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Different from the old-fashioned continuous wave communication system, the packet communication system (such as WLAN) adopts asynchronous data transmission technology. Analysing WLAN signals requires that specific asynchronous RF signal events be captured and effectively found in the captured records for analysis.
Tektronix R
The SA3408A real-time spectrum analyzer The early digital modulation developers wanted some tools that could analyze vector signals in the modulation domain better than current oscilloscopes or vector analyzers, which led to the development of a constellation analyzer. In the early days, the constellation analyzer was just a dedicated oscilloscope. Recently, the function of the constellation analyzer has been combined with a spectrum analyzer to facilitate down-conversion of the RF signal, making it a today's vector signal analyzer (VSA).
Although many vector signal analyzers have some sort of WLAN signal verification capability, asynchronous events such as packet collisions, intermittent signals, and power on/off transients require that the analyzer have the appropriate triggering capabilities to capture these events; at the same time it has real time correlation. Multi-domain analysis capabilities to diagnose these events.
These challenges have led to the development of real-time spectrum analyzers (RTSAs) designed to meet the measurement challenges associated with dynamic RF signals such as burst packet transmission used in systems such as WLAN and Bluetooth. The basic concept of real-time spectrum analysis is the ability to capture RF signals, seamlessly capture time-synchronized data into memory, and analyze the data in multiple domains. This makes it possible to reliably detect and characterize RF signals that change over time. 
Vector spectrum analyzer structure 
Figure 1. Real-time spectrum analyzer structure showing DSP functional differences.
Figure 1 is a simplified block diagram of the RTSA (Tektronix RSA3408A). The RF front-end can be tuned from DC to 8 GHz and the input signal is downconverted to a fixed intermediate frequency associated with the RTSA's maximum real-time bandwidth. The signal is then filtered, digitally converted by an analog-to-digital converter, and then passed to the DSP engine, which manages the instrument's triggering, memory, and analysis functions. Although many of the elements of this block diagram and acquisition process are similar to the traditional VSA architecture, the RTSA is specifically optimized for providing real-time triggering, seamless signal capture, and time-dependent multi-domain analysis. In addition, the development of analog-to-digital converters allows for high dynamic range, low noise conversion, allowing the RTSA to perform traditional frequency domain measurements that equal or exceed the basic RF performance of many swept analyzers.
The RTSA's frequency mask triggering feature allows engineers to see difficult-to-capture transients that are not visible in free-running mode. Real-time triggering technology is expected to reliably detect and capture intermittent RF signals even when there are much more powerful adjacent signals.
Unlike many vector signal analyzers that record continuously in real-time to take a “snapshot†of the modulated signal, the RTSA has no holes or blanks in the time domain records used to make measurements in the time, frequency, and modulation domains. The real time-correlated multi-domain analysis provided by the RTSA allows users to accurately correlate diagnostic data from multiple domains and quickly understand signal characteristics.
For example, a WLAN combo device drops 5% of WLAN packets under ideal signal conditions. Engineers may find it difficult to determine if this was caused by an uncontrolled packet collision or due to a logical problem in the Media Access Control (MAC) setup.
In traditional vector spectrum analyzers, using MAC as a trigger source not only takes a lot of time to connect, but if the MAC is part of the problem that needs to be diagnosed, this method itself has problems. Capturing 100 signal bursts in the VSA capture record and finding five bursts with problems is an inefficient, time-consuming diagnostic method. Using RTSA frequency template triggering will capture this issue for analysis without complex external triggers or time-consuming data searches.
The number of interference patterns of interfering devices exceeds that of typical transceivers. Engineers not only have to deal with in-band and out-of-band radiation regulations, they must also deal with the effects of RF radiation on receivers, transceivers, and high-speed microprocessors.
Without the triggering function of the RTSA, the intermittent nature of the WLAN packet makes it necessary to spend a lot of time to identify interference related problems. Asynchronous RF intermittent interference problems often cause project delays because engineers must work hard to see these unintentional sporadic interactions.
The WLAN-specific measurement RTSA has the technical ability to trigger and reliably capture intermittent signals. To be a valid diagnostic tool for combo devices, it must also have a full set of WLAN measurements: Some measurements are provided as options on the RSA3408A. All popular 802.11a/b/g measurement standards are preset in the analysis software to quickly characterize the signal. This sophisticated analysis software provides many measurement functions such as spectrum masks, EVM (Error Vector Magnitude), switching power transients, CCK constellation, OFDM (Orthogonal Frequency Division Multiplexing) constellation, subcarrier constellation, and more.
