Easiest way to generate bias for Jim Williams Pulse generator - Page 1
Forum section: electronics / projects

author: AlessandroAU
author_rank: Regular Contributor
timestamp: October 08, 2014, 08:04:23 am
content: I wanted to share an easy method for generating the +90v bias for a jim williams pulse generator without having to use an LT1071.I started with a classic joule theif circuit for power leds, 10+10 turns. I'm using a 300ohm resistor and a PN2222http://upload.wikimedia.org/wikipedia/commons/thumb/8/83/Joule_thief.png/250px-Joule_thief.pngWith 100 turns of 0.125mm copper as a secondary. A 1N4148 rectifies the output.The circuit draws 80ma from a AA and the secondary supplies 3ma short circuit. The open circuit voltage is 150v so I use a neon as a primitive voltage regulator, it caps the voltage to just above the breakdown voltage of the neon (100v). Plus it indicates if the circuit is on.The pulse generator uses a 2n2369 with a 10pf tank cap, is 10pf too large?Anyway here are some images

author: AlessandroAU
author_rank: Regular Contributor
timestamp: October 08, 2014, 08:05:16 am
content: Some more images

author: David Hess
author_rank: Super Contributor
timestamp: October 08, 2014, 12:32:08 pm
content: Quote from: AlessandroAU on October 08, 2014, 08:04:23 am10 picofarads is not too large if it works reliably. Transistor lifetime in avalanche mode depends on repetition rate, avalanche current, and pulse width. If the capacitance is small enough, then repetition rate is just limited by power dissipation in the transistor.

author: macboy
author_rank: Super Contributor
timestamp: October 08, 2014, 06:12:48 pm
content: Nice work, that is a super easy way to get the high voltage needed. It's also a super easy way todie, so be careful.Don't be fooled by this pulser circuit. You can't measure the rise time of a scope with a pulse that doesn't have a flat top. This does generate a rapidly rising pulse, but as soon as it peaks, it falls quickly too. You can only measure the rise time of the scope if the scope actually displays the top of the pulse. You can't be certain it does that, and in fact, you can be nearly certain that itdoes notunless you use a multi-GHz scope that has a rise time much greater than this pulse generator... in which case the whole exercise is for nothing. You can see the slew rate of the scope, butnotrise time, using this simple circuit.To measure rise time, you need to build the modified version of this pulse generator, which uses a length of 50 ohm coaxial cable as a pulse storage medium, rather than the capacitor. This allows the pulse to flatten out for the duration of the time it takes the pulse to travel along the coax. It is a very minor circuit difference. You already have the bias and trigger for the 2n2369, so you just need to add the coax (or preferably, 50 ohm hard line if you can get some) and termination. Read Linear Tech App Note 94 "Taming of the Slew" by Jim Williams. You don't need to build the described oscillator and low-jitter pre-trigger generator if you don't want them, the important part is the stuff hanging off the 2n2369 collector/emitter.

author: AlessandroAU
author_rank: Regular Contributor
timestamp: October 08, 2014, 08:17:13 pm
content: Quote from: macboy on October 08, 2014, 06:12:48 pm<3ma at a dead short is a tiny amount of current. Shorting the output with my palm I can't feel a thing from the 'high voltage' output. When the resistance of the human body is taken into consideration the current drops to imperceptible levelsI did use coax! ok, prehaps not on the image of the TDS210....But on the Tektronix you'll see a flat top. if you take the risetime (2.8ns) you'll get 125mhz. Which should be about right for a 100mhz scope no?

author: mazurov
author_rank: Frequent Contributor
timestamp: October 08, 2014, 09:44:18 pm
content: Be careful with the coax length. In my experience, 5 mm of extra cable is much better transistor killer than 25 pf of extra capacitance. The effect is more pronounced on pulsers made from nice RF transistors - buy extra.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 09, 2014, 01:30:37 am
content: The main thing I've noticed with extra capacitance is more "drool" (tail) in the pulse. The RC exponential decay is apparent.10pF into 50 ohms is a 500ps time constant. A scope 200MHz or less won't see it (and won't see the true voltage peak either).Tim