pins, thus:

    schematic

simulate this circuit – Schematic created using CircuitLab

  • This will ensure that the peak on the rising edge and trough on the falling edge of the ringing are damped - there will be some effect on the respective trough / peak of the ringing as well because of the excess energy of the peaks being dissipated across the diodes.
  • Finally, the breadboard, due to the nature of its construction, introduces capacitance, inductance, and all kinds of parasitic coupling. Even a simple perf-board will do better. Long leads simple exacerbate this problem, especially at high frequencies / sharp transitions, where even a simple wire lead is a source of coupling and inductive ringing.
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