Low phase noise L‐band oscillators based on novel general Chebyshev bandpass filters

Abstract

Oscillators act as signal sources for wireless communication systems, radars, wireless charging, and other industrial applications, whose phase noises directly determine system performance. In this paper, two low-phase noise microwave oscillators based on novel microstrip second-order band-pass filters are presented. These filters are characterized by good frequency selectivity owing to two transmission zeros near to the passbands. As a result, relatively high group delay can be achieved at center frequencies, which is vital to the phase noise performance of the oscillators to be designed. Simultaneously, the bandwidths can be made narrow enough so that actual oscillating frequency is very close to the designed one. In addition, the introduction of stepped impedance makes these filters capable of harmonic suppression, which efficiently suppresses the harmonics in the output such as the second harmonic. These filters are compact and also of easy design. Finally, two microwave oscillators at 2.0 GHz are designed on these microstrip filters. As the measured results show, their output frequencies are 2.002 GHz. The phase noises are −127.21 and −127.03 dBc/Hz@100 kHz, respectively. The second harmonic suppressions are as great as 30.45 and 39.21 dBc, respectively. KEYWORDS group delay, harmonic suppression, microstrip filter, microwave oscillator phase noise | INTRODUCTIONOscillators usually act as signal sources for wireless communication systems, radars, wireless charging systems, and other industrial applications. Their phase noises directly determine system performance. It is necessary to develop oscillators with low-phase noise. Up to now, there arise various types of oscillators. Usually, dielectric and cavity resonators with high-quality factors are adopted in classic oscillators. 1-4 However, they are somewhat bulky and not planar so that they are not easy to be integrated with other planar circuits and implemented in integrated circuits.As the well-known Leeson model shows, the loaded quality factor Q L of the frequency selection network play an important role in the phase noise performance of oscillators. 5 Recently, to use band-pass filters as frequency selection network within the feedback loop of oscillators receives much attention. By creating transmission zeros near the passbands of band-pass filters, it is likely to achieve group delay peak at oscillating frequency so that high quality factor is available for reduction of phase noise of oscillators. For example, 6,7 apply four-pole elliptic band-pass filters as frequency stabilization elements within microwave oscillators to reduce phase noise. In Chang and Tseng, 8 a trisection filter with a transmission zero near the passband is utilized to design a low-phase noise oscillator. Chao-Hsiung and Chih-Lin 9 uses