Oscilloscope question

[Allison]

>Subject: Re: Oscilloscope question
>   From: "John Allain" <allain at panix.com>

>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?

The probe is the "interface" between teh scope and the circuit.
The wrong type of connection will either pick up undesired 
signals, load the signal resistively or capacitively.  The 
latter is signigicant as digital signals are time based and
the capacitive load of raw coax cable and alter that timing
or worse introduce ringing into the circuit.


>Should a scope rating 2X the computer clock speed keep the
>readings useful?

Scopes are like telescopes, even the cheapest can see Mars,
slightly better can see the bands on Mars.  Relating the 
bandwidth to CPU clock is mostly meaningless.  The parameters
your are mesuring is timing of the waveform and possibly 
the waveshape.  If the bandwidth (and corosponding rise 
time of the scope amplifiers) is not adaquate you will distort
the waveshape and if the timebase(X or horizontal scan time)
is either porrly calibrates or cannout go fast enough it may 
be impossible to determine if a event occurs 450nanoseconds 
(10^-9) later.

Risetime is a measure of how fast the amplifer in a scope
can go from 10% to 90%.  Usually you want that to be faster
than the waveshape you wish to view by more an two and as 
much as 10 times.  Note most TTL have rise times well under
20ns and even some of the older logic families can be very 
fast rise and fall times. So this is why a 50mhz (465b) 
scope of better is used. The key here is risetime is the 
measure used to understand the faithfulness of the reproduction
for a given wave shape.  An example may be a very fast rise 
time square wave can end up looking more like a saws teeth
when reproduced using a scope with inadaquate risetime in the
most extreme case. 

For the math types 1/risetime+falltime give an approximate 
repetition rate or frequency 1/40ns=25,000,000 Reptitions/Second.
So 2x that is only 50mhz!  Also bandwidth and rise time are only 
related as a scope can have good rise time but limited bandwidth
or the reverse.


>I suspect the approact to an oscilloscope for the typical person 
>on this list wouldn't be the same as for a "rank dummy".

Yes.  In both cases there is a requirement to understand 
electronic basics and what waveforms result from basic 
circuits.  Complex circuits are just many simple ones 
combined in many ways.


Allison