oscilloscope probe questions

[Dwayne Carranza]

i bought a used scope and am looking for probes for it.
its my first scope and i mostly want to learn how to use it correctly
and become proficient in basic measurements.
its an old tektronix 5103N with a 5A21N diff amp, a 5A18N dual trace amp
and a 5B10N time base amp. the connections on the instrument all say
1Mohm, 47pf.

does that mean i need probes with those values?
could i use probes with more or less capacitance?

thanks in advance for your help,
dc

 

[Steve Minshull]

Dwayne Carranza wrote:

i bought a used scope and am looking for probes for it.
its my first scope and i mostly want to learn how to use it correctly
and become proficient in basic measurements.
its an old tektronix 5103N with a 5A21N diff amp, a 5A18N dual trace amp
and a 5B10N time base amp. the connections on the instrument all say
1Mohm, 47pf.

does that mean i need probes with those values?

Yes....you need to match the time constant in the scope (R*C) with that
of your probes. Most probes are 1Mohm and they have a small variable
capacitance that you can adjust with a screw driver. If you have too
much capacitance on the probe, the probe scope interface will act as a
high pass filter and too little capacitance it will act as a low pass
filter. Generally you calibrate the probes with what you know is good
quality square wave so that you see straight sides and a flat top on
your scope.

You can also get probes that you can switch between 1Megohm and 10Meg
ohms which is a good idea....the higher the input impedance to the scope
the less likely you are to effect the circuit being measured. Once you
switch to 10Megohms you might need to recalibrate the probe though.

could i use probes with more or less capacitance?

thanks in advance for your help,
dc

 

[ptw]

Dwayne Carranza wrote:

i bought a used scope and am looking for probes for it.
its my first scope and i mostly want to learn how to use it correctly
and become proficient in basic measurements.
its an old tektronix 5103N with a 5A21N diff amp, a 5A18N dual trace amp
and a 5B10N time base amp. the connections on the instrument all say
1Mohm, 47pf.

does that mean i need probes with those values?
could i use probes with more or less capacitance?

thanks in advance for your help,
dc

Nah. Just get a oscilloscope probe*. 1 Megohm 47 pF is the impedance of
the scope. If you stick a ohmmeter to the inputs of your scope, 1 megohm
is what you'll get.


*One end is a probe with a metal hook (the "red" lead), and a ground
alligator clip/hook. The other end is BNC male connector to plug into
scope. The cable shield is ground.

Note that if your scope plugs into wall AC receptacle, the chassis is
earth grounded. Therefore, the ground clip/hook is also earth grounded.
So don't connect ground clip/hook to non-zero source/signal!

 

[Jim Yanik]

Steve Minshull <s.r.minshull@shef.ac.uk> wrote in
news:e13dae$fnm$1@hermes.shef.ac.uk:

Dwayne Carranza wrote:
i bought a used scope and am looking for probes for it.
its my first scope and i mostly want to learn how to use it correctly
and become proficient in basic measurements.
its an old tektronix 5103N with a 5A21N diff amp, a 5A18N dual trace
amp and a 5B10N time base amp. the connections on the instrument all
say 1Mohm, 47pf.

does that mean i need probes with those values?

Yes....you need to match the time constant in the scope (R*C) with
that of your probes. Most probes are 1Mohm and they have a small
variable capacitance that you can adjust with a screw driver.

It's the 10 Megohm probes (10X) that have the comp adjustment,1Meg
probes(1X) are little more than coax with a fancy tip,and a lot of
capacitance.

If you
have too much capacitance on the probe, the probe scope interface will
act as a high pass filter and too little capacitance it will act as a
low pass filter. Generally you calibrate the probes with what you
know is good quality square wave so that you see straight sides and a
flat top on your scope.

You can also get probes that you can switch between 1Megohm and 10Meg
ohms which is a good idea....the higher the input impedance to the
scope the less likely you are to effect the circuit being measured.

Yes,less resistive and capacitive loading on the circuit under test.

Once you switch to 10Megohms you might need to recalibrate the probe
though.

You adjust it at 10X,not at 1X.

could i use probes with more or less capacitance?

thanks in advance for your help,
dc

--
Jim Yanik
jyanik
at
kua.net