# Looking at some oscilloscopes

1. Feb 8, 2006

### physmurf

Hi, I am looking at some oscilloscopes for a class I will be teaching. What should I look for when selecting a dual trace oscilloscope? What are the key differences between a 20MHz and a 100MHz oscilloscope? Is the 100 better than the 20?

Thanks!

2. Feb 8, 2006

### chroot

Staff Emeritus
Honestly, if you don't know what the difference is between a "20 MHz" and "100 MHz" oscilloscope, you probably don't even know how to use one, and therefore probably aren't really in a position to be purchasing them. Besides, I strongly doubt that anyone actually manufactures a 20 MHz oscilloscope anymore. (Agilent's crappiest model is 60 MHz.) Is this really just a homework problem? If so, please use the homework help forum; you don't need to "trick us" into helping you.

The numbers 20 and 100 MHz refer to the bandwidth of the oscilloscope. A 20 MHz oscilloscope is only capable of displaying signals with frequencies at or below 20 MHz. (The bandwidth is usually given at the -3 dB point, meaning that an oscilloscope with a bandwidth of 20 MHz will will actually attenuate a 20 MHz signal by 3 dB, or 50%. Signals with eve higher frequencies will be attenuated much more than 3 dB.)

Obviously, an oscilloscope with a higher bandwidth is better than one with a lower bandwidth, unless the signals you're interested in exploring are all of very low frequency.

Note that square waves and the like are composed of sinusoids of various frequencies added together, and a 20 MHz square wave will require at least 100 MHz bandwidth to be displayed reasonably accurately on the oscilloscope's screen.

- Warren