LBXXXA Series USB Powe
™
Product Manual PMA (Power Meter Application)
Revision: 08/02/11
- 24 -
Extended averaging can be very helpful in many situations. But, like any measurement you must have a general idea
of what the measurement is doing in order to properly manage the results. An alternative solution in this case might be
to make a more reasoned tradeoff between the number of averages and extended averaging.
Try resetting the extended averaging to 10 and then alternately connect and disconnect the RF signal. After a few
connections and disconnections you get some sense of how long your signal takes to settle. With some
experimentation you will find a combination of averaging and extended averaging and clicking the
Rst Max & XAvg
button that gives you an acceptable result. The following settings produced a reasonable result:
Averages = 1000
Extended Averages = 10
Even modest extended averaging helps the process along quite nicely.
Now we will go back to our measurement. The current results are:
Pulse = 16.7 dBm
DC = 0.033%
Pk = 18.2 dBm
Avg = -17.4 dBm
Clearly, a relationship should exist between the pulse/peak power, duty cycle and average power. Duty cycle is the
ratio of pulse on time to total time or PRT (1/PRF).
In the past, CW meters would provide a “duty cycle correction” to
the average power. But this meant you needed to know the duty cycle. The adjustment was fairly simple:
Pulse Power (W) = Average Power (W) / Duty Cyle
But this assumes a perfect pulse and no power during the off time. We can use this relationship to check the validity of
our previous measurement. Using the formula above we get:
Average Power = -17.4 dBm or about 18.2 uW
Pulse Power (W) = 18.2 uW/0.033% = 18.2e-6/0.00033 = 55.14 mW
Pulse Power (dBm) = 10 * Log(55.14 mW/1 mW) = 17.4 dBm
This is between the peak power (18.2 dBm) and pulse power (16.7 dBm) so it is a reasonable result. However, this
duty cycle, peak and pulse data is actual measured results. They are not assumed. Furthermore, the effects of wave
shape have been taken into account in the resulting measurements. And while we do not know the exact wave shape
we do know it is not perfectly square.
Finally, we will examine the Max Hold function. Start by presetting the unit and select
Pulse Power
mode. This sets
the averages back to 75 and disables the extended averaging. Our readings are bouncing around again. Now click the
Max Hold
button on the toolbar as shown below.
The readings should quickly stabilize. Now if you were to repeat the calculations above you would see that they do not
make as much sense. This apparent error is easily resolved when you understand that the maximum hold function is
applied to each reading independently. So while each reading is at its maximum, the maximums did not necessarily
occur at the same time or during the same measurement event. Knowing this
Max Hold
can be useful, but its use
must be tempered with an understanding of how the measurement is made.
