Vasile Surducan wrote: > Chinese tools must be allways watched with some circumspection. > I've bought an UT81B http://www.uni-trend.com/UT81b.html > for using on field and I was very dissapointed. It has low contrast, > it has a true analogic bandwith of 2MHz instead of 8MHz as it claims There's an old saying: "A spectrum analyser is only worth as much as its first local oscillator". The complement for oscilloscopes would seem to be "An oscilloscope is only worth as much as its front-end amplifier." As in, you can make a scope with the fastest, most accurate A/D converter in the world, but if the front-end amp can only do 2MHz, that's all you'll get. Also, you're measuring the bandwidth wrong. An 8MHz square wave has tons of frequency components above its frequency (feed one into a spectrum analyser and see for yourself, or look at the figures here -- ). To accurately display it, you need to capture those in addition to the fundamental frequency. Lose one or more of the harmonics, and you end up getting closer and closer to a pure tone, i.e. a sine wave. It's impossible to create or capture a "perfect" square wave in the real world, because there's always a bandwidth limit somewhere. Risetime and Falltime can never be absolutely zero, just very close to it. In short: 2MHz (square wave) is about all you can expect from an 8MHz bandwidth scope. Most good bench scopes (any LeCroy, almost all Tek DSOs/DPOs, or any Agilent DSO/DPO other than the entry-level boxes) will do sine waves up to their specified bandwidth, and give a good representation of a square wave at up to Bandwidth/4, i.e. about 50MHz for a good 200MHz scope. You really need to be aware of the limitations of your tools before you try and use them... it's really easy to get bitten in the backside and spend all day looking for a fault that doesn't actually exist ("ghost hunting"), or equally miss a fault that actually DOES exist because you're not using a scope with a high enough bandwidth for your application. -- Phil. piclist@philpem.me.uk http://www.philpem.me.uk/ -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist