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Industry News

Real-time Spectrum Analyzer Provides Seamless Signal Capture

July 11, 2010
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Interference affecting RF signals is a familiar problem for development engineers working in the field of RF engineering. It may be caused by sporadic or brief events within the frequency domain, the spectral behavior of signal sources during frequency switching, or by digital circuits. Identifying the causes behind problems like these is often challenging and can be very time-consuming. To meet such demanding challenges, Rohde & Schwarz has developed the new R&S FSVR, believed to be the first instrument on the market to combine a fully fledged signal and spectrum analyzer with real-time spectrum analyzer capability.


The R&S FSVR seamlessly captures and displays the frequency spectrum over a 40 MHz bandwidth; various display formats are available. Functionality such as the analyzer's spectrogram and persistence mode ensure that no events remain hidden to users. With its frequency-selective trigger, the analyzer is also capable of detecting and examining signals that occur sporadically within the spectrum. The instrument's signal and spectrum analyzer capabilities are based on the R&S FSV. They include the full complement of functions and properties that an advanced measuring device of this kind requires to meet modern demands.

Real-Time Spectrum Analysis Up To 30 GHz
In real-time mode, the R&S FSVR captures RF signals seamlessly in the time domain with a bandwidth of up to 40 MHz, transforms them into the frequency domain and displays them as a spectrum. To achieve high time resolution and therefore accurate level measurements, even of short-term or pulsed signals, the instrument can overlap the time windows for fast Fourier transformation (FFT) by at least 80 percent. Since all the data captured is processed in real-time without any gaps, users do not miss even very short signals. The R&S FSVR is thought to be the first analyzer to provide this real-time capability for input frequencies up to 30 GHz or, with an external mixer, up to 110 GHz.

The spectrum analyzer digitizes the RF signal with a sampling rate of 128 MHz and transforms it into the frequency range, computing up to 250,000 spectra per second in the process. Since the human eye cannot register such a large number of spectra, the R&S FSVR combines the data obtained in a detector and displays the results at a refresh rate of approximately 30 times per second. This is roughly the refresh rate the human eye is capable of processing.

The peak detector ensures that no RF signal in the monitored frequency range is lost and that every signal occurring during the monitoring period is displayed. When operating in this mode, the R&S FSVR combines multiple spectra in a single trace, which significantly reduces time resolution. To provide users with a clear picture of the spectrum variation versus time, the instrument offers a variety of display and measurement functions.

Persistence Mode
The persistence mode offers an effective means of visualizing ultra-short-term signals. The R&S FSVR superimposes the gapless spectra in a diagram. Depending on how often a specific signal with a given amplitude occurs, the spectrum analyzer changes the color of the corresponding pixel on the display. Signals that are continuously present are displayed in red, for example, whereas very infrequent signals are displayed in blue. If specific signals cease to occur, they disappear from the display when the chosen persistence time has elapsed. The persistence mode represents a kind of spectral histogram. It is a useful tool when investigating signals that occur irregularly.

For example, users are now able to analyze the fast transient response behavior of phase-locked loops (PLL). Full, gapless visualization of all frequencies and amplitudes occurring, complete with probability weighting, provides a new view of the dynamic behavior of a system within the frequency domain. Users can see whether a transmitter makes rapid frequency hops or whether there are significant changes in amplitude for brief periods of time. Effects like these, which can considerably affect the behavior of an entire system, are difficult to detect using sweeping spectrum analyzers.

Figure 1 Transient response of VCO for WLAN applications in persistence mode.

Figure 1 shows a typical measurement. The analyzer captures and displays even very short signals, thus providing a complete picture of the time variation of the frequencies and amplitudes occurring in the frequency domain.

Spectrogram Function
The persistence mode opens up new possibilities for users to analyze errors by visualizing the time variation of signals in the frequency domain. To accurately capture this time variation, the R&S FSVR includes a spectrogram function. It assigns a color to the signal amplitude, allowing the spectrum to be displayed with just a single horizontal line. The spectrogram is created by continuously appending the horizontal lines. In real-time mode, the spectrogram provides a gapless picture of the spectrum versus time.

In spectrogram mode, the instrument captures and records up to 10,000 traces per second and writes them to a ring buffer. Here, too, the analyzer uses a detector to compact the data for on-screen display. The ring buffer memory depth is sufficient to store up to 100,000 traces. Depending on the selected update rate, the R&S FSVR can measure continuously for a period of up to five hours.

Figure 2 Airport radar signal—the spectrogram shows the pulsed structure of the signal and allows the pulse repetition rate of 2.5 ms to be measured.

