To support warfighters in the field, engineers and technicians install, maintain, troubleshoot, and repair a wide range of mission-critical radar and communication systems. These tasks typically require measurements of cables, antennas, components, signals, and more. Often, this work must be done in non-ideal conditions such as rain or shine, hot or cold, aboard a ship, in an aircraft, or in a vehicle.
In many ways, this environment is inhospitable to benchtop instruments such as spectrum analyzers and network analyzers. Fortunately, strong alternatives are available in the form of rugged, handheld instruments that provide advanced all-inone capabilities (Figure 1).
With a focus on radar line-replaceable units (LRUs), this article will outline common test needs, describe the single-instrument solution, and present three example measurements.
As a general overview, LRU characterization requires testing in both the time and frequency domains. Some tests are absolute and others are relative. Absolute measurements made in the time domain include the peak power of a pulsed radar signal. This is often performed using a peak power sensor connected to a peak power meter. In the frequency domain, general spectrum analysis is needed to measure the quality of a signal source within an LRU. Vector network analysis is needed to measure magnitude and phase versus frequency along transmission paths, which include antennas, cables, and filters.
Relative measurements provide additional information about system performance. In the time domain, comparing two points in time provides essential pulse characteristics such as pulse width, rise time, and fall time. In the frequency domain, comparisons of amplitude versus frequency provide information about insertion loss, perhaps between two cables. Examples include relative amplitude and relative phase between multiple channels in a monopulse radar system.
In a lab or on a bench, several instruments might be needed to perform these tests. Even if it were feasible to carry multiple, full-sized units into the field, the test site would have to offer protection from the elements. Also, even under ideal conditions, some instruments require 30 minutes of warm-up time before providing fully calibrated results.
Measuring in the Field
As an alternative, all-in-one analyzers are becoming available, and are designed to provide precise measurements in harsh environments. For example, Keysight’s FieldFox handheld RF and microwave analyzers provide a fully sealed enclosure that meets the requirements of US MILPRF-28800 Class 2 and IEC/EN 60529 IP53. The design minimizes internal heat and provides an operating range of –10 to +55°C (14 to 131°F). It also enables an average operating time of 3.5 hours with the built-in lithium-ion battery. A vertical orientation makes the unit comfortable to hold, and the keypad layout enables easy operation with just thumbs, even when wearing gloves. With a nonslip rubber grip built into the case, the instrument won’t slide off the hood of a vehicle.
The 3.0-kg (6.6-lb.) unit can be configured as a cable-and-antenna tester, spectrum analyzer, vector network analyzer, or a combination analyzer. RF units have a maximum frequency of up to 6.5 GHz, and the microwave models can reach up to 26.5 GHz. Depending on the configuration, capabilities include power meter measurements, spectrum analysis, channel power measurements, pulse profiling, interference analysis, full two-port vector network analysis, and vector voltmeter measurements.
Measurements of S-parameters, frequency spectra, and more, match very closely with those made with benchtop instruments. In many cases, results correlate to within a few hundredths of a decibel.
Three examples illustrate the types of bench-quality measurements that can be made in the field: basic power measurements on a radar transmitter, the magnitude and phase characteristics of a rotary joint, and phase alignment of a stable local oscillator (STALO).
Measuring Transmitter Power
Figure 2 shows the measured power of an unmodulated 40-GHz radar pulse versus a function of time. The measurement was made by connecting a 40-GHz USB peak and average power sensor to a 26.5- GHz analyzer running in power-meter mode. Results can be presented as a measurement trace of magnitude versus time, or as a simple numeric readout of peak or average power.
The table at the bottom of the trace was produced using the automatic pulseanalysis capability. This calculates essential parameters such as peak and average power, rise and fall times, pulse width and duty cycle, and pulse repetition interval (PRI) and pulse repetition frequency (PRF).
Characterizing a Rotary Joint
In a radar system with rotating antennas, the elements of a rotary joint provide RF continuity. During periodic system maintenance, verifying magnitude and phase performance through the joint helps detect rotational variations that can affect radar performance.
Figure 3 illustrates a typical way to measure a multi-channel rotary joint. In this case, the signals of interest are the outputs from the monopulse antenna (sigma and delta) and the output from the omnidirectional antenna (omega).
After disconnecting the antennas from the rotary joint, a high-quality cable is used as a test jumper between the ports of the rotary joint. This cable should have good stability in amplitude and phase across the frequency range of interest.
The other side of the rotary joint is connected to the handheld unit, which is operating in its vector network analyzer mode. For the first measurement, the analyzer output is connected to the sigma port, and the output of the omega port is connected to the analyzer input; the test jumper is connected between the other sigma and omega ports of the rotary joint. For the second measurement, the analyzer output is connected to the delta port, and the jumper is connected between the other delta and omega ports.
For each setup, the analyzer measures the transmission characteristics through the series connection. Magnitude and phase through each signal path is measured as a function of angle as the rotary joint is manually rotated through 360 degrees. The limit line capability of the analyzer can be used to indicate “pass” or “fail” of magnitude or phase for any part of the rotary joint.
In some test environments, it may be difficult to control and observe the analyzer display while simultaneously rotating the joint. For example, trying to observe variation in the S21 parameter during a 360-degree rotation of the joint may require multiple people or long cable runs. With FieldFox, one possible solution is remote operation through an app that runs on iOS devices. This enables a single operator to wirelessly control and observe measurements as the joint is rotated.
Checking Phase in the STALO Transmission Path
Many radar systems have phase adjustments along the STALO path, and these can be used to rebalance the system during routine maintenance. The process centers on measurements of the phase differences between the sum and difference channels of the receiver.
In general, a standard network analyzer cannot be used because the RF receiver input and IF receiver output use different carrier frequencies. The solution is a vector voltmeter capability that can measure the ratio of the signals at the downconverted IF frequency.
Figure 4 shows the setup for an A/B ratio measurement using the analyzer’s vector voltmeter mode. In this case, port 1 is the A measurement and port 2 is the B measurement; the analyzer’s internal source is turned off because it isn’t needed.
In this arrangement, the omega channel serves as the reference for both ratio measurements, one versus the sigma channel and the other versus the delta channel. Because this test requires only the relative phase between the sum and difference channels, the Σ/Ω measurement is used to zero out the meter. Port 1 is then connected to the delta channel; the relative differences in amplitude and phase between the sigma and delta channels will be displayed on the readout.
Enhancing Readiness and Availability
Physically, a rugged handheld analyzer reduces the size and weight of the test equipment that must be carried into the field for technical support. Functionally, an all-in-one analyzer reduces the amount of equipment needed in the field kit. One that also provides fast, accurate RF and microwave measurements will minimize test time while helping maximize the uptime and performance of complex mission-critical systems.
This article was written by Wilkie Yu of Keysight Technologies Inc., formerly Agilent’s electronic measurement business. For more information, please visit www.keysight.com/find/fieldfoxapps .