A frequency divider is one of the key building blocks in phased-lock loops (PLL) and frequency synthesizers, which are essential subsystems in wireless communication and sensing systems. A frequency divider divides the output frequency of a voltage-controlled oscillator (VCO) in order to compare it with a reference clock signal. Among the different types of frequency dividers, the injection-locked frequency divider (ILFD) is becoming more popular due to its low power and high frequency characteristics. However, the ILFD has an inherent problem of narrow locking range, which basically defines a range over which a frequency-division operation is supported. In order to increase the locking range, one of the most obvious ways is to inject higher power. In fully integrated PLLs, however, the injection signal is supplied by an internal VCO, which typically has limited fixed output power and hence enhancing the locking range with large injection signal power poses difficulty. In this work, we present the development of a fully integrated 10.5-GHz divide-by-3 (1/3) injection-locked frequency divider (ILFD) that can provide extra locking range with a small fixed injection power. The ILFD consists of a previously measured on-chip 10.5-GHz VCO functioning as an injection source, a 1/3 ILFD core, and an output inverter buffer. A phase tuner implemented using an asymmetric inductor is proposed to increase the locking range through even-harmonic (the 2rd harmonic for this design) phase tuning. With a fixed internal injection signal power of only -18 dBm (measured output power of the standalone VCO with a 50-? reference), a 25% enhancement in the locking range from 12 to 15 MHz is achieved with the proposed phase tuning technique without consuming an additional DC power. The frequency tuning range of the integrated 1/3 ILFD is from 3.3 GHz to 4.2 GHz. The proposed 1/3 ILFD is realized in a 0.18-µm BiCMOS process, occupies 0.6 mm × 0.7 mm, and consumes 10.6 mA while the ILFD alone consumes 6.15 mA from 1.8-V supply. The main objective of this work is proposing a new technique of phase-tuning of the even-harmonics that can "further" increase the locking range with an "extra" amount beyond what can be achieved by other techniques. Since the developed technique can enhance the locking range further at a fixed injection power, it can be used in conjunction with other techniques for further enhancing the locking range. For instance, the locking range can be increased by using different injection powers and then further enhanced by tuning the phase of the even harmonics at each power level. The "extra" locking range, not achievable without the even-harmonic phase tuning, amounts to 25%, which is very attractive for PLL applications. Furthermore, additional tuning mechanisms such as use of a capacitor bank can be employed to achieve even wider tuning range for applications such as PLL.


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