RF Signal Capture and Analysis System Serves Commercial and Defense Applications
By Tektronix, Inc.
The ability to capture signals “off the air” over wide bandwidths and time periods is an essential tool in applications ranging from spectrum monitoring, radar and wireless system testing, signals intelligence (SIGINT), electronic intelligence (ELINT), communications intelligence (COMINT), and electronic warfare (EW). In each case, the captured signals must be digitized as an exact representation of the original so that they can be analyzed, characterized, possibly modified, and then played back at either their original or some other frequency. Accomplishing this is beyond the means of a spectrum analyzer alone as it is designed for capturing signal information only over short periods.
However, high-performance real-time spectrum analyzers are precision instruments, making them extremely useful when used as the “front end” of a specialized system designed to continuously store signal-capture data for hours or days. This is especially true when the spectrum analyzer can trigger on and mark many types of events even when obscured by stronger signals and has the ability to eliminate undesired instrument-generated signals from appearing in the file.
A system comprised of a Tektronix RSA5000B or RSA6000B real-time spectrum analyzer (Figure 1) and X-COM Systems IQC5000A RF record, storage, and playback system (Figure 2) performs these tasks extremely well. System analysis can be performed using software such as X-COM’s Spectro-X and RF Editor and Tektronix SignalVu and RFXpress for signal analysis and characterization.
The Proverbial Haystack
Identifying signals even over a relatively narrow bandwidth is an incredibly difficult task when they are interspersed with other signals with greater signal strengths along with random interference and other impediments. For example, capturing a swath of spectrum 160 MHz wide over 15 minutes in an urban area can reveal thousands of signals. Some will be from legitimate, licensed entities, and others such as harmonics and spurious emissions from other services. In military environments, additional signals can originate from enemy jammers and communication systems that are either operating in the open or attempting to evade detection by operating extremely close to or even beneath other carriers.
Finding the signals is itself a formidable task and identifying them adds another equally daunting layer of complexity. As a result, it is essential to have an extremely sensitive receiver and a system that provides error-free signal capture whose fidelity is maintained from when it is captured through analog-to-digital conversion and storage. Location, identification, and characterization require software that can find a waveform type regardless of its frequency and how randomly it appears, along with its characteristics (which may be time-variant). In addition, analysis, characterization, and file manipulation software can glean detailed information from each signal, and a new, highly modified file can be created that can be used for a variety of purposes.
Description of the System
A system designed to perform these tasks is shown in Figure 3. It employs the Tektronix RSA5000B or RSA6000B as well as X-COM’s IQC5000A, which is designed specifically for use in applications requiring long-term signal capture. The dual-channel IQC5000A can capture, store, and play back RF signal activity continuously over periods ranging from 300 min. at the RSA5000’s maximum bandwidth of 165 MHz to over 80 hr. with a 10 MHz bandwidth. The instrument measures only 12 x 3.5 x 10.5 in., weighs less than 10 lb., has up to 2 Tybtes of storage internally and 16 Tbytes externally, and operates from 120/240 VAC or a 12-VDC source such as a vehicle battery.
In the figure, the RSA5000B or RSA6000B at left acts as the receive front end, preselector, and downconverter, and presents a digital 16-b I&Q sample stream to the IQC5000A at up to 800 Mb/s. The data stream is stored by the IQC5000A either internally or externally depending on the length of the signal capture. For playback, the IQC5000A converts the I&Q samples to analog form and then streams them to an external source for re-modulation and retransmission. The source can be either a vector signal generator or the Tektronix AWG70000 series arbitrary waveform generator.
The RSA5000B and RSA6000B have capabilities found on no other similar instrument that make them uniquely qualified for the applications to which the IQC5000A is best suited. For example, spectrum analyzers typically use a superheterodyne receiver, one of the disadvantages of which is that in the process of harmonic mixing multiple “spectral windows” appear, and the signal present in these windows converts in the spectrum analyzer and may distort the input signal. It also allows numerous false signals to appear in the display, which makes it difficult or impossible to identify the desired signal from the rest. In all of the applications described in this article, the consequences of these images can range from annoying to disastrous.
Spectrum analyzers use a tunable preselector filter to remove unwanted mixer images and responses to LO harmonics, effectively closing all spectral windows except the one desired. However, as the preselector filters in traditional spectrum analyzers use YIG technology that has a maximum bandwidth of 40 MHz, the typical approach when wider bandwidths are required is to bypass the preselector. Without the filtering that the preselector provides, signals on the other side of the first LO can appear in the IF and thus in the display.
To avoid this problem, the RSA5000B and RSA6000B use a switched-filter preselector rather than a YIG-based approach over the entire bandwidth of the instrument. This allows it to accommodate wideband signal bandwidths as well as pulses with fast rise times and spread-spectrum signals that cover a broad range of frequencies. This technique is more difficult to implement but the advantages it provides are significant and even sometimes critical, as the analyzer is effectively image-free to the highest frequency in its range.
Preselection filters also keep internal signals from radiating from the RF input connector on the front panel that can produce signals strong enough to be detected by the enemy from quite a distance, making it possible to locate the system. The switched-preselector technology used in Tektronix real-time spectrum analyzers virtually eliminates this problem, as the instrument is always preselected. The effectiveness of this approach when compared to the approach of bypassing the preselector is shown in Figure 4.
LO leakage from an unpreselected swept spectrum analyzer (left) is compared to LO leakage from an RSA6000B (right). LO radiation is indicated by the upper blue band. The greatest portion of radiated signals stems from the LO’s fundamental, while the higher-order harmonics extend to frequencies beyond 20 GHz. Emissions from the unpreselected analyzer on the left are at such a high level that the instrument would effectively be broadcasting its presence. In contrast, the RSA6000B has almost no LO feedthrough radiation.
