Scope BW Limit - Why 20 MHz?

[TomKatt]

Just curious why the BW limit feature on most scopes seems to be 20 MHz?  Is that a hardware feature or controlled by software?  Because if it's software (which I would guess considering the ability to 'upgrade' scopes to higher BW), why not provide an arbitrary user adjustable BW limit?  ie you could apply a 100 MHz limit if desired?

[tooki]

I don't know the historical reasons for why the common BW limit is 20MHz and not some other frequency. But as for how:

I believe some scopes do it in software*, but more commonly, it's switchable hardware filters. (Yes, most bandwidth upgrades simply alter which hardware filter is enabled when it's not set to 20MHz mode. So the Rigol DS1000Z series, for example, is 100MHz hardware, with selectable 20, 50, and 70 MHz hardware filters. But the user interface only exposes 20MHz and whatever your licensed maximum is.)


*The R&S MXO4 I have at work has 20, 50, 100, 200, 350, 500 MHz, and 1GHz bandwidth filters (almost certainly in hardware) on hardware fundamentally capable of 1.5GHz. But it also can apply a bandwidth filter at arbitrary cutoff frequencies as a math channel.

[jjoonathan]

I always assumed it was TV signals, which are 5-ish MHz wide. Put that on a 10Mhz carrier and now you want 20MHz for a clean view. Or maybe you want to see the third harmonic? In any case, this would also explain why interfaces in this bandwidth range are called "video." Things often get named after their original driving application, and I think video was the original driving application in this bandwidth range, so it makes sense.

[BrianHG]

I thought is was for making the higher bandwidth x10 mode on your probe appear to have the same bandwidth as when you switch it to the x1 mode.  Or, at least, on my really old 100mhz analog scope, this was the approximate net effect.

[Aldo22]

See: https://www.eevblog.com/forum/beginners/the-popular-oscilloscope_s-20-mhz-bw-limit/msg1266847/#msg1266847

[TomKatt]

See: https://www.eevblog.com/forum/beginners/the-popular-oscilloscope_s-20-mhz-bw-limit/msg1266847/#msg1266847

OK that makes sense.  But it also seems somewhat outdated in today’s world now that most television has moved to the digital age…. Odd that a legacy feature like this  continues to be included in modern gear - I’d think there are different problematic frequencies in the modern world, or even better allow the filter to be user selectable.

[thm_w]

See: https://www.eevblog.com/forum/beginners/the-popular-oscilloscope_s-20-mhz-bw-limit/msg1266847/#msg1266847

OK that makes sense.  But it also seems somewhat outdated in today’s world now that most television has moved to the digital age…. Odd that a legacy feature like this  continues to be included in modern gear - I’d think there are different problematic frequencies in the modern world, or even better allow the filter to be user selectable.

The reason is outdated, but the spec lives on. You'll find a lot of stuff specifying "pk-pk noise at 20MHz" or similar. So even a modern scope, you want this specific 20MHz limit to be available.
Yes usually it is selectable, at least a few steps.

https://www.physicsforums.com/threads/why-to-scopes-bw-limit-to-20mhz.515072/