Compact 74AC14 pulse generator PCB  - Page 1
Forum section: electronics / projects

author: rfmerrill
author_rank: Regular Contributor
timestamp: February 20, 2021, 06:33:09 pm
content: A lot of people have seenshowing a fast edge pulse generator made with a 74AC14 and a few resistors and caps.I built the circuit up on a piece of copper clad but decided it was a bit too fragile and decided to make a small PCB.Unlike some of the other PCBs people have made, I decided to go with fairly small SMT passives for this to keep the signal length short and the overall size small. I also made it for an SMA connector as I have more SMA parts than BNC (which is the more popular choice). I still used the DIP version of the 74AC14 because I already had a bunch on hand and I don't know how much benefit there is to using the SMT versions--you would need to add vias anyway.I don't have a very high bandwidth scope but the rise time I see approaches the limit of what my scope can measure, so I think it works pretty well. Just like the deadbugged version I made, you do see a lot of ringing, but this isn't meant to be a perfect circuit.OSH park share link:https://oshpark.com/shared_projects/205KQbDAKiCAD project is attached

author: rfmerrill
author_rank: Regular Contributor
timestamp: February 20, 2021, 06:36:45 pm
content: One potential improvement I'm aware of is that C1 and the VCC pour should probably have gone on the top, as it gives a better decoupling path--you can implement this as a rework easily by removing solder mask on the top and using a wire. The thermal reliefs were left at their default and I think for this size board they're overkill and could even be eliminated.

author: MikeK
author_rank: Super Contributor
timestamp: February 20, 2021, 09:38:38 pm
content: Looks good. I think the surface mount package would have a shorter rise time.

author: MarkF
author_rank: Super Contributor
timestamp: February 21, 2021, 01:22:41 am
content: Here is what I did:https://www.eevblog.com/forum/beginners/is-this-standard-tolerance-for-square-wave-generator-output/msg3368814/#msg3368814waveform:https://www.eevblog.com/forum/beginners/is-this-standard-tolerance-for-square-wave-generator-output/msg3372374/#msg3372374Mine is much larger though with through-hole parts.I also use it as a buffer/driver with the jumper to select the external input instead of the pulse generator.

author: magic
author_rank: Super Contributor
timestamp: February 21, 2021, 12:14:57 pm
content: Quote from: rfmerrillBeen there, done that, got the t-shirt. I willy try to photograph my build later today, you will like itThe biggest limitation of those devices is power supply ESL. It's the origin of that 60MHz ringing and I solved it by using the tiny 74LVC2G14 chip and mounting bypass capacitors right on top of the package between its supply pins. Another problem is inductance of power cables, I found that it takes 47µF to suppress ringing at some kHz which shows up if you slow down repetition frequency.My result looks like below. I have a strong suspicion that the slow rise over 20ns is some sort of artifact of the Chinese DSO because it just doesn't make any sense to me otherwise. Besides that, there is only a bit of overshoot which settles down after 4ns and this too could be partly to blame on the scope.The gray trace is a reference I saved long ago.editAnd here's the post which gave the the idea of using small LVC gates:https://www.mikrocontroller.net/topic/217945#2205244

author: MikeK
author_rank: Super Contributor
timestamp: February 21, 2021, 01:37:48 pm
content: Would be nice if this had variable pulse width and period.

author: rfmerrill
author_rank: Regular Contributor
timestamp: February 21, 2021, 06:28:43 pm
content: I'm kind of a beginner to high speed stuff: With using 7414 type drivers, is there any point to length-matching the traces from the driver to the output?

author: magic
author_rank: Super Contributor
timestamp: February 21, 2021, 07:17:18 pm
content: Probably not much. Propagation velocity is on the order of 20cm/ns.Quote from: MikeK on February 21, 2021, 01:37:48 pmReplace R1 with a pot and you can control width and period, but not duty cycleAnd this is my "compact" build. As you see, I went a little nuts with minimization

author: MikeK
author_rank: Super Contributor
timestamp: February 21, 2021, 07:43:05 pm
content: Quote from: magic on February 21, 2021, 07:17:18 pmI like it. Always good to have less things in the way, or break off.

