Fast-switching frequency synthesizers are critical to many radar and electronic-warfare (EW) systems as well as in automatic-test-equipment (ATE) measurement systems where high throughput is important. But fast-switching microwave synthesizers are also widely used in radar simulators and EW training systems to emulate complex, multiple-signal threat environments. The requirements for these latter applications can be demanding, but the latest generation of compact, low-power, modular frequency synthesizers is enabling manufacturers of radar simulators and EW training systems to achieve smaller footprints and even portability in their systems while emulating complex signal environments with increasing accuracy.
Radar and EW simulators range from portable, lightweight radar signal simulator (RSS) to vehicle-sized suites of equipment for fully evaluating electronic combat (EB) and EW suites (Fig. 1). These simulators were developed for testing airborne radar-warning receivers (RWRs) but can also be used to evaluate vehicular and shipboard systems for their effectiveness in responding to the latest threat signals. Portable simulators usually consist of an embedded or laptop computer with custom software, some form of radio-frequency measurement device, and one or more wideband frequency synthesizers. The frequency synthesizers generate the threat RF emitters, which can be direct coupled to a system under test (SUT) or by means of free-space radiation using the RWR's antennas.
With a radar or threat simulator, a wide range of radar environments or signal threats can be recreated without an actual radar system. This saves the weight and cost of the actual system, and can simplify testing through the use of programmable synthesizers for running automated measurement setups. In contrast to radar and EW simulators that produce actual RF waveforms, digital threat simulators generate the digital code representations of threat emitters and bypass the receiver circuitry in a radar or EW system. Such digital testers use modeled waveforms and signal environments, but often ignore important effects in the signal environment, including multipath, signal reflections, and signal fading.
In order to mimic a radar system or EW threat, a radar simulator or EW trainer must cover a wide total bandwidth, generally from 500 MHz to 18 GHz and higher. It must provide multiple simultaneous channels of RF signal-generation capability in order to emulate the complex signal environments characterized by multiple emitter platforms. This is particularly true when testing a system's multi-path or simultaneous signal response. Generally, such systems mandate more than one synthesizer. Also, having more than one synthesizer reduces the number of dropped pulses in complex scenarios. Larger threat simulation systems have been developed with capability of generating as many as 64 simultaneous emitters, requiring banks of frequency synthesizers or digital RF memories (DRFMs) to recreate a threat scenario. Radar and EW simulators usually feature multiple buses, including IEEE-488, MIL-STD-1553B, and RS-232C interfaces for programmability and control.
As an example, EW Simulation Technology (www.ewst.co.uk), a Herley Co., supplies the Radar Environment Simulators (RES) and the Portable Radar Simulator (PRS) for EW testing. The RES system, which is available using either frequency synthesizers (for signals with relatively narrow envelope) or DRFMs (for wideband width or wide-envelop signals) as the signal sources, can produce a stunning array of signal waveforms, including phase, amplitude, monopulse, sum-and-difference, and rotating direction-finding (DR) return signals and a wide variety of fixed, agile, jittered, and hopped pulse signals. It can create complex pulsed waveforms, including pulse modulation on a pulse (PMOP) and frequency modulation on a pulse (FMOP) at frequencies from 500 MHz to 40 GHz.
The company's PRS unit operates over the MIL-STD-1553B bus and IEEE488 (GPIB) instrumentation bus, providing two broadband RF channels with frequency switching speeds of less than 1 µs. It can generate a wide range of complex RF emitters for testing am SUT via free-space radiation or by directing coupling the signals. In the case of the PRS, RF signals are generated by means of compact microwave synthesizers or optional phase-locked digitally tuned oscillators (DTOs).
Synthesizers for radar and EW simulation/training systems must meet a difficult set of requirements. To start, a frequency synthesizer for these applications must be extremely wideband, on the order of 500 MHz to 18 GHz, in order to cover the frequency range of threat emitters. As in the case of the EW Simulation Technology PRS unit, a wide frequency range can be covered by multiple phase-locked oscillators, such as the DTOs available with the PRS, a frequency synthesizer with stable crystal-oscillator reference source is more commonly the source of the RF/microwave signals. Although frequency synthesizers can be specified with both temperature-compensated crystal oscillators (TCXOs) and oven-controlled crystal oscillators (OCXOs) as the internal reference-frequency source, the superior stability of the latter across the wide military temperature range is generally preferable for a radar or EW simulator. The stability of this reference source ultimately translates into the frequency accuracy of a frequency synthesizer, which is characterized in terms of the parts-per-million (PPM) deviations in frequency.
