There are two broad categories of spectrum analyzers: swept-tuned analyzers and real-time analyzers. Both swept-tuned analyzers and real-time analyzers have been around for many years. However, within the past decade or so, spectrum analyzers have become much more sophisticated. These newer spectrum analyzers use digital signal processing to provide additional measurement capability—and let you interpret measurement results much more easily.
Both swept-tuned and real-time-spectrum analyzers display amplitude versus frequency. How they process and display this information, however, varies with the specific type of analyzer. A real-time spectrum analyzer displays the energy at all frequency components simultaneously. A swept-tuned spectrum analyzer displays measurement results sequentially—in other words, not in ''real-time.'' This is because a swept-tuned analyzer, in effect, uses a single narrow filter that is tuned across a range of frequencies to produce a spectrum display.
Swept-tuned analyzers have been the traditional choice for higher frequency applications—for example, 100 kHz and above. Real-time analyzers are generally used for lower frequencies—for example, audio-frequency and vibration measurements.
For an additional overview of spectrum/network measurements, see Spectrum and Network Measurements by Robert A. Witte (Prentice Hall, Englewood Cliffs, New Jersey, 1993).
Swept-tuned spectrum analyzers are descended from radio receivers. So it should come as no surprise that swept-tuned analyzers are either tuned-filter analyzers (analogous to a TRF radio) or superheterodyne analyzers. In fact, in their simplest form, you could think of a swept-tuned spectrum analyzer as nothing more than a frequency-selective voltmeter with a frequency range that's tuned (swept) automatically. It is essentially a frequency-selective, peak-responding voltmeter calibrated to display the rms value of a sine wave. The spectrum analyzer can show the individual frequency components that make up a complex signal. However it does not provide phase information, only magnitude information. The swept-tuned superheterodyne receiver technique used in Keysight spectrum analyzers can make a wide variety of frequency-domain measurements over a large dynamic range and a wide frequency range (30 Hz to 325 GHz).
Modern swept-tuned analyzers (superheterodyne analyzers, in particular) are precision devices that can make a wide variety of measurements. However, they are primarily used to measure steady-state, or repetitive, signals because they can't evaluate all frequencies in a given span simultaneously. The ability to evaluate all frequencies simultaneously belongs exclusively to the real-time analyzer.
Despite the high performance of modern superheterodyne analyzers, they still can't evaluate frequencies simultaneously and display an entire frequency spectrum simultaneously. Thus, they are not real-time analyzers. Also, the measurement times may be very long because the sweep speed of a swept-tuned analyzer is always limited by the time required for its internal filters to settle. As you will learn in the following discussion, there are many different designs and measurement processes used to provide real-time, dynamic signal analysis.
Another way to build a spectrum analyzer is to combine several bandpass filters, each with a different passband frequency. Each filter remains connected to the input at all times. This type of analyzer is called a parallel-filter analyzer. After an initial settling time, the parallel-filter analyzer can instantaneously detect and display all signals within the analyzer's measurement range. Therefore, the parallel-filter analyzer provides real-time signal analysis.
A particular strength of the parallel-filter analyzer is its measurement speed, which allows it to measure transient and time-variant signals (also called dynamic signals). However, the frequency resolution of a parallel-filter analyzer is much coarser than a typical swept-tuned analyzer. This is because the resolution is determined by the width of the bandpass filters. To get fine resolution over a large frequency range, you would need many, many individual filters—thus increasing the cost and complexity of such an analyzer. This is why all but the simplest parallel-filter analyzers are expensive.
Typically, parallel-filter analyzers have been used in audio-frequency applications.
The Fourier or FFT spectrum analyzer is another real-time spectrum analyzer implementation. The Fourier analyzer, also referred to as a dynamic signal analyzer, use digital signal processing to sample the input signal and convert it to the frequency domain. This conversion is done using the Fast Fourier Transform (FFT). The FFT is an implementation of the Discrete Fourier Transform, the math algorithm used for transforming data from the time domain to the frequency domain.
1 kHz FFT Analysis example - Digitize a time-domain signal and use FFT analysis to convert it to the frequency-domain.
FFT spectrum analyzers are powerful instruments, because their processing power can extract more information from an input signal than just the amplitude of individual frequency components. For example, FFT analyzers can measure both magnitude and phase, and can also switch easily between the time and frequency domains. This makes them ideal instruments for the analysis of communication, ultrasonic, and modulated signals.
If an FFT analyzer samples fast enough, all input data is evaluated and the analyzer makes a real-time measurement. When operating in real time, FFT analyzers can make the same measurements traditionally done with parallel-filter analyzers—and make these measurements, if desired, with far greater frequency resolution.
In the past FFT analyzers have had the disadvantage of their restricted frequency range—most FFT analyzers could not make measurements above 100 kHz. The limiting factor has been the speed of the analog-to-digital converter used to sample the analyzer's input signal. This is why swept-tuned superheterodyne analyzers are still used for RF and microwave measurements, though some newer-generation swept-tuned analyzers, such as the Keysight 8560 and PSA family of analyzers can also make FFT measurements. As you will see, this trend toward hybrid technology has gone one step further with the Keysight 89600 Vector Signal Analysis software.
In the past, swept-tuned and superheterodyne spectrum analyzers covered wide frequency ranges from audio, thru microwave, to millimeter frequencies. In addition, digital signal processing (DSP) intensive fast Fourier transform (FFT) analyzers provided high-resolution spectrum and network analysis, but were limited to low frequencies due to the limits of analog-to-digital conversion and signal processing technologies. Today's wide-bandwidth, vector-modulated (also called complex or digitally modulated), time-varying signals benefit greatly from the capabilities of FFT analysis and other DSP techniques. VSA's combine superheterodyne technology with high speed ADC's and other DSP technologies to offer fast high-resolution spectrum measurements, demodulation, and advanced time-domain analysis. The VSA is especially useful for characterizing complex signals such as burst, transient, or modulated signals used in communications, video, broadcast, sonar and ultrasound imaging applications.
The 89600 VSA provides these capabilities:
Dynamic Signal analysis (including time-varying and complex modulated signals)
Complex (I+Q) measurements
Real-time signal analysis
Simultaneous Multi-Domain Measurement Analysis
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