author: MarkF
author_rank: Super Contributor
timestamp: February 22, 2021, 03:05:11 am
content: I spun a compact version of my Pulse Gen loosely based on @rfmerrill's layout.

author: magic
author_rank: Super Contributor
timestamp: February 22, 2021, 08:54:05 am
content: The path through decoupling caps is easily twice what it needs to be. OP's is much better, though C1 could go to the top layer.It could be further reduced with little effort by switching to the SO14 package.Remember about RDS(on) of the outputs, you can't just put 4×200Ω and expect correct termination.editI found my old prototype built with 74AC14 in SO14. Apparently due to 50% decoupling ESL reduction, ringing goes from 120MHz to 250MHz and decays after 12ns. I used crappy carbon film resistors for that, perhaps SMD could be a little different in terms of rise time and overshoot.It also looks like five parallel 220Ω isn't exactly the correct termination either, because amplitude is too high.

author: florian-x
author_rank: Contributor
timestamp: February 22, 2021, 09:25:24 pm
content: Ah, I must add mine here! Dead-bug style

author: StillTrying
author_rank: Super Contributor
timestamp: February 22, 2021, 09:59:01 pm
content: I did once think of wiring 2 carefully chosen spare gates of a 74AC14 like that to get more power conections into the chip to hopefully reduce the ringing, leaving 4 gates to play with. I thought it was an original idea but I've since seen one other mention of it for the same IC, maybe it works.

author: rfmerrill
author_rank: Regular Contributor
timestamp: February 23, 2021, 01:27:30 am
content: Quote from: magic on February 21, 2021, 07:17:18 pmGosh, censor that porn, this is a family forum!

author: magic
author_rank: Super Contributor
timestamp: February 23, 2021, 07:58:28 am
content: Quote from: StillTrying on February 22, 2021, 09:59:01 pmThat has to be one of those original ideas everybody comes up with at one pointI tried it too, using two pins in the middle on SO14. IIRC there was an improvement, but not as good as SOT23.BTW, I just came up with something completely nutsWhat if termination is removed from the output and placed in series with each supply pin independently, between the chip and the capacitors? That will likely kill the ringing, but I suppose some sort of artifacts will appear when the chip draws strong pulses of current internally during switching.

author: Gyro
author_rank: Super Contributor
timestamp: February 23, 2021, 12:45:59 pm
content: Quote from: florian-x on February 22, 2021, 09:25:24 pmThere are times when a bit of 'dead buggery' beats a PCB implementation, hands down. You can manage things that you simply can't on a PCB.

author: rfmerrill
author_rank: Regular Contributor
timestamp: March 12, 2021, 07:50:59 pm
content: I added a 47nF C0G capacitor on the top side of the board. Not sure how much it improved things. Still usable.

author: magic
author_rank: Super Contributor
timestamp: March 12, 2021, 08:52:02 pm
content: Quote from: rfmerrill on March 12, 2021, 07:50:59 pmNot much would be my guess, I tried to convince you a few times that inductance of the package is absolutely the dominant factor here unless you make PCB parasitics worse

author: iMo
author_rank: Super Contributor
timestamp: March 12, 2021, 09:59:36 pm
content: You want 74AUC gates to be really fasthttps://hackaday.io/project/28833-microhacks/log/157535-just-how-fast-are-74auc-gates

author: rfmerrill
author_rank: Regular Contributor
timestamp: March 13, 2021, 03:43:11 am
content: Like this?

author: magic
author_rank: Super Contributor
timestamp: March 13, 2021, 07:59:00 am
content: With modern logic families people got good results using only two or three gates. (I have two and I think tggzzz tried three and reported 600ps rise time). I just used the minimum to ensure that absolute maximum ratings are not exceeded.BTW, if you plan to use 3.3V chips it may be a good idea to put an LDO on the board to eliminate low frequency ringing due to cable inductance. Alternatively, 47µF. I forgot to mention that my BNC plug generator does have a 47µF MLCC on the bottom of the board.

author: rfmerrill
author_rank: Regular Contributor
timestamp: March 13, 2021, 08:13:17 am
content: Quote from: magic on March 12, 2021, 08:52:02 pmYou said "power supply ESL" and I didn't quite get that you meant the power leads in the package.

