Won't make that mistake again - wrong resistor type... - Page 1
Forum section: electronics / beginners

author: JuiceKing
author_rank: Regular Contributor
timestamp: June 21, 2012, 12:51:47 am
content: So I'm keen on messing around with fast pulses, and thanks to this forum I discovered Jim William's note, "Simple nanosecond-width pulse generator provides high performance" in the November 11, 2004 issue of EDN. It's really easy to understand and only calls for a handful of components.One seemingly trivial component is a 50-ohm resistor to load the input pulse. I mistakenly ordered a wire-wound resistor for that part and realized that it would probably introduce all kinds of inductance problems only after it and the rest of the parts arrived from Digikey. Oh, well, I put in an order for metal film resistor to replace it.For laughs, I compared the pulse response of the wire-wound 50-ohm resistor with an ordinary 50-ohm pass-through terminator on my oscilloscope--see attached screenshot. The input pulse is 30ns, 3V. Overshoot with the wire-wound resistor (white trace) is nearly 100%! Compare this with the yellow trace that represents the response using the pass-through terminator.Next time I'll be more careful when ordering resistors...

author: free_electron
author_rank: Super Contributor
timestamp: June 21, 2012, 01:39:30 am
content: Caution as that metal film resistor may be spiral cut... And behave inductive....You need a thin film resistor that is lasercut with an l or r shape...

author: JuiceKing
author_rank: Regular Contributor
timestamp: June 21, 2012, 02:06:18 am
content: Quote from: free_electron on June 21, 2012, 01:39:30 amThanks, but ugh.At least at Newark (which I prefer to Digikey for its better UI), the thin-film resistors are either SMD (max 1/8w) or huge with heat-sinks...no through-hole versions.Do you know where I can find the right kind?Am I naive to try to build this with through-hole components?Thanks,- Ken

author: T4P
author_rank: Super Contributor
timestamp: June 21, 2012, 02:20:09 am
content: I assume it's going to be a big mistake when you find out the limiting factor in reducing inductance is the inductance of through hole leads ... better not find out later

author: Neilm
author_rank: Super Contributor
timestamp: June 21, 2012, 01:57:42 pm
content: Life is all about making mistakes. Living is learning from themNeil

author: free_electron
author_rank: Super Contributor
timestamp: June 21, 2012, 08:17:50 pm
content: Quote from: KenSchwarz on June 21, 2012, 02:06:18 amyes, because the wire in the hole already adds enough inductance and the coupling to the adjcent stuff enough capacitance to throw this off the scale..if you take that feed-thru-50 ohm terminator apart you will find an ALOX substrate ( alumina oxide , just like what is used for SMD resistors ) thin film, laser trimmed resistor . it is press fitted. so no soldering or lead termination.jim williams has somewhre a picture of his design. you will find that he was using alumia based resistors. I have SMD resistors that hold easily 3 watt. in 2512 body or 1210 body. no problem.Newark or farnell are the wrong places to look for such sttuff. Those guys deal only in 2 inch screws and 50 ampere fuses ... not in high tech components.. Newark and farnell are good for 'old skool' stuff. Need anything bleeding edge : they won't have it.. Digikey or mouser is the way to go.Here is a couple of good resistors for what you are trying to do :http://www.digikey.com/product-detail/en/HPC12JT100R/HPC12JT100RCT-ND/1650363put two of those in parallel and you have 50 ohms that can burn off 10 watts... and these are non-inductive.or the HPC12JB50R0 which s 5 ohms but not in stock right now ...http://www.mouser.com/ProductDetail/Bourns/CHF2010CNP500LX/?qs=N%252biqXe0%252bQtvQ8zB%2f9xxsNCpa4DFEK3YEIlbSnnKFt%252bQ%3dBefore you complain about the price of these ... have you looked at the cost of a good quality 50 ohm feed-through terminator ? that's not exactly chickenfeed either....there is an avalanche based pulse generator from jim williams as well. that generates picosecond rate pulses.uses a boost pump to make 90 volts form a 5 volts input and then avalanches a 2n3xxx transistor. can;t remeber the application note.It's in the big book from jim williams :http://www.amazon.com/Analog-Circuit-Design-Applications-Solutions/dp/0123851858/ref=sr_1_1?s=books&ie=UTF8&qid=1340310138&sr=1-1&keywords=jim+williamsa must have !

author: JuiceKing
author_rank: Regular Contributor
timestamp: June 22, 2012, 12:30:07 pm
content: Thanks to you all for this very helpful advice!I was planning to build this with ordinary breadboard and short leads. I have a standard pitch DIP adapter for the LT1721. I was hoping that the 74AHC08 would work as long as it got "reasonably clean" pulse edges from the LT1721, and that exactly how the sausage was made before the final output stage wouldn't be super-critical. Likewise, I figured that the LT1721 would be reasonably forgiving on its inputs.If this is hopeless, what fabrication strategy do you recommend? (I.e., what alternative to standard breadboard should I use? I've not built with surface-mount components before...)For your reference I've attached the schematic from the original article.Also, if all or some of these components are critical, do you have specific suggestions for the 8pF and 0.1uF capacitors? Finally, I was hoping to have a variable differential delay using a 1000R pot. Again, any parts recommendations here?Thanks again for pointing me in the right direction!- Ken

author: free_electron
author_rank: Super Contributor
timestamp: June 22, 2012, 01:30:17 pm
content: For this schematic the signal at the input is not super critical. The pulse is generated by the skew between the inputs of the 7408.Use np0 caps to have good temp stability.Make sure the i puts of the unused gates are not left floating. Decouple the 7408 with a 10nF in parallel with a 1uf ceramic cap. 100nf is an inductor at this kind of pulsewidths...Depending how fast you really go you may need to drop to 1nf decoupling cap.If you breadboard this : use a solid slab of copper clad pcb and build this dead-bug style. Flip chips upside down pins sticking up. The slab of copper is ground.Cut a narrow strip of pcb and glue that on. This becomes your power bar.The comparators need also good decoupling.

author: JuiceKing
author_rank: Regular Contributor
timestamp: June 22, 2012, 03:19:40 pm
content: As an experiment, I have run some tests on the 74AHC08 driven with pulses fed directly from my benchtop pulse generator. Input is yellow, output blue. Grounding the unused gates was indeed critical--thanks for that pointer. It works! I suspect I'm running into bandwidth limits with my scope (100Mhz) and that faster rise times than this will be immeasurable.