Unique Triggers Make the Difference
When looking for signals of interest, triggers can make the difference between detection and failure as they make it possible to identify specific points in a seamless I&Q capture record so that events of interest can be much easier to find. This is especially important when file contains data recorded over a long period of time.
While standard triggering techniques can detect signals that exceed an amplitude threshold, they cannot find a signal at a particular frequency if another signal of higher amplitude is present at the same frequency. Runt triggering addresses some of these signal-under-signal cases. However, only the Tektronix DPX density trigger can discriminate signals within a specific range of amplitudes and frequencies without the operator having knowledge of target signal characteristics besides where it might show up on the display.
When a target signal appears, the density value increases and as the trigger system monitors the density measurement it activates a trigger whenever the density value exceeds the adjustable density threshold. The threshold need only be set to a level somewhere between the normal density readings and the density caused by the offending signal. The instrument software can also compute the threshold value automatically. If there is a wideband modulated signal such as a chirp, and another signal of lesser amplitude is beneath it, both can be seen because the latter signal occupies the frequency all the time and the chirp sweeps through it infrequently.
“Trigger On This” allows the user to point and click to set up the DPX density trigger. For example, with a time-varying signal, right-clicking on a spot within the DPX spectrum display or pressing and holding a finger on the touchscreen for a second produces a menu. Selecting “Trigger On This” lets the trigger automatically adjust the threshold. The display will now only update whenever the automatic threshold is exceeded. The density threshold or the size of the measurement box can be adjusted until the event is acceptably captured.
Frequency Mask Triggering
Frequency mask triggering, also known as frequency domain triggering, compares the spectrum shape on the display to a user-defined mask so that the instrument can trigger on changes in spectrum shape. This ability makes it possible to trigger on weak signals in the presence of stronger ones, which is extremely useful when trying to detect random or intermittent signals in the presence of intermodulation products, transient signals, interference, and suspect emitters. The mask is created by defining a set of frequency points in their amplitudes, point by point either graphically or by drawing it on this display with a mouse, or even automatically based on user-defined amplitude and frequency offsets from a given spectrum trace. Triggers can be set to occur when a signal outside the mask enters within its bounds or when a signal inside the mask boundary exceeds it.
Another triggering feature extremely valuable in long-term signal capture is the time qualifier. This allows a trigger event to be qualified as either longer or shorter than a defined time period, or inside or outside a given time window and can be added to whatever type of trigger is selected. A power trigger can be set to enable when a signal exceeds the level of (for example) -30 dBm as well as when it exceeds -30 dBm for less or more than a user-specified time. This ensures that the instrument is triggering on the beginning of an event and not somewhere in the middle.
DPX technology employed by Tektronix real-time spectrum analyzers makes it possible to identify and measure transients by continuously observing a wide bandwidth in real time. With the ability to capture events as short as a few microseconds with 100% probability, this capability is invaluable for SIGINT, ELINT, and COMINT applications. A mode called Swept DPX allows the instrument to sequentially step the center frequency, “staring” for a time at each step, which makes it very helpful for finding transient events.
The instrument display shows 400,000 spectrums per second per section of the sweep. So for example, if a 4-GHz-wide radar chirp at 10 GHz with a 0.5% duty cycle is present along with many other signals, each one will be visible on the display at all times within a single sweep. The user need only determine which signal to examine. In many other systems this is impossible even if the instrument is set to sweep very slowly, as only the signals with the greatest amplitude will be shown.
Through the Looking Glass
Many of the applications in which a Tektronix/X-COM system will be used require highly detailed analyses the performed externally from the instrument, and X-COM and Tektronix provide software tools for this purpose. The RSA series instruments can simultaneously view signals of interest while recording. For example, after capturing the signals with the RSA5000B or RSA6000B and streaming them to the IQC5000A, they can be exported into X-COM’s Spectro-X signal analysis software. Spectro-X provides the capabilities required to locate and identify a signal or signals of interest within a dense electromagnetic environment using up to four independent search engines, even in very long capture files.
After a signal of interest is located and identified within Spectro-X, it can be exported to RFXpress software from Tektronix for re-modulation and re-generation. The software can also create and customize digitally-modulated IQ, IF, and RF waveforms, define baseband I&Q, IF, and RF signals using various modulation schemes, apply impairments such as interference and multipath, and perform many other signal modifications.
The captured I&Q data can also be examined within Tektronix SignalVu vector signal analysis software that allows the signals to be fully characterized to determine their time-variant behavior. These signals or signal segments can then be used to create entirely new signal scenarios using either RFXpress or X-COM’s RF Editor drag-and-drop RF file editing software.
RF Editor complements X-COM’s Spectro-X software and allows I&Q signals of any length to be modified and entirely new ones created. It can modify and build signal waveforms in the time and frequency domains and lets the user mix, trim, cut, join, repeat, and splice RF files in 10 independent tracks that align to create virtually any RF signal sequence. A new recording can then be built to accomplish specific goals (Figure 5). Snippets of recorded data can be dragged and dropped onto any of the tracks and can be repeated, lengthened or delayed, filtered, and shifted in frequency before playback.
Spectrum monitoring, radar and wireless system testing, SIGINT, ELINT, COMINT, and EW have unique measurement requirements, especially the ability to capture, record, and analyze signals over long periods. The ability to do this depends on the overall quality and capabilities of the equipment employed for the purpose, and the RSA5000B and RSA6000B real-time spectrum analyzers and X-COM’s IQC5000A RF capture recorded playback system provide an excellent solution for achieving these goals. The unique triggering abilities of the Tektronix real-time spectrum analyzers allow markers to be placed in the signal-capture file denoting key events, and their architecture virtually eliminates instrument-generated images.
this article to a friend!