Tektronix PSM3000 Series, User Manual

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Operating Basics
Pulse Measurements
NOTE. This information applies to the PSM4000 and PSM5000 series instruments
only.
The PSM4000 and PSM5000 series instruments use a detector, sampling system,
and signal processing to detect RF pulses and perform measurements on them. In
addition to overall average power, these instruments can measure the following:
Average power within the pulse
Peak power within the pulse
Duty cycle
Crest factor (which is also called peak to average power ratio)
The undersampling technique used to perform pulse measurements relies on
the pulses being repetitive. That is, these instruments will not perform “single
shot measurements”, nor will they operate well on signals in which modulation
constantly varies. Averaging and extended averaging can be used to increase
the measurement window, improve the quality of low-level measurements, and
increase the probability of sampling peak power.
The detection and sampling system in these instruments make it possible to
measure signals with modulation rates up to 10 MHz.
The real-time sampling rate of PSM4000 and PSM5000 series instruments is
500 kS/s, which is much lower than the video bandwidth of 10 MHz. Aliasing
may affect accuracy on signals with modulation bandwidth greater than 200
kHz. An anti-aliasing feature is provided to eliminate the effect of aliasing for
signals greater than 200 kHz. The anti-aliasing feature requires some additional
processing power and may slow down the measurement rate when it is active.
In order to perform pulse measurements, criteria for recognizing pulses have to be
established. This involves setting a threshold. The points at which the envelope
crosses this threshold determine the beginning and end of the pulse. The method
for setting the criteria differs somewhat between the Power Meter and the Pulse
Pro
fi ling applications, so consult the appropriate sections of this manual for
details. Both applications allow you to select an automatic setting that will work
for most applications.
RF and Microwave Power Sensors/Meters 15