WLAN combo devices share a unique set of measurement problems, which seem to be central to the future direction of communications. The advent of low-cost, high-speed logic devices and block error-detection and correction schemes has propelled the communications industry in the direction of packetised information transmitted via intermittent RF signal bursts. Unlike older continuous-wave communication systems, packetised communication systems such as WLAN use asynchronous data transmissions. The analysis of WLAN signals requires the ability to capture specific asynchronous RF signal events and efficiently find them in the captured record for analysis. Early digital-modulation developers sought tools that could do a better job of analysing vector signals in the modulation domain than the oscilloscopes or scalar analysers of the day. This led to the development of the constellation analyser. Initially, constellation analysers were little more than specialised oscilloscopes. More recently, the functions of the constellation analyser have been combined with those of a spectrum analyser to facilitate the down-conversion of RF signals, leading to what has become the present-day vector signal analyser (VSA). Many vector signal analysers have some ability to characterise WLAN signals. But factors such as packet collisions, intermittent signals, and startup/shutdown transients are asynchronous events that demand an analyser with triggering abilities suitable to capture these events and a truly time-correlated multi-domain-analysis ability to diagnose them.
Analysing multiple domains
Challenges such as these have led to the development of the real-time spectrum analyser (RTSA), which is specifically designed to address the measurement challenges associated with dynamic RF signals, such as the bursted packet transmissions used in systems like WLAN and Bluetooth. The fundamental concept of real-time spectrum analysis is the ability to trigger on an RF signal, seamlessly capture time-synchronised data into memory, and analyse it in multiple domains. This makes it possible to reliably detect and characterise RF signals that change over time. Figure 1 shows a simplified block diagram of an RTSA (Tektronix RSA3408A). The RF front end can be tuned from DC to 8 GHz, and the input signal is down-converted to a fixed intermediate frequency related to the maximum real-time bandwidth of the RTSA. The signal is then filtered, digitised by the analogue/digital convertor, and passed to the DSP engine that manages the instrument's triggering, memory, and analysis functions. While many elements of this block diagram and acquisition process are similar to those of the traditional VSA architecture, the RTSA is optimised to deliver real-time triggering, seamless signal capture, and time-correlated multi-domain analysis. In addition, advancements in analogue/digital convertor technology produce a conversion with high dynamic range and low noise, allowing the RTSA to make traditional frequency-domain measurements that equal or surpass the basic RF performance of many swept-spectrum analysers. The RTSA's frequency mask trigger allows the engineer to view elusive transient signals that are impossible to see in free-run mode. Real-time triggering makes it possible to reliably detect and capture intermittent RF signals, even when they occur in the presence of much more powerful adjacent signals.
Continuous real-time recording
Unlike many vector signal analysers that operate by taking "snapshots" of a modulated signal, the RTSA has no holes or gaps in the time-domain record that it uses to make time-, frequency-, and modulation-domain measurements. The true time-correlated multi-domain analysis provided by the RTSA allows users to precisely correlate diagnostic data across multiple domains to rapidly understand the nature of the signal. Take, for example, a WLAN combo device dropping 5% of the WLAN packets under ideal signal conditions. An engineer would find it difficult to determine if this was caused by an uncontrolled packet collision or a logic problem in the setup of the media access controller (MAC). Using the MAC as a trigger source for a traditional vector spectrum analyser would not only necessitate a time-consuming connection, but could be regarded as a questionable practice if the MAC is part of the problem to be diagnosed. Searching 100 signal bursts in the VSA's capture record to find the five that have a problem is an inefficient, time-consuming approach to diagnosis. Using the RTSA frequency-mask trigger will capture this problem for analysis without complicated external triggers or time-consuming data searches.
Interference
Combo devices have more modes of interference than the typical transceiver. The engineer has to cope not only with the in- and out-of-band emissions regulations, but also with the effects of the RF emissions on co-located receivers, transceivers and high-speed microprocessors. The intermittent nature of WLAN packets can make interference-related issues very time-consuming to identify without the triggering capability of the RTSA. Asynchronous RF intermittent interference problems frequently cause project delays, as engineers struggle to gain insight into these unintentional sporadic interactions.
