Tektronix P6015A, Instruction Manual

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In Detail
1–14
P6015A Instruction Manual
NOTE . If you want to check the presence or absence of signals from
low-frequency equipment, and if the equipment is line-powered and
plugged into the same outlet system as the oscilloscope, then the
common three-wire ground system provides the signal ground return.
However, this indirect route adds inductance in the signal path,
which can produce ringing and noise on the displayed signal, and
therefore is not recommended.
Don’t assume that the ground in the circuit under test is the same as
the oscilloscope ground. Check the circuit ground by first attaching
the ground lead of the probe to a known earth ground, then touch the
probe tip to the point you think is ground. If there is any voltage
differential, then the point that the tip is connected to is not a valid
ground point. (Due to the 1000X attenuation of the P6015A, you
may have to increase the sensitivity of the oscilloscope in order to
see small voltage differentials.) Perform this check before you attach
the ground lead of the probe to a ground point on the circuit under
test. The oscilloscope ground should always be earth ground as long
as you are using the proper power cord and plug.
Ground Lead Inductance
When making any kind of absolute measurement, such as amplitude,
rise time or time delay measurements, use the shortest grounding
path possible.
NOTE
. Even with the shortest ground lead, probe capacitance and
ground lead inductance form a series-resonant circuit that has the
potential to ring. Such ringing oscillations depend on the high-fre-
quency components of the transient you are measuring and will
distort the true waveform. The ability to see the ringing depends on
the oscilloscope bandwidth. The amount of displayed ringing can be
reduced by using the bandwidth limit function of the oscilloscope.
If you are going to be probing many different points in the same
circuit having signal frequencies less than 1 MHz, you can run a
ground wire from the circuit ground to the oscilloscope ground
terminal (if one is provided). Such a ground connection alleviates the
need to continually reconnect the probe ground lead.