Boonton 4240 RF Power Meter - Manual (Page 57)

Boonton 4240 Series RF Power Meter
ontinuous Response. Regardless of the overhead time or the digital filter length, the Model 4240 will output readings at a
Settling to 99% or 0.04 dB of final power
b. Power step of 10 dB
ecrease this time slightly; larger power steps in the
ownward direction will increase the time significantly. A 40 dB downward step, for example, will take several seconds to
settle to 0.04 dB.
igital Filter. The digital filter is a m e readings over the last x
rogrammed into the instrument for optimum general conditions. (Refer to Section 4.3 Filtering.) For diode sensors, the
ast Mode Measurement Time. The Fast Mode can be invoked over the bus to put the instrument into its fastest sampling
ency Accuracy
r would differ only by the inefficiency of the power sensor in converting
ensors.
rfect source and a power sensor are not known, but the maximum actual
C
maximum rate of about 200/second with the display operating. As the sensor and the digital filter settle, readings will ramp
up or down at that rate.
Overhead Time. Overhead time is <350 milliseconds for diode sensors and <450 milliseconds for thermal sensors under the
following conditions:
a.
c. Range does not change
d. Digital filter set to minimum
The power step may be upward or downward. Smaller power steps will d
d
D oving average or pipeline filter which simply integrates th
seconds, where x is the filter length. A step input to the filter will produce a linear ramp at the output, terminating when the
filter is full.
Default Filter Lengths. Although any filter length from 0 to 20 seconds may be chosen, default filter lengths are
p
range break-points are roughly in 10 dB steps, with the range 0 to 1 break-points at approximately -54 dBm.
Settled Measurement Time. In the free run settled mode, output data updates are held off until the measurements have
settled.
F
mode.
4.7 High Frequ
Power measurements, particularly at high frequencies, have a number of uncertainties which generally arise from imperfect
SWRs. If all power sources and power meters had impedances that were resistive and equal to Z
o
(the characteristic
impedance of the measuring system), most problems would disappear. The incident, dissipated, and maximum available
powers would all be equal, and the indicated powe
all dissipated power to indicated power. Tuning eliminates most of the SWR effects, but is cumbersome and is therefore
seldom done. The use of attenuator pads can mask imperfect SWRs, as can the use of a directional coupler to level the source
and reduce its reflection coefficient to a value equal to the directivity factor of the directional coupler. Boonton 51015 and
51033 power sensors have precision, built-in attenuators which improve the SWR over that of other power s
hen the complex coefficients of both an impe W
SWRs of both are known, the maximum positive and negative uncertainties of the measured power, Pm, can be determined
from Figure 4-11. For example, if the SWR of the source is known to be 1.2 and the SWR of the power sensor is 1.25, the
uncertainty derived from Figure 4-11 is 2%.
Operation
4-11