Bird Technologies VPM3 - Manual (Page 54)

46
Directional Power Sensor (DPS) Measurements
Description
The DPS series sensors utilize elements in order to make power measurements.
Each element has an arrow on it that represents the direction in which it mea-
sures power. The elements ignore power in the opposite direction with a direc-
tivity of at least 25 dB. The DPS series can make power measurements using
either 43 type or APM/DPM elements, and the readings available vary, based on
which elements are being used.
Since the DPS uses two elements, it can measure the quality of the system by
comparing the forward and the reflected power. This is usually presented in the
form of VSWR (voltage standing wave ratio) or Return Loss.
43 Type Elements
The 43 type elements are normally used to measure peak power. These ele-
ments can measure the peak power of a system with an accuracy of +/-8% of full
scale as long as the signal meets the following requirements:

At least 15 pps (pulses per second)

Minimum pulse width of 15 μs
(800 ns if frequency is greater than 100 MHz)

Minimum Duty Cycle of 0.01%
In addition, 43 type elements can be used to measure average power in signals
with a peak-to-average ratio close to 1, like a CW or FM signals. In these cases,
the average power is measured with an accuracy of +/- 5% of full scale.
APM/DPM Elements
The APM/DPM elements are used to measure true average power. True average
power means the sensor provides equivalent heating power of the signal,
regardless of modulation or number of carriers. These elements can measure
average power with an accuracy of +/-5% of reading from full scale down to
2.5% of full scale.
Note:
The equivalent heating power is dependent on the duty cycle
of a signal. If a system puts out 50 watts with a 50% duty cycle, the
APM/DPM elements will measure 25 watts.