WLAN-specific measurements
The RTSA has the technical ability to trigger and reliably capture intermittent signals. To be an effective diagnostic tool for combo devices, it must also have a complete set of WLAN measurements: such a feature is available as an option on the RSA3408A. Included in the analysis package are all the popular 802.11a/b/g measurement standards, preset for rapid signal characterisation. Measurements such as spectral mask, EVM (error-vector magnitude), on/off power transient, CCK constellations, OFDM (orthogonal-frequency-division multiplexing) constellations, sub-carrier constellations, and many others are part of this comprehensive analysis software.
Time-correlated multi-domain analysis
As indicated earlier, the DSP capabilities of the RTSA provide true time-correlated multi-domain analysis for the entire signal stored in memory. Switching between time-, frequency-, modulation-, code-domain and spectrogram displays to view signal characteristics in the most logical way is possible without loss of timing information. Moreover, the seamless signal capture of the RTSA enables markers set in the spectrogram to be precisely time-correlated with those in other domains. Completely time-correlated displays (Figure 2) mean that an event captured in the spectrogram or frequency-mask-trigger display can be viewed in the modulation domain for evaluation of its impact on error performance. The marker is simply placed on the event in a spectrogram, and the corresponding symbol is viewed on the constellation diagram. This ability to identify anomalies in one domain and instantly evaluate their impact at that exact time in another domain allows for rapid diagnostic.
Real-world combo-device problems
Test equipment must have a "combo" of analysis measurements available for each standard in the combo device to be tested. The modern RTSA is capable of supporting most modern wireless modulations including 802.11a/b/g, GSM/EDGE, W-CDMA, HSDPA, cdma2000, 1xEV-DO, TD-SCDMA, and more. With 36 MHz of real-time RF capture bandwidth and over 78 dB dynamic range (TOI), the real-time spectrum analyser is ideally suited for combo-device work and supports many popular WLAN-companion transceivers. Like the WLAN analysis option, all the supported wireless standards are powerful, multi-domain analysis packages. In addition to the time-correlated multi-domain ability, these other analysis packages also benefit from the patented frequency-domain-triggering ability.
Application example
Combo devices that use both Bluetooth personal area networks and 802.11b/g WLAN networks have a unique set of RF interference problems because they share the same 2.4 GHz industrial/scientific/medical (ISM) frequency band. Often advertised as compatible and complementary, Bluetooth and WLAN modulation formats can interfere with each other when the transceivers are located less than a few meters apart. This is clearly the case when they coexist in a combo device. Evaluating the packet-interference performance of a Bluetooth/WLAN combo device can be difficult. The need to capture the collision of signals requires a trigger capable of catching the event. The RTSA can detect signals colliding in a band using the frequency-mask-trigger facility, which allows for rapid measurement setup and capture of the asynchronous WLAN/Bluetooth packet collisions. Frequency-mask triggering can efficiently identify the collision information and store only the packets of interest into memory. This eliminates the time-consuming process of searching through a long record for occasional errors, and reduces the capture-memory requirements. Pre- and post-trigger delays can be used to ensure complete capture of the WLAN/Bluetooth burst, if desired. It is worth noting that the unlicensed ISM band has many possibilities for interference scenarios. Hospitals, stock exchanges, and manufacturing plants often have immensely complicated ISM-band RF environments. In the field, it has traditionally been difficult to determine the interference sources precisely. Using the same procedure outlined above for the development laboratory, it is now possible to quickly identify interference sources in the field (Figure 3). Unlike other sources of interference that are continually present or occur at predictable times, interference caused by packet communications devices is sporadic and asynchronous, making it very difficult to pinpoint the root causes and to engineer solutions. The RTSA can easily capture thihis interference, and can greatly aid the engineer in determining the level of spectral control necessary to prevent interference.
