RF Primer: Spectrum Analyzers Scan RF Performance

When choosing a portable benchtop spectrum analyzer, the best value is in matching the performance and capabilities of an instrument to the waveforms under investigation.

Spectrum analyzers are among the most versatile of RF/microwave test instruments, providing a window into the performance of highfrequency components and systems. Selecting a new spectrum analyzer for a test lab or production line is not simple, given the number of choices. Making the right choice is a matter of knowing what is available and matching performance and features to an application.

In contrast to an oscilloscope, which displays signal amplitude as a function of time, a spectrum analyzer shows signal information in the frequency domain. Although modern digital oscilloscopes tend to be associated with high-speed pulsed signal waveforms, a properly equipped spectrum analyzer can display information about pulsed signals as well, in addition to continuouswave (CW) and modulated signals. The input impedance of RF/microwave spectrum analyzers is typically 50 ohms, although analyzers are available with 75-ohm input impedance for testing cable-television (CATV) systems.

The most basic performance parameter for a spectrum analyzer is frequency range, followed by amplitude measurement range. For both parameters, accuracy is important, with high accuracy generally associated with high price. While the frequency range requirement for a given measurement may seem obvious, it often depends on whether harmonic frequencies are considered. For certain components, such as frequency mixers and oscillators, a suitable spectrum analyzer may need a frequency range that extends well beyond the third harmonics of a band of interest.

The frequency range for a given measurement is set by the frequency span adjustment which, for most instruments, can be set for the full span of the spectrum analyzer, across a user-defined frequency span, or for a zero-span measurement.

Any display of frequency information in a spectrum analyzer is affected by a user's choice of the instrument's filtersresolution-bandwidth (RBW) filters and video-bandwidth (VBW) filters. Considering a spectrum analyzer as a receiver, the RBW filters are essentially the receiver's IF filters, and are used to select how much information about a signal will be displayed on the analyzer's screen, and with what frequency resolution. VBW filters are applied after a signal has been detected and prior to the display screen, to help remove noise. For most analyzers, the 3-dB points are used to define each filter's bandwidth. Both RBW and VBW filters are not continuously variable, but selected in steps, such as a 1-3 or 1-3-5 sequence.

With a spectrum analyzer, there is a trade off when using narrowerbandwidth filters, in increased sweep time. Selecting the narrowest RBW filter will result in the lowest displayed average noise level (DANL) but the longest sweep time. Wider RBW filters result in faster sweep speeds but higher DANLs. The type of signal to be displayed also dictates how filters are set during a test. For a modulated signal, the RBW filter must be set wide enough to include sideband information.

The amplitude measurement range and amplitude accuracy are functions of a spectrum analyzer's analog components, such as its logarithmic amplifier, as well as its digital components, such as the analog-to-digital converter (ADC). An analyzer's displayed dynamic range is determined by how high the reference level can be set and the instrument's DANL. For an instrument with reference level at 0 dBm and DANL of -80 dBm, the dynamic range is 80 dB. Because spectrum analyzers also include frontend attenuators, any amount of added attenuation must be considered.

An analyzer's amplitude accuracy can hinge a great deal on the type of detectors incorporated in the instrument, with a peak detector allowing the analyzer to measure the highest peak power level within a given measurement interval. But that same peak detector is not suitable for measuring the mean power of pulsed signals, which is better handled by a sample detector or root-mean-square (RMS) type detector. Ideally, a spectrum analyzer to be used for CW, modulated, and pulsed measurements is equipped with each type of detector.

Spectrum analyzers are available in both portable and rack-mount or benchtop models. In the portable arena, for example, B&K Precision Corp. offers model 2650 for on-site measurements from 50 kHz to 3.3 GHz. Additional portable spectrum analyzers include the 10-kHz-to- 6-GHz model FSH3-03 from Rohde & Schwarz, the 10-MHz-to-8-GHz model HF-6080 from Spectran, the model 9103 from Willtek with coverage of 100 kHz to 7.5 GHz, and the Spectrum Master instruments from Anritsu Co., with models covering as wide as 9 kHz to 20 GHz.

A much wider selection of benchtop spectrum analyzers is available, including models from Agilent Technologies, Advantest, Anritsu, Rohde & Schwarz, and Tektronix. Agilent offers economy (ESA series) and high-performance (PSA series) benchtop spectrum analyzers as well as analyzers in the MXA series that provide both spectrum and vector signal analysis.

For example, the model N9020A is an MXA series analyzer (Fig. 1) is available with frequency ranges of 20 Hz to 3.6 GHz, 8.4 GHz, 13.6 GHz, and 26.5 GHz. It captures bandwidths as wide as 25 MHz and offers connections for in-phase (I) and quadrature (Q) baseband signals with bandwidths to 40 MHz. The analyzer can scrutinize complex modulated signals used in modern communications systems, capturing RF events as short as 14 ms.

The RSA6114A (Fig. 2) real-time spectrum analyzer (RTSA) from Tektronix is something of a hybrid instrument, blending the characteristics of an oscilloscope and its time-related triggering capabilities with the frequency-display capabilities of a traditional spectrum analyzer. Rather than sweeping across a selected bandwidth, the RSA6114A can capture all of the signal information within bandwidths as wide as 40 MHz (and 110 MHz as an option) across a frequency range of 9 kHz to 14 GHz, including for pulsed and modulated signals. For spans beyond the real-time bandwidth, the RTSA steps through the desired frequency range. The stepped-sweep of the RTSA can be orders of magnitude faster than traditional swept-analyzers, especially for wide sweeps with narrow resolution bandwidths.

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