Series 8650B Universal Power Meters
2-28
Publication 31470-001, Rev C, November 2, 2017
2.4.6
High Power Level Measurements
High power amplifiers and transmitters can damage standard sensors. Use only high power sensors to measure
these devices without using attenuators and measurements.
For example, if the output of an RF source is amplified to +30 dBm (1 Watt), this signal cannot be measured
directly using a standard sensor because the sensor’s maximum input level is +23 dBm (and any level above +20
dBm is potentially harmful to a standard sensor). The signal would have to be attenuated, and the attenuation
would have to be corrected for by means of a measurement offset. However, if a 5 Watt high power sensor is
used, any power level up to +37 dBm can be measured directly without the use of an attenuator.
2.4.7
Modulated Measurement Modes
The 8650B series of power meters expands upon the capabilities of the previous 8540 power meters in a
number of ways. In the past, power measurements of modulated signals (Pulse, Multi-tone, AM, etc.) required
that the signals be attenuated to levels less than -20 dBm to avoid errors due to sensor nonlinearity. The 8650B
eliminates this restriction when used with a modulation sensor, and brings the speed and accuracy of diode
sensors to the power measurement of modulated signals. Basic measurement procedures are presented below,
along with some useful tips on how to get the most out of the modulated measurement modes.
The modulated measurement modes are available through the sensor setup menu when the active sensor a
modulated series. The 8650B features three modulated measurement modes:
•
Modulated Average Power (MAP)
•
Pulse Average Power (PAP)
•
Burst Average Power (BAP)
MAP and PAP modes measure the true average power of modulated and pulsed signals. PAP mode differs from
MAP mode only in that it allows specifying a duty cycle figure, which is automatically factored into the
measurement. In BAP mode, the true average power within the pulse is measured (the pulse pattern is detected
automatically, so there is no need to specify the duty cycle).
2.4.7.1
MAP Mode
The Modulated Average Power (MAP) mode measures RF signals, which are amplitude modulated, pulse
modulated, or both. In the MAP mode the 8650B calculates the average RF power received by the sensor over a
period of time controlled by the time constant of the internal digital filter. The result is comparable to
measurement by a thermal power sensor.
In this mode, the 8650B measures the average power of CW and modulated signals, such as:
•
AM
•
Two-tone
•
Multi-carrier
•
Pulse modulation
•
Digital modulation (QPSK, QAM, etc.)
For example, if an RF signal pulse modulated at 50 Hz with a 10% duty cycle is measured with the averaging
factor set to 128, the measured power reading will be 10% of the peak power during pulse ON periods. If the
signal is modulated at a low pulse rate (below about 1 kHz), the 8650B will synchronize the readings precisely
with the start of a pulse so that each displayed reading is averaged over a whole number of pulses (There are no
fractional pulses included in the measurement). This eliminates a significant amount of noise from the readings.
An important reminder, even though the filter settling time has been set to a long time constant of 2.56
seconds, the update rate of the meter will be much faster, even the first reading will be very close to the fully
settled value.
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