Agilent Technologies N1913a User Manual page 201

Using U2000 Series USB Power Sensors 7
Accuracy
The value of –12 dBm lies in the lower range of the U2000/1/4A sensor.
In autoranging mode ("AUTO"), the power meter determines the average
power level is below –10 dBm and selects the low power path. However,
the peak amplitude of –6 dBm is beyond the specified, square law
response range of the low power path diodes.The high power path
(–10 dBm to +20 dBm) should be used to ensure a more accurate
measurement of this signal. However, range holding in "UPPER" (the high
power path), for a more accurate measurement, results in a considerably
increased number of filtering processes.
Speed and Averaging
The same signal also requires that consideration is given to measurement
speed. As shown above, in autoranging mode the power meter selects the
low power path in the U2000/1/4A sensor. With auto- averaging also
configured, minimal filtering is applied. Values of one to four for average
power levels above –20 dBm are used in the low power path. (Refer to
"U2000 Series auto- averaging settings" on page 170.)
If the range is held in "UPPER" for more accuracy, the measurement is
slower. More filtering is applied due to the increase in noise susceptibility
at the less sensitive area of the high power path. Values of one to 128 for
average power levels less than –10 dBm are used. (Again, refer to "U2000
Series auto- averaging settings" on page 170.) Manually lowering the filter
settings speeds up the measurement but can result in an unwanted level
of jitter.
Summary
Attention must be paid to signals whose average power levels are in the
low power path range whilst their peaks are in the high power path range.
You can achieve best accuracy by selecting the high power path or best
speed by selecting the low power path.
N1913A/1914A EPM Series Power Meters User's Guide User's Guide 177