Spectral purity is important in frequency synthesizers for radar simulators and EW training systems in order to generate emitter signals without unwanted artifacts. Spectral purity is usually specified in terms of the noise characteristics of the synthesizer's output signals, including harmonic levels, nonharmonic spurious levels, and single-sideband (SSB) phase noise. Phase noise is the amount of phase instability normalized to a 1-Hz bandwidth for a given offset distance from the carrier signal. Phase noise decreases with increasing distance from the carrier, resulting in a plot with negative slop that descends to the ultimate noise floor, usually at 10 MHz or more from the carrier (Fig. 2). Typically, synthesizer phase noise may be specified at offset frequencies anywhere from 10 Hz to 10 MHz (as a function of the manufacturer's measurement capabilities).
Frequency switching speed is another key synthesizer parameter for radar simulators and EW trainers required to generate multiple-emitter environments. Unfortunately, switching speed, which is essentially the time required for a synthesizer to tune a given-sized frequency step and settle to a given resolution of a new frequency, is not specified by any standard means from one manufacturer of frequency synthesizers to another. Not all suppliers measure switching speed by the same step size, or the same resolution of the new frequency, making it difficult to accurately compare different units. In addition, switching speed can apply to amplitude as well as frequency.
For any attempt at comparing switching speed, it is important to think of switching speed as the delay time required by the synthesizer to change between two frequencies or two power levels or both. The delay is a combination of the time required to react to a tuning command, the switch/blanking time, the dwell time, and the settling time. The switching time usually refers to the time to settle within some value of a final output phase (such as 0.1 rad) or within some tolerance of a final frequency. The blanking time refers to the capability of turning off the output power during a switch from one frequency to the next in order to avoid generation of unwanted signal components. The settling time may also include user-programmed delays, in order to slow the switching speed in accordance with the requirements of a simulation. Any comparison of frequency synthesizers for radar and EW simulators should try to match specifications under the same conditions, bearing in mind that for the same synthesizer, switching larger steps requires longer switching times.
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Fast-switching synthesizers employ a variety of different architectures, including those based on direct-analog and direct-digital synthesis. Using a direct-analog approach, output frequencies are produced by multiplying and dividing a low-noise reference oscillator, then selecting the desired signal by switches and filters. This method is extremely fast, essentially limited only by the speed of the switches and any delays through the filters. But the technique is also expensive, with a large number of components, and often with reliability problems because of the large component count. The digital alternative is the use of a direct-digital synthesizer (DDS), which uses phase and frequency accumulators to define different signal frequency and phase states and a high speed digital-to-analog converter (DAC) to produce the output RF/microwave waveforms. Both approaches are capable of achieving nanosecond-switching speeds for frequency and amplitude.
Indirect synthesizers based on fast-tuned oscillators, such voltage-controlled oscillators (VCOs), can approach the switching speeds of the two direct methods, eliminating complex microwave circuitry required to remove and avoid spurious signals inherent to direct synthesis approach. VCO-based synthesizer architectures can achieve microsecond-speed switching, generally good enough for most radar simulators and EW trainers.
Both radar and EW simulators require switching speed on the order or microseconds, usually with frequency resolution of 1 MHz or better. As noted with the discussion on phase noise, radar and EW simulators generally do not need frequency synthesizers with the fine resolution required by communications systems with their closely spaced carriers.
Depending upon the type of radar or EW simulator or trainer (portable or fixed installation), the size and weight of a frequency synthesizer may be an issue. Especially for systems designed for vehicular or portable uses that rely on several synthesizers to create multiple-emitter threat scenarios, size, weight, and power consumption are important considerations. Larger, rack-mount synthesizers tend to require operating power levels of 400 W or more per unit, which may be acceptable in a fixed installation, but can overstress a vehicular power supply.
A more practical frequency synthesizer solution for portable or mobile radar and EW simulators is the use of small, modular frequency synthesizers that do not take up the size or draw the power of those traditional rack-mount frequency synthesizers. Modern compact frequency synthesizers such as the fast-tuning series of synthesizers from Wide Band Systems enable developers of radar and EW simulators to design much more compact simulators than in the past for a variety of applications. As an example (Fig. 3), the company has developed compact frequency synthesizers measuring only 6X .5 6.25 1.050 in. but providing 3 µs typical switching speed (5 µs maximum switching speed) over a frequency range from 2 to 18 GHz. The power consumption is only 18 W maximum, making the synthesizer ideal for portable or vehicle-powered systems. The synthesizers provide the spectral purity required by most EW trainers and radar simulators, with spurious content of –50 dBc, harmonics of –26 dBc, and typical single-sideband (SSB) phase noise of –90 dBc/Hz offset 1 kHz from the carrier and –121 dBc/Hz offset 1 MHz from the carrier.
The availability of such compact frequency synthesizers is changing the way that developers of EW trainers and radar simulators approach their systems. For such systems requiring the generation of complex signal environments with multiple emitters, integration of multiple synthesizers no longer demands room-sized racks of signal sources and over-sized power supplies.