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Electronics => Beginners => Topic started by: Akj on March 05, 2021, 05:51:55 am

Title: What is the difference between the bandwidth of a scope and bandwidth limit?
Post by: Akj on March 05, 2021, 05:51:55 am

Duh this hantek scope datasheet is confusing me!
They've mentioned their scope bandwidth as 100mhz as you can see in the image attached but there is also a bandwidth limit parameter of 20mhz.Can anyone please explain me what does this mean?

Title: Re: What is the difference between the bandwidth of a scope and bandwidth limit?
Post by: rfclown on March 05, 2021, 05:56:07 am

Scopes will often have a Bandwith Limit function which inserts a low pass response. It is useful when trying to look at a signal which has high frequency noise on top of it.

Title: Re: What is the difference between the bandwidth of a scope and bandwidth limit?
Post by: Electro Fan on March 05, 2021, 12:03:29 pm

Quote from: Akj on March 05, 2021, 05:51:55 am

Duh this hantek scope datasheet is confusing me!
They've mentioned their scope bandwidth as 100mhz as you can see in the image attached but there is also a bandwidth limit parameter of 20mhz.Can anyone please explain me what does this mean?

The specs for bandwidth limit at the top of the spec sheet represent what the particular oscilloscope model can support, typically at -3dB.  You can think of this as the maximum signal bandwidth reliably tested and measured with the oscilloscope, although some models will exceed their specification. 

Depending on what you want to test/measure/observe you might want some headroom, for example if you are observing/measuring rise times. 

https://www.tek.com/support/faqs/how-bandwidth-related-rise-time-oscilloscopes (https://www.tek.com/support/faqs/how-bandwidth-related-rise-time-oscilloscopes)

https://blogs.keysight.com/blogs/tech/bench.entry.html/2019/08/16/supercharge_yourosc-pBDK.html (https://blogs.keysight.com/blogs/tech/bench.entry.html/2019/08/16/supercharge_yourosc-pBDK.html)

The reference to bandwidth limit further down the spec sheet is not the overall bandwidth limit of the oscilloscope but a feature (often invoked with a button) that enables a user to set a lower bandwidth, generally as a filter to help minimize noise.

https://www.tek.com/blog/scope-tip-use-bandwidth-limiting-reduce-noise-captured-signals (https://www.tek.com/blog/scope-tip-use-bandwidth-limiting-reduce-noise-captured-signals)

https://www.eevblog.com/forum/beginners/the-popular-oscilloscope_s-20-mhz-bw-limit/ (https://www.eevblog.com/forum/beginners/the-popular-oscilloscope_s-20-mhz-bw-limit/)

Note that these specs are in MHz (Mega for millions of Hz) rather than mHz (milli for thousandths of Hz), ie upper case M vs lower case m.

edit - typo fixed

Title: Re: What is the difference between the bandwidth of a scope and bandwidth limit?
Post by: Akj on March 05, 2021, 12:20:28 pm

Thank you for such a detailed explanation! I clearly understand this now  :)

Title: Re: What is the difference between the bandwidth of a scope and bandwidth limit?
Post by: Vovk_Z on March 05, 2021, 05:00:37 pm

We usually use scope with "20MHz BW limit" On, when there is no need in full bandwith.