350/rise time questions... - Page 1
Forum section: electronics / testgear

author: alank2
author_rank: Super Contributor
timestamp: August 03, 2013, 08:50:31 pm
content: Hi Everyone,I've seen the 350/risetime in ns formula for calculating the bandwidth, but it got me thinking, how fast is a transition and what would make the transition from an off state to an on state faster. I've got a 150 MHz oscillator that has a risetime of 1.425nS. What would be a good circuit or ic that has a very fast rise time - so fast that it itself wouldn't be any limiting factor, but a scopes input channel would be for example.Thanks,Alan

author: Mechatrommer
author_rank: Super Contributor
timestamp: August 03, 2013, 09:08:52 pm
content: search avalanche pulser

author: DaveW
author_rank: Frequent Contributor
timestamp: August 03, 2013, 10:20:03 pm
content: Page 93,http://cds.linear.com/docs/en/application-note/an47fa.pdf

author: dfmischler
author_rank: Frequent Contributor
timestamp: August 03, 2013, 11:12:51 pm
content: Checkthisfor an applicable discussion on another forum (Winfield Hill weighs in BTW). I haven't yet tried the suggested NC7NZ34.

author: edavid
author_rank: Super Contributor
timestamp: August 04, 2013, 02:07:36 am
content: Here's a search that will get you lots of interesting reading, in addition to the links already posted:http://www.google.com/search?q=fast+rise+pulse+generatorThe main methods used by hobbyists are:1. Tunnel diode - easy to build, and very fast with the right part. The supply of fast tunnel diodes had mostly dried up, but recently some Russian TDs have turned up on eBay - unfortunately the good ones are kind of overpriced.2. Avalanche transistor - uses only cheap, common parts, but requires medium voltage (usually some kind of step up converter), has inherent jitter, and is hard to make really fast.3. Fast IC... which could mean anything from 74AC to 10EP to exotic GaAs or SiGe. The fast parts get expensive, are hard to buy, and are in hard to use packages.

author: BravoV
author_rank: Super Contributor
timestamp: August 04, 2013, 02:55:40 am
content: Quote from: alank2 on August 03, 2013, 08:50:31 pmGet your self a cup of coffee and spend your time here ->EEVblog #306 - Jim Williams Pulse GeneratorScope shot from one of the results in that thread, see the measured t-Rise below here.Result posted by Alan (w2aew)'s 33GHz scope.

author: alank2
author_rank: Super Contributor
timestamp: August 09, 2013, 08:59:42 pm
content: Can you also measure the fall time to test this? I don't have one of Vincent's fine generators (yet!), but I did try setting up a tactile switch that switches to ground and feed a voltage to it so that when pressing it it snaps to ground pretty quickly. I was able to measure a fall as fast as 1.22nS on my DS2072 (in 200 MHz mode)...

author: Orange
author_rank: Frequent Contributor
timestamp: August 09, 2013, 09:32:31 pm
content: Quote from: alank2 on August 09, 2013, 08:59:42 pmin the early days, Tektronix used a wet mercury relay pulsed at 50 or 60 Hz to create fast pulses. After that tunnel diodes became in use. I have a TD pulser that does 70 pSec its the Tek 284, probably one of the fastest pulsers.The thing with the avalance pulser as described by J. Williams is that it does not use a transmission-line but a simple capacitor. This shows in the pulse it produces (not realy flat). If you want a clean and flat pulse, you need to replace the c with a 50 ohm coax line. The length of the coax determines the pulse length. as transistor use a 2N2369 from Motorola or a 2N2501If you need even faster pulses you can use a snap diode after the avalanche pulser, then you can go into the pico Second domain.Tek used this technique in their sampling heads