To allow completed measurements to be examined in detail, the analyzer provides markers that users can move along the time and frequency axes. For example, this makes it very convenient to measure the duration of events or the intervals between events for a given frequency (see Figure 2), and thus greatly simplifies the gapless monitoring of frequency bands. This is not just valuable in the context of frequency monitoring; it is also highly useful for engineers searching for intermittent interfering signals.

Even for radio transmissions that involve frequent frequency changes (as is the case with RFID or Bluetooth®), the R&S FSVR is a valuable tool when it comes to tracking frequency hops and determining transmitter characteristics. It makes finding rare errors in the frequency processing of transmitters or sporadic interference from digital circuits much easier and greatly reduces the time required.

Triggering On Events
The information acquired by means of the spectrogram can also be used to define a trigger in the spectral domain, known as a frequency mask trigger (FMT). The FMT reacts to events in the spectrum. The R&S FSVR evaluates every single spectrumóat a rate of up to 250,000 spectra per secondóand compares each spectrum with a predefined, frequency-dependent mask. If a trace violates the mask, the spectrum analyzer generates a trigger event and displays the current spectrum, or makes the captured data available for further processing (in a test application, for example).

Figure 3 Input window for setting the frequency mask trigger limit line (shown here triggering on an error signal approximately 400 kHz above the actual signal frequency of a swept source).

The mask for the spectral trigger can be defined easily via the R&S FSVR's touch screen. Clearly structured tables and graphics provide users with the means to adapt masks quickly to changing situations. Figure 3 gives an impression of how easily and efficiently users can work with the analyzer. Users are able to define both a lower and an upper limit line. The trigger conditions just described can be especially useful when the signal being monitored has to remain within a given tolerance band.

The R&S FSVR has an I/Q memory depth of 200 Msamples. This means that, even with large bandwidths and high sample rates, it can record spectra gaplessly over prolonged periods of time.

Full-Featured Signal and Spectrum Analyzer
When real-time operation is not activated, the R&S FSVR operates like a sweeping spectrum analyzer or a signal analyzer. It sweeps the selected frequency range (up to 30 GHz) and displays the spectrum. It has a minimum sweep time of under a tenth of a second for the full 30 GHz frequency range. It offers a wide range of resolution bandwidths from 1 Hz to 10 MHz. Unlike in real-time mode, the resolution bandwidth is user-selectable and not linked to the selected span. In addition to the fast sweep filters, the analyzer also provides channel filters and root raised cosine filters to support mobile radio standards.

Like all of the company's spectrum analyzers, the R&S FSVR offers as standard an extensive range of measurement functions, including adjacent channel power, spectrum emission mask, third-order intercept (TOI), CCDF and spurious emission measurements. With a level measurement uncertainty of 0.4 dB up to 7 GHz, the R&S FSVR delivers precise and reliable results. With the R&S FSV-K9 option, it can also be connected directly to R&S NRP power sensors, obviating the need for a separate power meter in situations requiring especially high measurement accuracy.

The R&S FSVR provides more than 1,000 sweeps per second. This high measurement performance not only helps speed up manufacturing systems, it also reduces processing time in R&D or conformance testing in applications where large numbers of measurements have to be averaged (as required by numerous standards). In addition, the instrument offers options for measuring basic physical characteristics, including phase noise (R&S FSV-K40), noise figure (R&S FSV-K30), and the parameters of signals with analog (AM/FM/PM) and digital modulation (R&S FSV-K70).

As well as classical spectrum analysis, the R&S FSVR is suitable for verifying compliance with mobile radio standards. It currently supports the following standards: GSM/EDGE/EDGE Evo, WCDMA, TD-SCDMA, CDMA2000, WLAN 802.11a/b/g/n, WiMAX and LTE (TDD/FDD).

Conclusion
Combining real-time spectrum analyzer capability with full-featured signal and spectrum analysis, the R&S FSVR is a unique instrument. Its extensive, easy-to-use real-time functions mean users have new and highly effective analysis resources at their disposal, yet at the same time can continue to work with the same signal and spectrum analysis functions they are familiar with. The intuitive user interface, designed along the same lines as those of spectrum analyzers, simplifies working with the instrument and successfully integrates real-time analysis within a cohesive overall design concept.

Rohde & Schwarz,
Munich, Germany,
Tel: +49 89 4129 13774,
e-mail: customersupport@rohde-schwarz.com,
www.rohde-schwarz.com;

North America:
Tel: +1 888 837 8772,
e-mail: customer.support@rsa.rohde-schwarz.com;

Latin America:
Tel: +1 410 910 7988,
e-mail: customersupport.la@rohde-schwarz.com;

Asia:
Tel: +65 65130 488,
e-mail: customersupport.asia@rohde-schwarz.com.
RS No. 302

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