Signal generators can be among the most versatile of test instruments, since their main function is to emulate the signals handled by modern electronic systems and components. At one time, a signal generator was simply a test source. It was not designated for "commercial" applications or for "military" testing. But such test sources were also not designed for complex modulation formats, typically providing only amplitude modulation (AM), frequency modulation (FM), and pulse modulation. With digital modulation widely adopted in both commercial and military applications, and specific bandwidths used for specific systems, newer signal generators are often tailored for mainly commercial wireless applications. What, then, is a military signal generator?
Depending upon the system, military applications can consume considerably more bandwidth within one system than a number of commercial systems combined. Military electronic warfare (EW) and airborne radar systems typically occupy 2 to 18 GHz, and signal generators from decades earlier often aimed at this bandwidth to minimally serve the frequency-range requirements of military applications. With modern military applications extending beyond 40 GHz, signal generators for military testing must command wide bandwidths with adequate output power, in some cases, to substitute for an RF input signal from an antenna or a local oscillator (LO) signal from the system under test.
Performance requirements for test military hardware may vary from system to system, but some key parameters include frequency resolution and accuracy, output-power flatness, spectral purity, frequency and amplitude switching speed, and modulation capabilities. Frequency resolution indicates the precision of the frequency steps used in tuning the signal generator, such as 1 Hz or 1 kHz.
Unlike older signal generators, which often relied on a free-running oscillator, such as a YIG-tuned source or a voltagecontrolled oscillator (VCO), as the source of signals, newer signal generators use synthesized frequencies, generated by a number of different techniques. The fastest sources are either direct-analog or direct-digital synthesizers (DDSs). The former employs multipliers, filters, and switches to generate a comb of different frequencies from a low-noise oscillator, and then select a desired output by means of a switched filter. The frequency switching speed is essentially limited only by the speed of the switches. In a DDS-based signal generator, a high-speed digitalto- analog converter (DAC) is used to generate an analog waveform. This technique, commonly employed in arbitrary waveform generators, can also provide a wide range of modulation formats. In addition to direct-analog or direct-digital approaches, signal generators typically employ indirect synthesis using fractional-N or phase-lock-loop (PLL) techniques.
The IBS series of signal generators from Elcom Technologies, for example, covers 50 MHz to 20 GHz using the firm's proprietary VCO and PLL-based indirect frequency synthesis techniques. Designed for EW systems and for radar-cross-section (RCS) testing (where a large number of measurements must be made), these sources achieve frequency tuning of less than 200 μs (an option can cut the switching speed to 100 μs). They tune with frequency resolution of 1 to 8 Hz and deliver +11 dBm output power with 1 dB amplitude flatness.
The sources have two levels of phasenoise performance: standard and optional low-noise version. For a standard unit, the phase noise is -95 dBc/Hz offset 1 kHz from a 2-GHz carrier and -75 dBc/Hz offset 1 kHz from a 20-GHz carrier. The phase noise drops to -115 dBc/Hz offset 100 kHz from a 2-GHz carrier and -95 dBc/Hz offset 100 kHz from a 20-GHz carrier. The low-noise option provides phase noise of -120 dBc/ Hz offset 1 kHz from a 2-GHz carrier and -105 dBc/Hz offset 1 kHz from a 20-GHz carrier. The phase noise drops to -132 dBc/Hz offset 100 kHz from a 2-GHz carrier and -120 dBc/Hz offset 100 kHz from a 10-GHz carrier. The IBS signal generators mount in 19-in. racks and include an Ethernet port for connection to a personal computer.
The company also has developed its RS series of rugged frequency synthesizers more for original-equipmentmanufacturer (OEM) type applications, such as sources on military unmanned aerial vehicles (UAVs). These PLK-based synthesizers can switch frequencies in under 50 μs with frequency coverage as broad as 1 to 23 GHz (custom designs can be specified to 40 GHz). They are designed for low DC power consumption and qualified for use at altitudes to 70,000 ft.
