Specifications

40
Time gating can be achieved using three different methods that will be
discussed below. However, there are certain basic concepts of time gating
that apply to any implementation. In particular, you must have, or be able to
set, the following four items:
• An externally supplied gate trigger signal
• The gate control, or trigger mode (edge, or level)
• The gate delay setting, which determines how long after the trigger
signal the gate actually becomes active and the signal is observed
• The gate length setting, which determines how long the gate is on and
the signal is observed
Controlling these parameters will allow us to look at the spectrum of the
signal during a desired portion of the time. If you are fortunate enough to
have a gating signal that is only true during the period of interest, then you
can use level gating as shown in Figure 2-34. However, in many cases the
gating signal will not perfectly coincide with the time we want to measure
the spectrum. Therefore, a more flexible approach is to use edge triggering
in conjunction with a specified gate delay and gate length to precisely define
the time period in which to measure the signal.
Consider the GSM signal with eight time slots in Figure 2-35. Each burst is
0.577 ms and the full frame is 4.615 ms. We may be interested in the spectrum
of the signal during a specific time slot. For the purposes of this example,
let’s assume that we are using only two of the eight available time slots, as
shown in Figure 2-36. When we look at this signal in the frequency domain in
Figure 2-37, we observe an unwanted spurious signal present in the spectrum.
In order to troubleshoot the problem and find the source of this interfering
signal, we need to determine the time slot in which it is occurring. If we wish
to look at time slot 2, we set up the gate to trigger on the rising edge of burst
0, then specify a gate delay of 1.3 ms and a gate length of 0.3 ms, as shown
in Figure 2-38. The gate delay assures that we only measure the spectrum
of time slot 2 while the burst is fully on. Note that the gate delay value is
carefully selected to avoid the rising edge of the burst, since we want to allow
time for the RBW filtered signal to settle out before we make a measurement.
Similarly, the gate length is chosen to avoid the falling edges of the burst.
Figure 2-39 shows the spectrum of time slot 2, which reveals that the spurious
signal is NOT caused by this burst.
RF signal
Gate signal
Figure 2-34. Level triggering: the spectrum analyzer only measures the frequency spectrum
when gate trigger signal is above a certain level