4 May
2010
4 May
'10
11:26 a.m.
On Saturday 01 May 2010 03:36:59 pm Chris M wrote:
I happen to have a probe I picked up from somewhere
in my garbage garage. The inputs on these 2 scopes require 1Mohm wit either 22pF or 47 pF
of capacitance.
No...
What does this mean? There's simply a capacitor
between the coax? The resistance part is strait forward presumably. If that's the
case, then you can just jury rig an acceptable probe, no?
It means that the input impedance of that connector is 1 Mohm. And that it has the
equivalent of 22-47pF of capacitance across it. The reason it's important to know
this is that if you want a waveform to display properly you need to compensate for that in
the design of the probe that's attached to that input. Usually it's a 10X probe
(meaning it attenuates the signal by a factor of 10) and has a series resistor of 9Mohm or
so, plus some capacitance in the cable. There's typically a trimmer capacitor in the
probe somewhere to adjust this so that square waves are actually shown as being square.
Too much one way and the corners get rounded off, too much the other way and there's
overshoot...
--
Member of the toughest, meanest, deadliest, most unrelenting -- and
ablest -- form of life in this section of space, a critter that can
be killed but can't be tamed. --Robert A. Heinlein, "The Puppet Masters"
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Information is more dangerous than cannon to a society ruled by lies. --James
M Dakin