Unlike the VSA, the RTSA converts the time-sampled data into the frequency domain in real time, prior to signal capture (Fig. 2c). This permits a pre-analysis of the signal spectrum before triggering a capture to memory. The instrument previews the signal and triggers only on spectral events of interest. A frequencymask trigger (FMT) enables the analyzer to detect and trigger on signals well below the largest signal level in the spectrum (Fig. 3). FMT technology goes beyond simple IF level triggering and provides a reliable way to look at intermittent RF signals embedded in complex spectrums.
Not long ago, the cost for such superfast real-time digital signal processing was prohibitive for most engineers. The computational speed needed to execute a 1024-point FFT in less than 12μs, before the next data frame is ready, requires a respectable amount of DSP horsepower. Advances in DSP technology and clever use of a combination of real-time FFTs for triggering with post-processing FFTs for measurements have enabled RTSA capability that fits even a modest budget.
Also, the modern single-box portable RTSA is much smaller than early models that occupied an entire equipment rack. Another unique RTSA feature is a full-bandwidth, streaming digital I-Q data-output port. The real-time spectrum analyzer is optimized for transient RF signals and has extensive time-correlated multidomain analysis capability.
Internally, the SA, VSA, and RTSA all feature special characteristics that enhance their performance for different applications. So which spectrum analyzer best fits each demanding application?
KEY ADVANTAGES OF EACH ANALYZER
The swept-tuned spectrum analyzer is optimized for analog continuous signal measurements. The high dynamic range available on upper models is great for applications like testing linearity in a multicarrier power amplifier (PA).
The cost advantages of multicarrier PA designs have made them very popular in the cellular industry. But 3G multicarrier PAs require exceptional linearity to prevent unwanted intermodulation products from interfering with adjacent-channel signals. Testing these extraordinary amplifiers requires leading-edge analyzer dynamic range (Fig. 4). This prevents distortion within the spectrum analyzer itself, masking the amplifier's linearity characteristics during intermodulation and adjacent-channel power (ACP) measurements.
The swept-tuned analyzer also excels at integrated phase-noise measurements. Sensitive applications such as highcapacity point-to-point telecom radios have particularly difficult phase-noise requirements. The complex high-order modulations used by these radios demand an analyzer with superior phasenoise performance. High-end, swepttuned spectrum analyzers offer the best phase-noise performance. Most swept analyzers also come with optional phasenoise integration software not commonly found on other analyzers.
Like the swept-tuned spectrum analyzer, the VSA is optimized for some unique applications. For example, it dominates coherent two-channel measurements. At present, the VSA is the only analyzer that can perform cross-spectrum measurements between two channels. This capability allows for precise phase measurement and time correlation of pulses. The dual-channel VSA can make network-analyzer-like measurements with an arbitrary signal—even a noise spectrum. Crosschannel spectrum measurements are useful for phased arrays, radar, and interferometer applications.
Similar to the SA and VSA, the realtime spectrum analyzer is optimized for transient RF signals. The RTSA is preferred for signals that switch on and off rapidly or change frequency rapidly. The cost and integration advantages of the transmit/receive (T/R) switch used for time-division duplexing (TDD) have made transient RF signals like those used in WLAN, 3G, and many RFID devices quite popular. With its precapture spectral analysis and FMT, the RTSA is ideal for intermittent signal diagnostics.
The RTSA's FMT can reliably capture spectral emissions from an RFID tag that responds intermittently. Many existing proprietary anticollision schemes make it possible for an RFID tag interrogator to communicate with one tag without interference from other tags. Testing the tag's responses can be difficult in complex spectral environments. The RTSA's frequency mask trigger routinely captures asynchronous signals in dense RFID signal environments.
The RTSA's concentration on today's complex TDD modulations has given it a particularly rich multidomain analysis package. In RFID work, power-efficient modulations like frequency-shift keying (FSK) are often used. The RTSA tends to be better equipped for analysis of these signals, offering features such as built-in symbol decoding for FSK and amplitudeshift keying (ASK) modulations, while other analyzers offer only analog waveform measurements (Fig. 5).
The RTSA technology supported WLAN signals very early. Consequently, it offers a well-developed set of WLAN measurements. Advanced features like autodetection of complementary code keying (CCK) or orthogonal frequency-division multiplexing (OFDM) modulations are found only on the RTSA. Place the marker on a signal burst, and the RTSA figures out the appropriate WLAN demodulation settings (Fig. 6).
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