Low THD bandpass-based oscillator using multilevel hard limiter
In order to reduce the total harmonic distortion (THD) of a bandpass-based oscillator, instead of using a conventional hard limiter, a multilevel hard limiter (MHL) is proposed which inherently removes the third and the fifth-order harmonics from the frequency spectrum of its output signal. The input–output characteristic of the proposed MHL contains slope values of only zero and infinity, making it easy to implement. The optimal height and width of each stair of the MHL characteristic are derived. Measurement results show that for the same bandpass filter, the proposed approach shows 14 dB improvement in the THD of the output signal with respect to the conventional two-level hard limiter (comparator). The oscillator has been fabricated in TSMC 0.35 µm CMOS process and the die occupies an area of about 3.15 mm2. The oscillator chip prototype operates at the frequency of 10.7 MHz, consumes 40 mA current from a 3.3 V power supply and yields a THD of −53 dB.
