The ability to analyze Radio Frequency (RF) signals is critical in applications involving military radios and avionics equipment. Staff at the EMPF have successfully measured the signal content of a survival radio waveform for the AN/ARS-6 commercial-off-the-shelf / open architecture upgrade. As part of the airborne rescue platform for a Combat Search and Rescue System, the AN/ARS-6 is used to locate the position of a survivor’s handheld radio. RF signal measurements were performed on the AN/ARS-6 using several instruments, including wireless communication, vector signal, and spectrum analyzers.

Background
RF measurement techniques can generally be divided into three major categories of analysis – spectral, vector, and network. Spectrum analyzers, which provide basic measurement capabilities, are the most popular type of RF instrument in many general-purpose applications. With a spectrum analyzer, the user can view power vs. frequency information and can sometimes demodulate analog formats such as AM, FM, and PM. Vector instruments include vector or real-time signal analyzers. These instruments analyze broadband waveforms and capture time, frequency, and power information from signals of interest. Network analyzers are typically used for making S-parameter measurements, which are often performed on RF components.

Spectrum, vector, and network analyzers offer different functionality to the end user. All analyzers are generally built on either a scalar or a vector architecture, each of which has its advantages and disadvantages. Because it is narrowband in nature, a scalar architecture is not well suited to analyzing the broadband signals which are becoming more prevalent in today’s marketplace. In addition, a scalar architecture gives only a 2-dimensional (power vs. frequency) view of the acquired signals, and is typically slower than vector-based.

Vector analysis
To accurately capture and characterize broadband signals, it is necessary to change from narrowband measurement equipment to broadband vector instruments. Using vector instruments with a real-time bandwidth equal to or wider than the bandwidth of the transmitter, it is possible to capture all signals of interest from the device under test.

Though typically more expensive than scalar instruments, vector instruments provide faster measurements and more complex signal analysis and generation.Vector instruments use wider filters than narrowband instruments such as spectrum analyzers. Because this width reduces the number of times the filter must be retuned, a vector instrument can sweep across the frequency spectrum more quickly. With vector architecture, it is possible to generate complex signals such as the modulated waveforms used in most modern cellular communications systems.

In addition to capturing broadband signals, vector instruments deliver other key advantages to your measurement application. When performing spectral sweeps or other measurements that span a large frequency range, the wide real-time bandwidth of a vector analyzer can dramatically improve test time. For example, the new PXI-5660 RF Signal Analyzer (see Figure 6-1) from National Instruments features a 20 MHz real-time bandwidth and delivers measurement throughput advantages from 30 to 200 times that of traditional instrumentation. The EMPF has utilized this PXI (PCI eXtensions for Instrumentation) based signal analysis system to characterize military communications waveforms. Figure 6-2 shows a block diagram of how the individual modules within the RF signal analyzer characterize incoming signals.

Spectrum analyzer background
At the most basic level, a spectrum analyzer can be described as a frequency-selective, peak-responding voltmeter calibrated to display the root-mean-square (RMS) value of a sine wave. It is important to understand that the spectrum analyzer is not a power meter, even though it can be used to display power directly. As long as we know some value of a sine wave (i.e., peak or average) and know the resistance across which we measure this value, we can calibrate our voltmeter to indicate power. With the advent of digital technology, modern spectrum analyzers have been given many more capabilities.

Types of measurements
Common parameters measured by a spectrum analyzer include frequency, power, modulation, distortion, and noise. Understanding the spectral content of a signal is important, especially in systems with limited bandwidth. Transmitted power is another key measurement. Too little power may mean the signal cannot reach its intended destination. Too much power may drain batteries rapidly, create distortion, and cause excessively high operating temperatures.

Measuring the quality of the modulation is important to ensure a properly working system and correctly transmitted information. Tests such as modulation degree, sideband amplitude, modulation quality, and occupied bandwidth are examples of common analog modulation measurements. Digital modulation metrics include error vector magnitude, IQ imbalance, phase error vs. time, and a variety of other measurements.

In communications, measuring distortion is critical for both the receiver and transmitter. Common distortion measurements include intermodulation, harmonics, and spurious emissions. Noise is often the signal that needs to be measured. Any active circuit or device will generate excess noise. Tests such as noise figure and signal-to-noise ratio (SNR) are important for characterizing the performance of a device and its contribution to overall system performance.

Spectrum analyzer fundamentals
The output of a spectrum analyzer is an X-Y trace on a display, which is mapped on a grid (graticule) with 10 major horizontal divisions and 10 major vertical divisions. The horizontal axis is linearly calibrated in frequency that increases from left to right. Setting the frequency is a two-step process. Using various frequency control settings (such as span, start, and stop) on the spectrum analyzer, it is possible to determine the absolute frequency of any signal displayed and the relative frequency difference between any two signals.

The vertical axis is calibrated in amplitude using a linear scale calibrated in volts or a logarithmic (log) scale calibrated in dB. The log scale is used more frequently because it has a much wider usable range. The log scale allows signals as far apart in amplitude as 70 to 100 dB (voltage ratios of 3200 to 100,000 and power ratios of 10,000,000 to 10,000,000,000) to be displayed simultaneously. The linear scale can be used for signals differing by no more than 20 to 30 dB (voltage ratios of 10 to 32). Using reference settings on the vertical axis, it is possible to measure either the absolute value of a signal or the relative difference in amplitude between any two signals.

Conclusion
Spectrum analyzers are excellent instruments for inspecting and characterizing RF signals for existing military systems in need of technology upgrades. It is also possible to measure those same signals with commercially-available open architecture which is reconfigurable using virtual instrumentation software. If you have any questions about RF signal testing and analysis, please contact the EMPF Helpline at (610) 362-1320. A design engineer will be able to offer technical insight and appropriate advice regarding your concerns.