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

Question

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:

And the waveform 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?

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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:

Overshoot measured w/ DC Coupling:

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's soldered version:

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Edit 2:

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

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Edit 3:

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

Answer 1

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:
  

  

^{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.

Answer 2

I'm writing this as an answer because I didn't think there would be enough room in comments. Having said that, it's likely that several of the points I'm making could be the cause of your problems: -

Are you using a x10 scope probe? What does the output from pin 2 look like - schmitt triggers will not all trigger at the same point on a badly shaped squarewave from pin 2 - I can see evidence of this in the scope trace - it begins to settle then shoots off again. Chip decoupling from the picture is a little flaky.

Are you actually using 7414s - I'd recommend the 74AC14 for best speed - also double check the output current these devices can supply - in particular, some devices may not produce a decent o/p from the oscillator section given 6k8 load and 5 other inputs.

If you disconnected one of the 220R resistors and hung the scope directly onto the output (say pin 4) what does it look like?

What Vcc are you using - you say the overshoot is 200mV on top of the peak of 500mV - this seems strange - are you sure all the inverters are switching. From a 5V supply I'd expect to see a 5V peak with any overshoot on top of this.

Food for thought.

Answer 3

Per other answers and comments, I focused on bringing the overshoot down with some of the suggestions provided.

I did the following:

• shortened the leads going to and from the breadboard,
• adjusted compensation on the probes (one was slightly under compensated)

This reduced measured overshoot from ~2.4V to 1.8V (over 5V).

@AndrejaKo's suggestion had the greatest effect, however. I put the tip ground spring on the probe and measured again, this time only seeing 680mV overshoot.

Until this circuit is soldered to a PCB, I certainly don't expect much better. But this is a significant improvement from the original.

Measuring square wave output at pin 2:

Short ground path with tip spring:

The photo makes it look as though the resistor is touching the ground spring, but it isn't.

I'm not convinced that the overshoot has ever really been as high as measured (or even is really at 680mV), but that improper measuring methods have been to blame. If nothing else though, this has shown definitively that trying to measure high speed events really does require attention to things like lead length (impedance), stray capacitance, and careful analysis.

Note: I removed the resistors to the other five Schmitt triggers for the photo; the results were basically the same with/without them.

Answer 4

You have a power supply problem. Edit 3, showing VCC (yellow) and the OUT node (blue) is the smoking gun. Add capacitance between VCC and supply rail, as close as you can to the IC pins. Capacitor leads are currently much too long. I would use about 100 microfarad electrolytic, bypassed with a .01 microfarad film cap and a small ceramic, say 600 pF. Line these up as close as you can to the pins, and land the smallest one right on the pins if you can. BTW, many audio amps display this same problem. You can test them by connecting a speaker between VCC and ground, in series with a small value cap to block DC. You will hear music on the supply rails. Your objective is to reduce or eliminate this music.

Answer 5

In the original tutorial/video by W2AEW from where this circuit came Alan does mention that the circuit achieves fairly close to 50 ohm "Output**" impedance.

Your earlier post actually answered your own question but I suspect you didn't realize that you already had the answer.

From your earlier post: "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"

Your 220 ohm resistors are forming the Output impedance for the launched energy, they are not the load impedance. You then needed to feed that final output signal into a corresponding characteristic impedance to fully deplete/consume the launched energy and prevent reflections. Solution: Just add the 50 ohm load either as a load resistor or, if your scope supports it, just use the Scope's 50 ohm input impedance selection. There will also be parasitic capacitance/inductance effects but the impedance mismatching will be the dominant element at present.