Boonton 4240 RF Power Meter - Manual (Page 56)

Boonton 4240 Series RF Power Meter
Noise Error Examples. Figures 4-8 and 4-9 show the computed error for the 51013 diode sensor at different power levels,
for 2.8 and 10 second filters. To attain these results, the sensor must be at a stable temperature, and zeroing must be done
immediately before the measurement is taken.
Integration of Power. With long filtering, instrument readings may seem erroneous because the filter has not been cleared.
For example, with a 20 second filter, if a 2 second RF pulse is applied, the instrument display will indicate a nonzero level for
18 seconds after the pulse has terminated. Additional pulses will be integrated along with the first until, by the process of
selective deletion, the pulses are removed one at a time from the filter. Actually, measurement samples are deleted, not the
pulses, giving rise to a ramping effect at the instrument display/output. This is shown in Figure 4-10. In all senses, the filter
is a simple integrator.
Figure 4-10. Integration of Power
Clearing of Filter. When long filter times are used, it may become troublesome at times to wait for the filter to clear. If the
Auto filter function is selected, the filter is cleared after significant power changes, and filtering then resumes. Clearing can
also be accomplished by changing the filter length to any different value and then resetting it using the interface bus;
however, with bus operation, most of the trigger modes clear the filter at trigger time.
Partial Results. Measurement time is affected by the filter since valid readings to within a certain error band can be obtained
only when the filter is full. If the filter has been cleared, data is available at reduced accuracy immediately after the first 50
millisecond sample period. The filter uses the number of samples as a divisor when computing the average, and the
output/display does not ramp but homes in on the result instead as the samples accumulate.
.6 Measurement Time
a power input step is the sum of the overhead time and the length of the digital
4
tep Response. The measurement time from S
filter, where the overhead time is defined as the time delay due to sensor response time and measurement software
(processing).
Operation
4-10