NARDA 8718B - Manual (Page 84)

83 Chapter 14 Probe Zero
• Whenever the meter continues to show low-level values
and never seems to get back to a zero-field condition,
positive zero drift is a possibility. This can be confirmed
by shielding the probe to see if the meter goes back to a
zero reading. See Shielding the Probe on the following
page for suggested methods.
Temperature Concerns
It is important to allow the meter and probe to become
conditioned to the ambient temperature before attempting to
zero the system. This is especially important when the meter
and probe have been transported or stored at a different
temperature such as in a vehicle, for example. Diode
sensor-based probes require only a few minutes to stabilize.
The thermo-couple sensors used in microwave band and ultra
broadband probes may require 15-20 minutes to stabilize.
Establishing a Zero Field
It is important to “zero” the probe while the probe is not exposed
to a significant RF field level. For accuracy, the field level should
be lower than the lowest value that the probe is capable of
reading. If previous measurements have indicated that the
ambient field level is very low, the probe can be zeroed without
shielding. This is often the case in a typical office environment.
It is recommended that the ambient field level be at least 30 dB
below the full scale measurement range of the probe. When it is
necessary to zero a probe in higher field areas, the probe must
be shielded.
R E - A U T O - Z E R O
1 2 . 1 % S T D
2 7 . 5 M A X C F 1 . 0 0
A L T M A X R M E N U F R E Q
F1 F2 F3 F4