To meet the needs of even more broadband applications, the R&S SMA100A microwave signal generator from Rohde& Schwarz can cover a maximum frequency range of 100 kHz to 43.5 GHz in a single unit (see figure). It provides impressive output-power performance of typically +25 dBm, and is designed for low phase noise, with performance of typically -102 dBc/Hz offset 10 kHz from a 10-GHz carrier (an option can drop the phase noise even lower). Nonharmonic spurious content is as low as -100 dBc at 1.5 GHz.
The R&S SMA100A signal generator features generous modulation capabilities, with analog AM, FM, pulse, and phase modulation through 6 GHz. An optional pulse generator provides numbers that should impress most engineers involved in military component and system testing, with typical on/off ratio of 100 dB and typical rise/fall time of 10 ns.
The normal switching/settling times for the R&S SMA100A signal generator are 1.3 ms and 1.0 ms, respectively for frequency and amplitude, although these times can be reduced to less than 450 μs by using a list mode with programmed frequencies and levels. The synthesized source provides GPIB, Ethernet, and Universal Serial Bus (USB) ports for flexible connection to a personal computer for remote control.
Additional high-performance signal generators well suited to military testing include units from Giga-tronics, Agilent Technologies, and Anritsu Co. The 2500B series of fast-switching signal generators from Giga-tronics include a model (the 2550B) capable of tuning from 2 to 50 GHz with 0.001-Hz resolution. They include AM, FM, and pulse/square-wave modulation, and better than 550 μs frequency and amplitude switching speed for a frequency jump of less than 500 MHz. The signal generators provide +13 dBm leveled output power at 50 GHz and feature low close-in phase noise, with performance of -96 dBc/Hz offset 1 kHz from a 10-GHz carrier and -109 dBc/Hz offset 1 kHz from the same carrier.
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The N5183A MXG line of signal generators from Agilent Technologies includes options for coverage from 100 kHz to 40 GHz with 0.01-Hz resolution. They deliver +7 dBm output power (+12 dBm with an option) to 40 GHz with typical phase noise of -86 dBc/Hz offset 20 kHz from a 40-GHz carrier. The phase noise drops considerably for lower-frequency carriers.
The MG37020A series of signal generators from Anritsu are built for speed, with frequency and level switching speed of 100 μs from 10 MHz to 20 GHz. The minimum step size is 0.001 Hz and the phase noise is typically -86 dBc/Hz offset 10 kHz from a 20-GHz carrier and -94 dBc/Hz offset 100 kHz from the same carrier.
Of course, not all signal generators suitable for military testing are housed in traditional 19-in. rack-mount packages. The NI PXIe-5673 vector signal generator from National Instruments, for example, is a 6.6-GHz vector signal generator packed into a PXI Express card format. With better than 7.5-ms tuning speed and as much as +10 dBm output power, the signal generator on a card exhibits typical phase noise of -112 dBc/Hz offset 10 kHz from a 1-GHz carrier.
In addition to fast-switching rackmount signal generators, Aeroflex also offers its PXI 3020 series of PXI card signal generators, with 250-μs switching speed and coverage from 1 MHz to 6 GHz. The HMCT2100B signal generator from Hittite Microwave Corp. is a portable unit capable of delivering +27 dBm signals from 10 MHz to 20 GHz while running 4 hours on battery power. Ideal for in-field testing, it achieves 10-kHz resolution with 300-μs switching speed and phase noise of -113 dBc/Hz offset 100 kHz from a 1-GHz carrier. It weighs just 11 lbs.
These test sources have come a long way from the vacuum-tube-based AN/ URM-25 signal generators developed for military testing and troubleshooting in the time immediately following World War II. Covering 10 kHz to 50 MHz, they were used for evaluating the performance of a variety of military systems. A more advanced version of that test generator, the man-portable MTS-300A EW simulation source from Advanced Systems Concepts, was used by US Army Special Operations troops during the 1990-1991 Gulf War to check the performance of EW systems in fixed-wing aircraft, helicopters, and ships. The MTS-300A works from 700 MHz to 18 GHz with 1-MHz tuning resolution with continuous-wave (CW) or pulsed operation.