Using the Power Sensor
Measurement Considerations
MA24104A UG
PN: 10585-00016, Rev. A
3-15
3-5
Measurement Considerations
Time Varying Signals
Case 1: Modulated signals with pulse or pattern repetition times
≤
1 ms (PRF
≥
1 KHz)
If you obtain a steady power reading of a modulated signal (no significant fluctuations of the displayed power)
with no averaging, then it is likely that the pulse or pattern repetition rate is greater than 1 KHz. In this case,
most of the averaging of the envelope power is performed in the front end of the sensor (before being digitized).
When this is the case, the MA24104A will provide an accurate indication of the average power with no special
considerations.
Case 2: Modulated signals with pulse or pattern repetition times between 1 ms and 50 ms
(100 Hz < PRF < 1 KHz)
In this case, the signal is varying too slowly to be averaged in the front end of the sensor, so averaging must be
performed after digitalization by increasing the averaging number in the power meter application (or
calculating the average of several measurements if controlling the sensor over the bus). A large amount of
averaging must be used for some pulse/pattern repetition frequencies to get a steady reading. If Low Aperture
Time (LAT) mode is selected, the maximum recommended pulse repetition time is about 10 ms. If High
Aperture Time (HAT) mode is selected, signals with pulse repetition periods as long as 50 ms can usually be
measured.
Case 3: Modulated signals with pulse or pattern repetition times greater than 50 ms
In this case, it can be difficult to get an accurate average power reading even by averaging many readings. The
sample rate of the sensor and the pulse repetition rate of the signal may be close enough that they can “beat”
together resulting in low frequency modulation of the power indication. If averages are not calculated over
many of these beats, or an integer number of beats, errors can result. This is not unique to the MA24104A and
can be an issue with any power sensor/meter and any sampled data system.
Multitone Signals
The MA24104A is a True-RMS sensor that can measure very wide bandwidth modulation. The only limitation
is the frequency flatness of the sensor. Because the sensor’s sensitivity is not identical for all frequencies and
when measuring multi-tone signals, the frequency entered into the sensor’s application should be the average
frequency of all significant tones. The MA24104A has an error of 0.1 dB for every 100 MHz bandwidth at
frequencies between 1 GHz and 3 GHz, and an error of 0.5 dB for every 100 MHz bandwidth at frequencies
below 1 GHz and above 3 GHz.
Noise and Averaging
When there is a need to achieve a required reading resolution, particularly at low power levels, averaging is
often needed to reduce noise and steady the displayed power reading. Use the noise vs. resolution table in the
sensor manual (Using the Power Sensor|Making Measurements|Optimizing the Readings) to determine the
number of averages that will typically be required for a given resolution. Alternatively, determine the number
of averages through calculation by using the noise specifications and the fact that noise will be proportional to
the square root of N, where N is the number of averages.
For example, a CW tone at +40 dBm is to be measured to 0.01 dB resolution. Using Table 3-1 and Table 3-2,
the required number of averages is 5 averages using High Aperture Time mode (the same measurement would
require more than 78 averages in Low Aperture Time mode).