author: rfmerrill
author_rank: Regular Contributor
timestamp: March 13, 2021, 08:17:58 am
content: Quote from: magic on March 13, 2021, 07:59:00 amI'm kinda burned out but what do you mean by this?QuoteI've got 0805 cap footprints across each of the driver chips now, so I can absolutely cap the heck out of it XD. Probably going to put 0.1uF 0402 right on top of them as well. Local regulator might be a good thing to try tho.The board I'm fabbing out has really short and matched traces, I'm planning on using 0201 resistors (Vishay has a whitepaper where they show better frequency response with smaller package size) as well.

author: StillTrying
author_rank: Super Contributor
timestamp: March 13, 2021, 09:41:13 am
content: Quote from: magic on March 13, 2021, 07:59:00 amWhen I was using a 74AC14 to 1us pulse 5mm LEDs I had the 3 or 4 parallel gates of the 74AC14 up to 9V and 800mA.It was gettingquite warmwith the LED only on 0.1% of the time.

author: magic
author_rank: Super Contributor
timestamp: March 13, 2021, 10:26:15 am
content: Quote from: rfmerrill on March 13, 2021, 08:13:17 amYes, sorry, everything counts. As far as I understand, it's simply a resonance of the LC circuit formed by the internal capacitance of all the MOSFETs in the IC and any inductance present along the way between the power supply terminals on the IC (package pins + PCB + external capacitors). This resonance causes the power supply voltage seen inside the chip to vary and hence the output varies too. BTW, the newer logic families likely have less internal capacitance which could be another reason why the 74LVC1Gxx gates ring less.Quote from: rfmerrill on March 13, 2021, 08:17:58 amLook up "absolute maximum ratings" in the datasheetThere is also "recommended operating conditions"; I think I actually designed to stay within the recommended limits under normal operation and below absolute maximum under output short circuit to ground.Quote from: rfmerrill on March 13, 2021, 08:17:58 amBe careful with power dissipation. I used only two channels and two resistors and made them with doubled 0805 to be on the safe side.By the way, I think you should add vias near GND pins of the driver chips (assuming the bottom is ground). Otherwise the return current has to take a round trip around the whole bunch.

author: rfmerrill
author_rank: Regular Contributor
timestamp: March 13, 2021, 07:59:41 pm
content: Quote from: magic on March 13, 2021, 10:26:15 amWhen you said you "used the minimum", the minimum of what?Are you saying there's an absmax rating that is exceeded if you have too many gates? Not enough gates?My guess is that what you meant was: using only a single gate can exceed the absmax output current

author: rfmerrill
author_rank: Regular Contributor
timestamp: March 13, 2021, 09:11:53 pm
content: Quote from: magic on March 13, 2021, 10:26:15 amthe top is ground, the bottom is VCC, and I have a void in the middle on both sides where the high speed traces run, someone suggested it.I made this to learn anyhow, so if it has problems that's not a failureI'm relying on the small size (the drivers are SC70) to keep the return paths relatively short, plus I kept extra margin on the sides of the board so the plane has more surface area.

author: rfmerrill
author_rank: Regular Contributor
timestamp: March 23, 2021, 09:22:45 pm
content: Parts finally came and it works pretty well! No more ringing. Time to borrow a better scope to see how good it really isI used 10uF 0805s for all of the bypass caps, plus added a 100uF electrolytic at the input. The output voltage is lower since the AUC chips run at 2.7V, but voltage wasn't really what I was looking forAt 2.7V with a 50 ohm load, each of the 220 ohm 0201 resistors dissipate at the very most ((2.7V/50 ohm)/5)^2 * 220 ohm = 25mW, which is well below their 50 mW rating

author: rfmerrill
author_rank: Regular Contributor
timestamp: March 25, 2021, 04:51:14 am
content: Measured on a Keysight 6 (8?) GHz scope (but bandlimited to 500 MHz according to the UI anyway--I guess it doesn't support higher bandwidth with just a 50 ohm input?)410 ps rise time!Only three cycles of ringing and no lower-frequency ringingFalling edge has no ringing, but fall time appears somewhat longer?