Boonton 4540 Peak Power Meter User Manual - Manual (Page 46)

Boonton 4540 Series RF Power Meter
Getting Started 3-18
3.6.2 Pulse Mode
For periodic or pulsed signals, it is often necessary to analyze the power for a portion of the waveform, or a certain region of
a pulse or pulse burst. For these applications, the 4540 Series has a triggered Pulse Mode. Operation in this mode is similar to
a digital storage oscilloscope - power samples are stored in a circular memory buffer until a trigger signal is received. The
samples, with the desired relationship to the trigger signal, are then selected and processed to obtain a power-versus-time
trace.
The trigger signal can be either internal, triggered from a rising or falling edge on the measured signal; or external, triggered
from a rear-panel BNC input. The trigger level and polarity are both programmable, as is the trigger delay time and trigger
holdoff time. Displays of both pre- and post-trigger data are available, and an auto-trigger mode can be used to keep the trace
running when no trigger edges are detected. An ―auto peak-to-peak‖ trigger level setting can be chosen to automatically set
the trigger level based on the currently applied signal. The timebase can be set from 10 ns/Div to 60 min/Div. The 4540
Series graphical display has 10 horizontal and 8 vertical divisions. Vertical units can be set in dBm, watts, volts, and dB
volts. Setting vertical resolution does not affect the sensitivity of the instrument and is provided for ease of viewing.
Programmable markers can be moved to any portion of the trace that is visible on the screen, and these can be used to mark
regions of interest for detailed power analysis. The instrument can display power at each marker, as well as average,
minimum and maximum power in the region between the two markers. This is very useful for examining the power during a
TDMA or GSM burst when only the modulated portion in the center region of a timeslot is of interest. By adjusting trigger
delay and other parameters, it is possible to measure the power of specific timeslots within the burst. Trigger holdoff allows
burst synchronization even if there is more than one edge in the burst which may satisfy the trigger level. Simply set the
holdoff time to slightly shorter than the burst’s repetition interval to guarantee that triggering occurs at the same point in the
burst each sweep. Figure 3-7 shows one timeslot of a GSM signal with marker measurements.
Figure 3-7 GSM Signal with Marker Measurements
Pulse Mode is only available when using a peak power sensor, and is best choice for most pulse modulated and periodic
signals. Pulse mode requires a repeating signal edge that can be used as a trigger, or an external trigger pulse that is
synchronized with the modulation cycle. Pulse mode performs measurements that are synchronous with the trigger - that is
the measurements are timed or ―gated‖ so that the same portion of the waveform is measured on each successive modulation
cycle. Multiple modulation cycles may be averaged together, and measurement intervals may span both before and after the
trigger. Pulse Mode is best for the following types of measurements:
• Moderate signal level (above about -40dBm except when modulation is ―off‖).
• The signal is periodic.
• A time snapshot of a single event is needed (minimum single-shot time is 200 nanoseconds).
• Typical modulation and signal types: NADC, GSM (and extensions), TDMA, RADAR, SatCom, TCAS, Bluetooth,
iDEN, NTSC, Wireless LAN.