What an oscilloscope is not? - Page 1
Forum section: electronics / beginners

author: JustAnotherGuy999
author_rank: Contributor
timestamp: February 27, 2016, 04:49:06 pm
content: Actually its shocking for a beginner when he discovers the difference between the "ideal oscilloscope" he has in his mind, and the "real world oscilloscope" in the real life, for example i never thought that i might need at least 5n Hz bandwidth oscilloscope to visualize n Hz square wave, and also the fact that the amplitude of the waveform will diminish to 70% as the frequency of a sine waveform get near the bandwidth of oscilloscope?are there any other facts about the "real world oscilloscope" that should be known?

author: ataradov
author_rank: Super Contributor
timestamp: February 27, 2016, 05:37:37 pm
content: Quote from: JustAnotherGuy999 on February 27, 2016, 04:49:06 pmAlways keep in mind effects of undersampling and aliasing.Also, all those bandwidth limitations are not that bad. Oscilloscope is not a precise measurement tool. If you know you should have 100 MHz square wave and on a 100 MHz scope you see a sine, then in most practical cases you should not worry and the actual hardware is probably fine. It is still good for estimating timing parameters of the signal.And at 5 n you won't really have a square wave, it will be a less perfect sine wave. To visualize perfect square wave you need an infinite bandwidth. Obviously impossible in practice, so you have to sacrifice something.

author: JustAnotherGuy999
author_rank: Contributor
timestamp: February 27, 2016, 05:43:35 pm
content: thanks

author: macboy
author_rank: Super Contributor
timestamp: February 27, 2016, 08:35:19 pm
content: Quote from: ataradov on February 27, 2016, 05:37:37 pmHuh? If you put a "perfect" 20 MHz square wave into a 100 MHz scope, you will see a square wave on the screen not a "slightly less perfect sine wave". The real limitation will be the rise time; on a 100 MHz scope, this is about 3.5 nsec. This results in a square wave with sloped edges rathe than perfectly vertical, but it will bear no resemblance to a sine.edit: Since "a picture is worth a thousand words", here is one of a ~20 MHz square wave (1.7 ns rise time) on a 100 MHz analog scope.

author: alsetalokin4017
author_rank: Super Contributor
timestamp: February 27, 2016, 08:48:32 pm
content: Now try it with a100 MHzsquare wave, like ataradov said in his post, and see what you get.

author: vk6zgo
author_rank: Super Contributor
timestamp: February 28, 2016, 06:09:35 am
content: An "ideal" square wave consists of a fundamental & aninfinitenumber of odd harmonics.The time honoured "rule of thumb" is if you have response out to the 7th harmonic you won't see the difference between that an infinite response.We can't make ideal square waves or ideal Oscilloscopes,so we do the best we can.In most cases,response to the 5th harmonic will do.

author: Kilrah
author_rank: Unknown
timestamp: February 28, 2016, 06:12:44 am
content: Quote from: macboy on February 27, 2016, 08:35:19 pmAnd on that picture you can perfectly see that the edges aren't vertical but a bit curved, which perfectly illustrates the point. It's for sure good enough to see it's supposed to be a square, but it isn't a perfect square.

author: AG6QR
author_rank: Frequent Contributor
timestamp: February 28, 2016, 06:47:54 am
content: Quote from: JustAnotherGuy999 on February 27, 2016, 04:49:06 pmI won't discuss frequency response, as that's already been covered. But it's sometimes worth noting that an ideal oscilloscope doesn't load the circuit it's measuring, while a real one does load the circuit. Most people understand the DC loading, usually a megaohm for the scope itself, or 10 megaohms for a scope with 10X probe. That's rarely a serious problem and can usually be safely ignored. But it's important to remember that the resistive DC loading is in parallel with some capacitance, and the capacitance can easily dominate at higher frequencies.For example, 20 pF of capacitance at 100MHz has a capacitive reactance of around 80 ohms. That's often something that shouldn't be ignored.

author: pickle9000
author_rank: Super Contributor
timestamp: February 28, 2016, 06:49:47 am
content: Is that the scope or the signal causing that?Good question.

author: vk6zgo
author_rank: Super Contributor
timestamp: February 28, 2016, 08:33:50 am
content: Quote from: alsetalokin4017 on February 27, 2016, 08:48:32 pmBut later in his post,ataradovreferred to"5n",which implied an Oscilloscope with response to the 5th harmonic of the square wave fundamental,when he said:-"And at 5 n you won't really have a square wave, it will be a less perfect sine wave.This is the part of his commentmacboywas referring to.Although it doesn't fit either the strict definition,or the "rule of thumb" I referred to above,a 20MHz square wave is still recognisably a square wave on a 100MHz 'scope.