make a square wave even squarer? - Page 1
Forum section: electronics / projects

author: dentaku
author_rank: Frequent Contributor
timestamp: July 18, 2016, 12:16:56 am
content: I've been messing with a circuit called the Hip Bass Drum.http://electro-music.com/forum/phpbb-files/hip_bd_schema_724.gifAt the moment I'm using my little DDS function generator to trigger it and was wondering what would happen if I could make this square wave even sharper (faster rising edge).I checked to see what a 4050 buffer does to the signal and it's not any sharper, in fact it's slightly rounder but I expected that.Question:Are there any other techniques for making a square wave have a sharper rising edge?It probably doesn't make any difference in my case but it's just an experiment to see what I get when I give the gate-to-trigger part of the circuit (the filter and comparator to the left) as much high frequency content as possible.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 18, 2016, 01:04:18 am
content: How sharp are we talking? 4050 should be ~100ns at modest voltages, unless you've loaded the output with a lot of capacitance.Which leads to a better question: how much capacitance do you need to drive?I don't see how a drum sensor should care, as long as it's sub-ms (typical timing for piezo drum pads, assuming it's made to use them).For the circuit in question, rise time only needs to be short enough that the input coupling / differentiator does its job, so, sub-ms. And there's very little capacitance (other than the one), so it won't take much drive.You do need a minimum pulse width, probably 10s of microseconds. You may have better results with a one-shot timer to extend very short pulses, or simply set your DDS to give longer pulses..?Tim

author: Someone
author_rank: Super Contributor
timestamp: July 18, 2016, 04:15:49 am
content: Quote from: T3sl4co1l on July 18, 2016, 01:04:18 amWell done for not reading the link the OP posted.Back on topic the input is sent through the TL07x as a comparator so it will be determining the edge rate seen by the rest of the circuit. It won't be affected much by the rise times of the external trigger signal. You could experiment with using your digital pulses directly in place of that OPAMPs output if you want to experiment with different trigger slew rates.

author: vk6zgo
author_rank: Super Contributor
timestamp: July 18, 2016, 06:08:43 am
content: Or,if you want to do it "retro-style",feed your square wave into a double sided clipper ("slicer") then amplify it back to the original level in a gain stage with much better rise time.The new rise time of your signal will be,near as dammit to the time between the two clipping levels.

author: Berni
author_rank: Super Contributor
timestamp: July 18, 2016, 11:42:32 am
content: If you want it fast then put it trough a fast logic gate like 74F or 74LVC and you will get roughly 1 to 5 ns rise times. Keep in mind that you need a reasonably fast scope to even see rise times this quick.Not sure why you would even need a rise time this quick tho.

author: dentaku
author_rank: Frequent Contributor
timestamp: July 18, 2016, 01:39:09 pm
content: Quote from: Berni on July 18, 2016, 11:42:32 amOh I don't really need anything that fast for this circuit but I was just wondering if there would be any appreciable difference if I could create a faster edged gate signal.I'll look through my 74xx gates and see what I've got and compare it to the old 4050 buffer, again... just as an experiment, because there's no other reason for this exercise other than education

author: mikerj
author_rank: Super Contributor
timestamp: July 18, 2016, 02:06:35 pm
content: Quote from: dentaku on July 18, 2016, 01:39:09 pmAs 'someone' already said, it's won't make any difference. The first TL072 op-amp stage is configured as a comparator, so no matter how fast the edges are on the trigger pulse, the TL072 output won't get any faster.

author: dentaku
author_rank: Frequent Contributor
timestamp: July 18, 2016, 02:09:57 pm
content: Quote from: Someone on July 18, 2016, 04:15:49 amYup, the TL084CN (in my case) is the limiting factor but I kind of just got sidetracked wondering about ways of making the gate signal as fast as possible before it hits the opamp.I guess there's many things I could experiment with like changing the values for the differentiator/HP filter,even if the comparator doesn't careor using a different/faster opamp or even a real comparator.It's times like this I wish I had a modern scope so I could actually make measurements.

author: KerryW
author_rank: Regular Contributor
timestamp: July 19, 2016, 01:31:38 am
content: One thing that is easy to try is to add some hysteresis to the 4050. Say a 1K in series with the signal and a 10K from output to input. Play with the values to see what happens.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 19, 2016, 06:06:04 am
content: Quote from: Someone on July 18, 2016, 04:15:49 amWell done for not reading the post I wrote. The first half was phrased in general, laying out what I would expect for the situation. The second half was in reference to the circuit.Tim

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 19, 2016, 06:14:19 am
content: Quote from: dentaku on July 18, 2016, 02:09:57 pmHow modern?Heck, dime store logic is pretty peppy these days -- or, since whenever the 74F, 74AC, 74LVC and other families came out. Output swing is a few volts (the TTL families, like 74LS, 74F, 74ALS and others, are about GND to half VCC; the CMOS families offer more-or-less full GND to +V output swing), and all of these do... uh, under 2ns I think (for a lightly loaded output). 74LVC is the most advanced that'll still tolerate 5V supply, I think? (Newer CMOS families get even faster, and more powerful and efficient, but only run on 3.3V supplies, or even less!) (Note that conventional families, like 74LS and 74HC, are several ns, so aren't really screaming fast, or anything. Let alone CD4000, which struggles to deliver 10s of ns...)Of course, ECL (with sub-1ns edges) has been around forever, but it doesn't offer much voltage swing, so it's kind of less impressive, and also, perhaps a little harder to use (not like buffering a signal is hard in any family, though).So in any case, there's no shortage of nanoseconds to be found.If you want to take a stroll through discrete circuits instead, you can get 10s of ns quite easily -- a 2N4401 will do 30ns already, though you have to push it pretty hard to get there! Back-to-back MOSFETs (like a 2N7002 paired with a BSS84, as a discrete CMOS NOT gate) get pretty peppy, but you have to drive them pretty hard (because they're rather oversized transistors for merely generating logic signals!).Tim

author: Fungus
author_rank: Super Contributor
timestamp: July 19, 2016, 09:05:39 am
content: Quote from: dentaku on July 18, 2016, 01:39:09 pmThat part is simple: Nope. Any frequencies you could possibly add will be way above what you can hear(and way above what your speakers can reproduce).

author: dentaku
author_rank: Frequent Contributor
timestamp: July 19, 2016, 12:58:41 pm
content: Quote from: Fungus on July 19, 2016, 09:05:39 amIt was still interesting as an experiment to see if it was possible.Like others have said, including myself, it's the opamp being used as a comparator that could possibly make a difference in the sound of the thud or click at the start of the bass drum sound in this circuit but I was still interested in learning about making faster edges for my gate signal anyway.I usually ask question on these forums out of curiosity rather than a practical need

author: Fungus
author_rank: Super Contributor
timestamp: July 19, 2016, 03:23:02 pm
content: To move a bass speaker cone instantaneously from one place to the other would require infinite energy.(and would kill everybody in the room).Moving it in a nanosecond? Still not easy.

author: dentaku
author_rank: Frequent Contributor
timestamp: July 19, 2016, 09:31:17 pm
content: Quote from: Fungus on July 19, 2016, 03:23:02 pmMaybe this circuit came out of the research Kate Bush was working on in the Experiment IV video in the 80's.https://youtu.be/NTUcoR8_pyE

author: tautech
author_rank: Super Contributor
timestamp: July 19, 2016, 09:53:25 pm
content: Ha, that's a quite young Hugh Laurie in that vid.

author: vk6zgo
author_rank: Super Contributor
timestamp: July 20, 2016, 07:15:58 am
content: Quote from: dentaku on July 18, 2016, 02:09:57 pmWhat kind of 'scope do you have?You don't need to be "spoon fed" the measurements---if you have an old Oscilloscope with fairly good specs,you can easily measure risetimes,with just the graticule & eyeball.

author: Fungus
author_rank: Super Contributor
timestamp: July 20, 2016, 12:25:26 pm
content: Quote from: tautech on July 19, 2016, 09:53:25 pmAnd a young Dawn French as well...