[sdiy] Owon scope oddity...

[Neil Johnson]

Hi,

Justin Owen wrote:
> That's interesting...
>
> ...so is it possible to say whether it's my scope limiting the
> bandwidth or the probes when set to x1?

It will be a combination of the two together that form the "measurement 
system".

> Or to put it another way -  if I was to upgrade my probes e.g. to the
> x1 3900 you've mentioned above - would I get the full 25MHz bandwidth
> of the scope back (or thereabouts) at x1 setting?

It would certainly be better than with the probes that come with the 
scope.  Suppose both scope probe and scope have a bandwidth of 25MHz. 
Now connect them together.  The tip-to-trace bandwidth is now about 
13MHz.  This is because you have two 25MHz low-pass filters in series, 
which puts the attenuation at 25MHz at -6dB, so a rough hand-wavey 
guestimate puts the -3dB point at about half that.

For making amplitude measurements that would put you at around 1.3MHz 
top frequency.  Above that and the attenuation starts to affect the 
amplitude and the errors start to become significant: at 13MHz (or 
wherever the -3dB point really is) the amplitude as shown on the scope 
screen will be about 70% of what the tip sees.

> For the record, it's been a perfectly adequate scope - but I am
> interested in seeing if I can push it a bit further...

Well, that x1 probe will be OK for audio (good up to 400kHz), as long as 
you don't probe any high-impedance or otherwise sensitive points.  If 
you want to get more out of the scope get the best probes you can, 
certainly aim for wider bandwidth than the scope.  Although really you 
need to decide what bandwidth you actually need, otherwise you'll be 
spending money on fancy probes that won't give you any benefits in the 
work you're doing.

Cheers,
Neil
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