Subquestion
Number one for me would be the reasoning why those
probes are so important. How bad are readings taken with just
direct wired connections?
Very! Using a normal piece of cable will load the signal
under test
,mostly due to the capacitance of the cable.
Of course, this depends on "the cable". Back when I used a 'scope
(basically, back when I had a mostly-working one handy), I normally
connected it not with a "cable" but with two pieces of loose wire, one
to (signal) ground and one to the signal being examined. The
capacitance of that sort of "cable" is probably on the order of the
capacitance you introduce just by standing near it - the scope itself
usually swamps it. (I'm sure such a thing introduces other kinds of
error potential, especially when measuring high-impedance and/or
high-speed signals....)
Now, this was mostly audio-range, or relatively slow digital stuff, and
almost always relatively low-impedance stuff. For that kind of signal,
you can get away with almost anything. :-) But I also used that setup
to look at the signal from an oscillator built by connecting an odd
number of inverters in a loop, and unless you use a lot of them, that's
a fairly high-frequency signal, even if somkewhat low-impedance....
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