Frequency synthesizers for electronic-warfare (EW) systems must meet one of the more rigorous sets of specifications for any application. Such parameters as fast switching speed and low phase noise have always been important in EW systems, but with a trend toward increasing integration in military electronics systems, designers of frequency synthesizers are now being asked to deliver high performance levels in smaller packages, and with less power consumption.

Synthesizers for EW systems and simulation or training tools must be extremely broadband, such as 0.5 to 18.0 GHz, to match the range of possible threat emitters. Although the range can be covered with multiple digitally tuned oscillators (DTOs), it is more typically the province of a broadband frequency synthesizer locked to a stable reference source, such as an oven-controlled crystal oscillator (OCXO).

They must also provide clean signals, gauged in terms of a synthesizer’s spectral purity or noise characteristics, which usually includes its harmonic levels, nonharmonic spurious levels, and singlesideband (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. This type of noise decreases further from the carrier, finally leveling off at the noise floor, generally about 10 MHz from the carrier. Because frequency synthesizer manufacturers may specify phase noise at various offset frequencies from the carrier, from as close to 10 Hz to as far as 10 MHz, it is important to compare synthesizer SSB phase noise for different models at the same offset frequencies.

Frequency switching speed—the delay time required by a synthesizer to switch from one frequency to another—is a key synthesizer parameter for EW applications. It can be difficult to compare from one synthesizer manufacturer to another because of a lack of standardization in how it is tested. Switching speed may be defined as the time required to reach some phase value of a final frequency or to within some percentage, such as the final 10 percent, of a selected frequency. Not all synthesizer suppliers use the same frequency step size when evaluating switching speed. And switching speed can also apply to a change in amplitude.

To change frequencies, a synthesizer must react to a tuning command, such as a binary-coded-decimal (BCD) signal, and then go through several steps before reaching the new frequency, including the blanking time when the output power is attenuated between the frequencies to minimize spurious generation, the dwell time, and the settling time. In many EW systems, a frequency synthesizer must sometimes program delays as part of change in frequencies, when trying to match the characteristics of a detected emitter.

In many applications, frequency synthesizers may be required to provide phase-coherent or phase-continuous outputs. For phase-coherent switching, a synthesizer shifts from one frequency to another and back to the original frequency, at the same phase of the original frequency had it never switched away from it. Phasecoherent frequency-switching capability is important in coherent pulse Doppler radar systems that rely on coherent pulse detection for predetection integration. It is also important in EW systems that require an analog-like (continuous rather than in discrete steps) frequency sweep across a bandwidth with minimal noise generation.

Frequency synthesizers for EW applications can be constructed in a variety of ways, using different types of tunable oscillators, including voltagecontrolled oscillators (VCOs) and YIGtuned oscillators. In an indirect frequency synthesizer architecture, these microwave oscillators are locked to the phase of a lower-frequency reference oscillator, such as an OCXO, with superior stability and phase-noise characteristics. Such synthesizer designs can employ a single phaselocked loop(PLL for simplicity, although multiple loops are usually required when it is necessary to achieve good low-noise performance.

When frequency switching speed is critical, direct-analog and direct-digital synthesizers can provide the microsecond or even nanosecond switching speed required for EW systems. In a directanalog synthesizer, output frequencies are produced by multiplying and dividing a low-noise reference oscillator, creating a comb of different-frequency signals, then selecting desired signals by means of switching and filtering. The speeds of the switches (usually based on high-speed PIN diodes) essentially dictate the speeds that different frequencies can be selected, since all of the frequencies are present at all times. Unfortunately, this architecture is complex and expensive, requiring a large number of components. In the digital realm, a direct-digital synthesizer (DDS) uses phase and frequency accumulators to define different frequency and phase states for a desired signal, and a high-speed digital-to-analog converter (DAC) to produce output waveforms. Early DDS products were plagued by high spurious content due to the limited bit resolution of available DACs. But improvements in digital components have also led to improvements in the noise performance of high-switching-speed DDS products.

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