ANEKA INFO TEKNIK: SPECTRUM ANALYZER; Trace Averaging

Trace Averaging
Digital displays offer another choice for smoothing the display: trace averaging. This is a completely different process than that performed using the average detector. In this case, averaging is accomplished over two or more sweeps on a point-by-point basis. At each display point, the new value is averaged in with the previously averaged data:Thus, the display gradually converges to an average over a number of sweeps. As with video filtering, we can select the degree of averaging or smoothing.
We do this by setting the number of sweeps over which the averaging occurs. Figure 2-31 shows trace averaging for different numbers of sweeps. While trace averaging has no effect on sweep time, the time to reach a given degree of averaging is about the same as with video filtering because of the number of sweeps required.

In many cases, it does not matter which form of display smoothing we pick. If the signal is noise or a low-level sinusoid very close to the noise, we get the same results with either video filtering or trace averaging. However, there is a distinct difference between the two. Video filtering performs averaging in real time. That is, we see the full effect of the averaging or smoothing at each point on the display as the sweep progresses. Each point is averaged only once, for a time of about 1/ VBW on each sweep. Trace averaging, on the other hand, requires multiple sweeps to achieve the full degree of averaging, and the averaging at each point takes place over the full time period needed to complete the multiple sweeps.

As a result, we can get significantly different results from the two averaging methods on certain signals. For example, a signal with a spectrum that changes with time can yield a different average on each sweep when we use video filtering. However, if we choose trace averaging over many sweeps, we will get a value much closer to the true average. See Figures 2-32a and b.

Figure 2-31. Trace averaging for 1, 5, 20, and 100 sweeps, top to bottom (trace position offset for each set of sweeps)

Figure 2-32a. Video filtering

Figure 2-32b. Trace averaging

Figure 2-32. Video filtering and trace averaging yield different results on FM broadcast signal

Time gating

Time-gated spectrum analysis allows you to obtain spectral information about signals occupying the same part of the frequency spectrum that are separated in the time domain. Using an external trigger signal to coordinate the separation of these signals, you can perform the following operations:

Measure any one of several signals separated in time; for example, you can separate the spectra of two radios time-sharing a single frequency Measure the spectrum of a signal in one time slot of a TDMA system Exclude the spectrum of interfering signals, such as periodic pulse edge transients that exist for only a limited time

Why time gating is needed

Traditional frequency-domain spectrum analysis provides only limited information for certain signals. Examples of these difficult-to-analyze signals include the following signal types:

Pulsed RF

Time multiplexed

Time domain multiple access (TDMA)

Interleaved or intermittent

Burst modulated

In some cases, time-gating capability enables you to perform measurements that would otherwise be very difficult, if not impossible. For example, consider Figure 2-33a, which shows a simplified digital mobile-radio signal in which two radios, # 1 and # 2, are time-sharing a single frequency channel. Each radio transmits a single 1 ms burst, and then shuts off while the other radio transmits for 1 ms. The challenge is to measure the unique

frequency spectrum of each transmitter.

Unfortunately, a traditional spectrum analyzer cannot do that. It simply shows the combined spectrum, as seen in Figure 2-33b. Using the time-gate capability and an external trigger signal, you can see the spectrum of just radio # 1 (or radio # 2 if you wished) and identify it as the source of the spurious signal shown, as in Figure 2-33c.

Figure 2-33a. Simplified digital mobile-radio signal in time domain

Figure 2-33b. Frequency spectrum of combined signals. Which radio produces the spurious emissions?

Figure 2-33c. Time-gated spectrum of signal # 1 identifies it as the source of spurious emission

Figure 2-33d. Time-gated spectrum of signal # 2 shows it is free of spurious emissions

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 it ms:

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.

Figure 2-34. Level triggering: the spectrum analyzer only measures the frequency spectrum when gate trigger signal is above a certain level

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.

Figure 2-35. A TDMA format signal (in this case, GSM) with eight time slots

Figure 2-36. A zero span (time domain) view of the two time slots

Figure 2-37. The signal in the frequency domain

Figure 2-38. Time gating is used to look at the spectrum of time slot 2

Figure 2-39. Spectrum of the pulse in time slot 2

There are three common methods used to perform time gating:
Gated FFT
Gated video
Gated sweep

Gated FFT
Some spectrum analyzers, such as the Agilent PSA Series, have built-in FFT capabilities. In this mode, the data is acquired for an FFT starting at a chosen delay following a trigger. The IF signal is digitized and captured for a time period of 1.83 divided by resolution bandwidth. An FFT is computed based on this data acquisition and the results are displayed as the spectrum. Thus, the spectrum is that which existed at a particular time of known duration.
This is the fastest gating technique whenever the span is not wider than the FFT maximum width, which for PSA is 10 MHz.

To get the maximum possible frequency resolution, choose the narrowest available RBW whose capture time fits within the time period of interest. That may not always be needed, however, and you could choose a wider RBW with a corresponding narrower gate length. The minimum usable RBW in gated FFT applications is always lower than the minimum usable RBW in other gating techniques, because the IF must fully settle during the burst in
other techniques, which takes longer than 1.83 divided by RBW.

Gated video
Gated video is the analysis technique used in a number of spectrum analyzers, including the Agilent 8560, 8590 and ESA Series. In this case, the video voltage is switched off, or to negative infinity decibels during the time the gate is supposed to be in its blocked mode. The detector is set to peak detection. The sweep time must be set so that the gates occur at least
once per display point, or bucket, so that the peak detector is able to see real data during that time interval. Otherwise, there will be trace points with no data, resulting in an incomplete spectrum. Therefore, the minimum sweep time is N display buckets times burst cycle time. For example, in GSM measurements, the full frame lasts 4.615 ms. For an ESA spectrum analyzer set to its default value of 401 display points, the minimum sweep time for GSM gated video measurements would be 401 times 4.615 ms or 1.85 s. Some TDMA formats have cycle times as large as 90 ms, resulting in long sweep times using the gated video technique.

Figure 2-40. Block diagram of a spectrum analyzer with gated video

Gated sweep

Gated sweep, sometimes referred to as gated LO, is the final technique. In gated sweep mode, we control the voltage ramp produced by the scan generator to sweep the LO. This is shown in figure 2-41. When the gate is active, the LO ramps up in frequency like any spectrum analyzer. When the gate is blocked, the voltage out of the scan generator is frozen, and the LO stops rising in frequency. This technique can be much faster than gated video

because multiple buckets can be measured during each burst. As an example, let's use the same GSM signal described in the gated video discussion earlier in this chapter. Using a PSA Series spectrum analyzer, a standard, non-gated, spectrum sweep over a 1 MHz span takes 14.6 ms, as shown in Figure 2-42. With a gate length of 0.3 ms, the spectrum analyzer sweep must be built up in 49 gate intervals (14.6 divided by 0.3), or. If the full frame of the GSM signal is 4.615 ms, then the total measurement time is 49 intervals times 4.615 ms = 226 ms. This represents a significant improvement in speed compared to the gated video technique which required 1.85 s for 401 data points. Gated sweep is available on the PSA Series spectrum analyzers.