Time-dependent multi-domain analysis As mentioned earlier, the RTSA's DSP function provides true time-correlated multi-domain analysis of the entire signal stored in memory. It can switch between time domain, frequency domain, modulation domain, frequency domain map and code domain display map to view signal characteristics without losing timing information. In addition, RTSA seamless capture signal function can set the scale in the spectrum diagram. , establish a precise time correlation with the scales in other domains. 
Figure 2. Multi-domain analysis shows.
A completely time-dependent display (Figure 2) means that events captured in the spectrum or frequency mask trigger display can be viewed in the modulation domain to assess its effect on bit error performance. You can simply place the scale on an event in the spectrogram and see the corresponding symbol on the constellation diagram.
The ability to identify anomalous events in one domain and then immediately determine the impact of that point in another domain is a key component in quickly obtaining diagnostic information.
Actual Environment Combo Device Problem The test device must provide the 'combo' analysis measurement function for each standard in the combo device to be tested. Modern RTSA can support most modern wireless modulation technologies, including 802.11a/b/g, GSM/EDGE, W-CDMA, HSDPA, cdma2000, 1xEV-DO, TD-SCDMA, and so on. With a real-time RF capture bandwidth of 36MHz and a dynamic range (TOI) greater than 78dB, the real-time spectrum analyzer provides an ideal solution for combo device operation and supports many popular WLAN matching transceivers.
As with the WLAN analysis option, all supported wireless standards are implemented using powerful multi-domain analysis software. In addition to the time-related multi-domain functions, the patented frequency domain trigger technology brings many benefits to other analysis software.
Application Example: WLAN/Bluetooth Packet Collision Combo devices that use both a Bluetooth personal area network and an 802.11b/gWLAN network have a unique set of RF interference problems because they share the same 2.4GHz industrial/scientific/medical (ISM) frequency band. Bluetooth and WLAN modulation formats are often claimed to be compatible and complementary, but when their transceivers are only a few meters apart, they interfere with each other, and when they must co-exist in a combo device, this will be very obvious. It may be difficult to assess the packet interference performance of Bluetooth/WLAN combo devices. To capture signal collisions, triggers are required to be able to catch the event.
The RTSA can use the frequency mask trigger tool to detect signal collisions in the band. It can quickly set up measurements and capture asynchronous WLAN/Bluetooth packet collisions. The frequency template trigger can effectively identify the collision information and only store the interested group into memory. This eliminates the need to search for long records for long periods of time to find occasional errors, reducing the capture memory requirement. When needed, pre-trigger delays and post-trigger delays can be used to ensure proper acquisition of WLAN/Bluetooth bursts.
It is worth mentioning that the unlicensed ISM band has many possible interference situations. Hospitals, stock exchanges, and manufacturers often have an unusually complex ISM band RF environment. At the scene, it has traditionally been difficult to determine the exact source of interference. By using the same procedure listed above for the development lab, it is now possible to quickly identify sources of interference in the field (Figure 3). 
Figure 3. Frequency Mask Triggering Technique Captures 802.11b/g CCK signal packets that collide with Bluetooth FHSS signals and interference from microwave ovens.
Unlike other sources of interference that are always present or occur at predictable times, the interference caused by packet communication equipment is sporadic and asynchronous, so it is difficult to determine the root cause and design a solution. The RTSA can easily capture this interference and effectively assist the engineer in determining the level of spectrum control necessary to prevent interference.
The WLAN combo device brings a series of unique measurement challenges. RTSA provides a unique solution for WLAN combo equipment engineers. Due to its frequency mask triggering, seamless signal capture, and time-dependent multi-domain analysis, the real-time spectrum analyzer provides many unique tools for designing and debugging packet-based RF communication systems.
Real-time spectrum analysis for emerging RF standards
WLAN combo equipment faces a series of unique problems, which seems to have become the core content of future communications development. The emergence of low-cost, high-speed logic devices and error block detection and correction schemes has prompted the communications industry to burst packetized information via intermittent RF signals.