HIGH-PERFORMANCE MICROWAVE signal generators must deliver excellent spectral purity by minimizing phase noise, harmonics, and spurious signals. In fact, spectral-purity performance often differentiates high-performance, mid-range, and entry-level signal generators. In a white paper titled, "Comparing Spectral Purity in Microwave Signal Generators," Giga-tronics, Inc.'s Leonard Dickstein discusses the factors that comprise spectral-purity specifications. The paper's goal is to help engineers make more informed comparisons of microwave signal-generator performance for test and measurement applications.
Among the factors that make up spectral purity is phase noise. The paper works to provide a definition of phase noise and some of its attributes. For example, the horizontal axis is the offset from the carrier frequency. Offsets 1 to 100 Hz from the carrier are defined as "close-in" phase noise. In contrast, offsets 100 kHz to 10 MHz from the carrier are referred to as "far-out" phase noise. Phase noise that is close in to the carrier creates problems in applications that use the signal as a local oscillator (LO) and can limit receiver sensitivity. If the signal is used as a clock, close-in noise could even limit bit-error-rate (BER) performance. Similarly, phase noise that is far out from the carrier creates issues for wideband communications systems.
On datasheets, 10 kHz from the carrier is usually used for phase-noise comparisons. However, a manufacturer may optimize phase-noise performance for an impressive specification at 10 kHz offset while compromising performance in both the close-in and far-out regions. Due to differences in multiplication factors, it also is important to note that better performance at 10 GHz does not necessarily translate into better performance at 40 or 50 GHz.
When it comes to harmonic and spurious performance, different applications may be sensitive to spurious at different amplitudes and frequencies. While one synthesizer may have a problematic spur in one specific application and another synthesizer does not, the opposite will be true in a different application. Even if a signal generator can meet or exceed its spurious specification, it may still exhibit a problematic spur if it falls into a sensitive region in any application. Overall, however, better spurious specifications result in this issue arising less frequently. Finally, keep in mind that spurious signals vary as a function of manufacturing variations as well as the external environment. They also may vary with the output power level and other settings.
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