## 1 Introduction  
The development of medical technology has seen rapid growth in the use of radio frequency (RF) applications in medical services [1]. Medical device systems that use microwaves are extensively employed in treatment and diagnostics [2, 3, 4, 5, 6]. In the former, they are used to treat inflammation and tumors, while in the latter, they aid in the detection of tissue-based substances, such as tumor cells, DNA, cortisol, and glucose [2, 3, 4, 5, 6]. Markers are substances that help to track the location of a tissue (such as a tumor) [1, 7, 8]. Since sensors work by generating electrical signals of the required frequency, oscillators are employed in marker sensors. The oscillator used by the sensor to supply the correct frequency signal must have low phase noise. A method for designing a low-phase-noise oscillator can be a resonator having a high Q\({}_{\text{L}}\) for the oscillator. Therefore, the design of a resonator with a high Q\({}_{\text{L}}\) is very important.  
Dielectric resonators (DRs) are promising elements because of their high-Q\({}_{\text{L}}\) characteristics. However, their three-dimensional (3-D) structures not only limit their implementation to system-on-chip (SoC) and integrated circuit (IC) systems, but also render them unsuitable for mass production [9, 10].  
Resonators with planar structures (microstrip lines) are being extensively studied, with a specific focus on increasing their \(\mathrm{Q_{L}}\). The representative microstrip-line resonator is a hairpin structure, which has a physical length of \(\lambda_{\mathrm{g}}/2\). While hairpin structures are easy to fabricate [11], their \(\mathrm{Q_{L}}\) is too low in performance to apply them to an oscillator. To overcome this disadvantage, spiral resonators have been considered as an alternative. However, although they are small and have higher \(\mathrm{Q_{L}}\) values, they are still insufficient for application in oscillators [11].  
To further increase the \(\mathrm{Q_{L}}\), a substrate-integrated waveguide (SIW) structure and an open-loop resonator with a T-stub inserted into it were studied in 2014. An analysis of their characteristics showed significantly improved values of \(\mathrm{Q_{L}}\). The SIW structure is patch-shaped and the structural characteristics of its feeding-line part make the \(\mathrm{Q_{L}}\) adjustable [12; 13; 14; 15]. The \(\mathrm{Q_{L}}\) is increased by virtue of the dual structure used in the state where the T-stub is applied [16]; however, it also has the undesired effect of enlarging the resonator. This increase in area means that applying it to the oscillator would hinder system integration.  
A triangle-folded resonator with a coupled line has a smaller size, with a physical length of \(\lambda_{\mathrm{g}}/4\)[17]. Since resonators do not use vias, this one has excellent performance because there is no energy loss to the via. However, although this device enabled the oscillator to achieve a low phase noise, its \(\mathrm{Q_{L}}\) was relatively low.  
In this paper, a low-phase-noise oscillator with a high-\(\mathrm{Q_{L}}\) resonator, using a combination of a split-ring structure and an open-loaded T-type stub, is presented. The resonator is a combination of a split-ring structure and an open-loaded T-type stub, where a high-\(\mathrm{Q_{L}}\) characteristic can be obtained by adjusting the length of the open-loaded T-type stub. The characteristic of the resonator is the band-stop type of resonance (\(\mathrm{S_{21}}\)). Owing to the sharp band-rejection characteristic of the new resonator, very low phase noise can be obtained for the oscillator. This design strives to obtain excellent performance so that it can be applied to tissue- and tumor-tracking sensors.