Keysight E6640A, Measurement Applications User & Programmer Reference

Page: 1584 / 2870

11 Occupied Bandwidth Measurement
Sweep/Control
Periodic Timer Triggering:
This feature selects the internal periodic timer signal as the trigger. Trigger occurrences are set by the
Periodic Timer parameter, which is modified by the Sync Source and Offset.
The figure below shows the action of the periodic timer trigger. Before reviewing the figure, we’ll explain
some uses for the periodic trigger.
A common application is measuring periodic burst RF signals for which a trigger signal is not easily
available. For example, we might be measuring a TDMA radio which bursts every 20 ms. Let’s assume that
the 20 ms period is very consistent. Let’s also assume that we do not have an external trigger source
available that is synchronized with the period, and that the signal-to-noise ratio of the signal is not high
enough to provide a clean RF burst trigger at all of the analysis frequencies. For example, we might want to
measure spurious transmissions at an offset from the carrier that is larger than the bandwidth of the RF
burst trigger. In this application, we can set the Periodic Timer to a 20.00 ms period and adjust the offset
from that timer to position our trigger just where we want it. If we find that the 20.00 ms is not exactly right,
we can adjust the period slightly to minimize the drift between the period timer and the signal to be
measured.
A second way to use this feature would be to use Sync Source temporarily, instead of Offset. In this case,
we might tune to the signal in a narrow span and use the RF Burst trigger to synchronize the periodic timer.
Then we would turn the sync source off so that it would not miss-trigger. Miss-triggering can occur when
we are tuned so far away from the RF burst trigger that it is no longer reliable.
A third example would be to synchronize to a signal that has a reference time element of much longer
period than the period of interest. In some CDMA applications, it is useful to look at signals with a short
periodicity, by synchronizing that periodicity to the "even-second clock" edge that happens every two
seconds. Thus, we could connect the even-second clock trigger to Ext1 and use then Ext1 as the sync
source for the periodic timer.
The figure below illustrates this third example. The top trace represents the even-second clock. It causes
the periodic timer to synchronize with the leading edge shown. The analyzer trigger occurs at a time
delayed by the accumulated offset from the period trigger event. The periodic timer continues to run, and
triggers continue to occur, with a periodicity determined by the analyzer time base. The timer output
(labeled "late event") will drift away from its ideal time due to imperfect matching between the time base of
the signal being measured and the time base of the analyzer, and also because of imperfect setting of the
period parameter. But the synchronization is restored on the next even-second clock event. ("Accumulated
offset" is described in the in the Offset function section.)
1584 W-CDMA & HSPA Measurement Applications Reference