Dean KK4DAS asked me why we have these switches on our 'scopes. I didn't know. Dean asked Alan Wolke W2AEW. Alan knew:
Alan Wolke wrote:
Interesting background on the 20MHz vertical BW selection. This feature has existed on the vast majority of all oscilloscopes since the 50s or 60s (both analog & digital). When I explored the history of this, I spoke to some of the folks at VintageTek.org, and wound up having a chat with THE engineer that did it first! Tt was John Addis, designer at Tektronix.
At the time, Tektronix was located in Portland Oregon. While working on a wideband vertical preamp for a new scope (the 7A11 vertical plugin for the 7000 series scope), John Addis was plagued with interference from the local television broadcast station in the 50MHz band. So, he popped in a 20MHz low pass filter that he could switch in/out so that he could complete the work on the preamp. Since it was deemed useful, it was left in the design.
And, since Tektronix added a switchable 20MHz low pass filter in their scope, and Tek was the leader in oscilloscope technology, other manufacturers followed suit, and this feature has "stuck" as a staple in vertical setting controls.
The main reasons you'd use this filter would be to improve the signal to noise ratio (SNR) for signals when their frequency content is below 20MHz. You've probably noticed that, even without any signal connected, the thickness of the trace is thinner when you engage the 20MHz filter.
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Alan sent a link to a Wiki page about the 7A11 that John Addis was designing. Alan says this places the initial inclusion of the 20 MHz BW Limit filter to the mid 1960s:
https://w140.com/tekwiki/wiki/7A11.
Thanks Dean, thanks Alan!
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Very kind of you not to mention "why" I asked. It's because I had unintentionally had that switch turned on ever since I started homebrewing about 5 years ago. At that time I didn't even know what bandwidth meant. I think I read somewhere that the switch would reduce high frequency noise. I turned the switch on and never thought about it again. All this time I've been blaming my signal generated for pooping out at 28 MHz. I relearned the valuable lesson that that then when something goes wrong it's usually me! Moral of the story:
ReplyDelete.Know your test equipment.
Another reason is because Power supply noise measurements are also made with 20MHz bandwidth.
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