HOW A REAL-TIME SPECTRUM ANALYZER WORKS
A modern real-time spectrum analyzer can acquire a passband, or span, anywhere within the analyzer's input frequency range. At the heart of this capability is an RF down-converter followed by a wideband IF section. An ADC digitizes the IF signal, and the system performs all further steps digitally. An FFT algorithm implements the transformation from time domain to frequency domain, where subsequent analysis produces displays such as spectrograms, codograms, and more.
Several key characteristics distinguish a successful real-time architecture:
- An ADC system that can digitize the entire real-time bandwidth with sufficient fidelity to support the desired measurements.
- An integrated signal analysis system that provides multiple analysis views
of the signal under test, all correlated in time.
- Sufficient capture memory and DSP power to enable continuous real-time
acquisition over the desired time measurement period.
- DSP power to enable real-time triggering in the frequency domain.
This section contains several architectural diagrams of the main acquisition and analysis blocks of a proprietary RTSA. The principles of operation that are described typically apply to real-time spectrum analysis in general. However, some ancillary functions, such as minor triggering-related blocks and display and keyboard controllers, have been omitted to clarify the discussion.
Digital Signal Processing in Real-Time Spectrum Analysis
A modern RSA uses a combination of analog and digital signal processing to convert RF signals into calibrated, time-correlated multidomain measurements. This section deals with the digital portion of the RSA signal processing flow. Figure 10-13 illustrates the major digital signal processing blocks used in a modern RSA. An analog IF signal is band-pass filtered and digitized. A digital down-conversion and decimation process converts the analog to digital converted samples into streams of in-phase (I) and quadrature (Q) baseband signals. A triggering block detects signal conditions to control acquisition and timing. The baseband I and Q signals as well as triggering information are used by a baseband DSP system to perform spectrum analysis by means of FFT, modulation analysis, power measurements, timing measurements, and statistical analysis.
An RSA typically digitizes a band of frequencies centered on an IF. This band or span of frequencies is the widest frequency for which real-time analysis can be performed. Digitizing at a high IF rather than at DC or baseband has several signal processing advantages, such as overcoming spurious performance, DC rejection, and improved dynamic range. But this can result in excessive computation to filter and analyze the data if processed directly. An alternative technique, shown in Figure 10-13, is to employ a DDC and a decimator to convert the digitized IF into I and Q baseband signals at an effective sampling rate just high enough for the selected span.
As shown in Figure 10-13, the IF signal in an RSA is digitized with a sampling rate designated as FS. The digitized IF is then sent to a DDC. A numeric oscillator in the DDC generates sine and cosine signals at the centered frequency of the band of interest. The sine and cosine signals are numerically multiplied with the digitized IF to generate streams of I and Q baseband samples that contain all the information present in the original IF signal. The I and Q streams then pass through variable-bandwidth low-pass filters where the cut-off frequency of the low-pass filters is varied according to the selected span.
I and Q Baseband Signals
Figure 10-14illustrates the process of taking a frequency band and converting it to baseband using quadrature down-conversion. The original IF signal is contained in the space between three halves of the sampling frequency and the sampling frequency itself.