Ultra-low-voltage mixer and VCO in 0.18-m CMOS, In IEEE Radio Frequency Integrated Circuits Symposium Digest, June 2005, pp. 167-170. 8. C. Hermann, M. Tiebout, and H. Klar, A 0.6-V 1.6-mW transformerbased 2.5-GHz downconversion mixer with ϩ5.4-dB gain and Ϫ2.

In a software-defined radio (SDR) receiver it is desirable to minimize RF bandfiltering for flexibility, size and cost reasons, but this leads to increased outof-band interference (OBI). Besides harmonic and intermodulation distortion (HD/IMD), OBI can also lead to blocking and harmonic mixing. A wideband LNA [1, 2] amplifies signal and interference with equal gain. Even a low gain of 6dB can clip 0dBm OBI to a 1.2V supply, blocking the receiver. Hard-switching mixers not only translate the wanted signal to baseband but also the interference around LO harmonics. Harmonic rejection (HR) mixers have been used [3,1,4], but are sensitive to phase and gain mismatch. Indeed the HR in [4] shows a large spread, whereas other work only shows results from one chip [3,1]. This paper describes techniques to relax blocking and HD/IMD, and make HR robust to mismatch.

Abstract-Transmitter circuits using large signal swings and hard-switched mixers are power-efficient, but also produce unwanted harmonics and sidebands, which are commonly removed using dedicated filters. This paper presents a polyphase multipath technique to relax or eliminate filters by canceling a multitude of harmonics and sidebands. Using this technique, a wideband and flexible power upconverter with a clean output spectrum is realized in 0.13-m CMOS, aiming at a software-defined radio application. Prototype chips operate from DC to 2.4 GHz with spurs smaller than 40 dBc up to the 17th harmonic (18-path mode) or 5th harmonic (6-path mode) of the transmit frequency, without tuning or calibration. The transmitter delivers 8 mW of power to a 100-load (2.54 V pp-di voltage swing) and the complete chip consumes 228 mW from a 1.2-V supply. It uses no filters, but only digital circuits and mixers.

Both ring oscillators and relaxation oscillators are subsets of RC oscillators featuring large tuning ranges and small areas. Figure 19.5.1 shows a typical relaxation oscillator with a capacitor and two switched current sources. Such relaxation oscillators have two advantages with respect to ring oscillators: 1) they have a constant frequency tuning gain; and 2) their phase can be read out continuously due to their triangular (or sawtooth) waveform. A major disadvantage of practical relaxation oscillators is their poor phase-noise compared to ring oscillators [1,2,4].The 1/f 2 phase-noise performance of oscillators can be compared using the FoM definition given in Fig. 19.5.3 [1]. Navid et al. have shown that at 290K thermodynamics limits the FoM of ring oscillators and relaxation oscillators to -165.3dB and -169.1dB, respectively [2]. Interestingly, they have also shown that the FoM of practical ring oscillators is generally better than about -160dB, while the FoM of practical relaxation oscillators is about 10dB worse. So in theory relaxation oscillators can be better, but in practice they are not. Part of the explanation is given in [2]; the noise added by the comparator, which is present in relaxation oscillators (cmp osc in Fig. 19.5.1) but not in ring oscillators, increases the phase-noise. We will show below that filtering this noise by exploiting a switched-capacitor discharge mechanism, the FoM of a practical relaxation oscillator can be as good as the FoM of ring oscillators. Fig. 19.5.1, I 1 charges capacitor C 1 . However, C 1 is not grounded, but connected across an OTA, and the discharge process exploits a switched capacitor, C 2 , which is reversed periodically. The operation of the circuit is described in the next sentences. has been subtracted from C 1 . V 3 = V + -V -is a sawtooth waveform. Subtracting this fixed charge packet filters out the noise of the oscillator comparator. The operation is illustrated in Fig. 19.5.3, which shows the control signal X, V 3 and the output of the comparator cmp out , V out , is also shown, which produces an edge whenever voltage V 3 reverses polarity. Suppose now that cmp osc is noisy and C 2 is reversed at t 4 instead of at t 3 . Although the duty cycle of V out is changed (at t 5 ), the active edge of V out at t 6 is unaffected and so is the phase-noise. This filter technique is similar to the anti-jitter circuit (AJC) technique used in open-loop jitter filters [3]; note that we apply a switched-capacitor circuit to subtract the charge packet, which is very power-efficient.Filtering out the noise of the oscillator comparator has two consequences: 1) the power dissipated by the oscillator comparator and its reference can be reduced without deteriorating the phasenoise; and 2) the two remaining contributions to the 1/f 2 phasenoise are the white noise of the charging and discharging mechanisms. It can be shown that the resulting FoM of such a relaxation oscillator is given by the equation in Fig. 19.5.3, where k is the Boltzmann constant, T the absolute tem...

Amplifiers and mixers not only produce a useful amplified or frequency-shifted signal, but also many unwanted harmonics and sidebands caused by nonlinearity and time-variance. Filters are commonly used to clean up the spectrum, but are application specific and often difficult to integrate. Zero-order-hold filtering in a mixer-DAC [2] reduces DAC-related spurs, but does not remove harmonics generated in the mixer. A harmonic-rejection mixer canceling the third-and fifth-order harmonics [3] does relax analog filter requirements. In this paper, we exploit the practical potential of the polyphase multipath circuit theory proposed in [1] to further reduce or even completely eliminate filters by canceling a very large multitude of harmonics and sidebands. As a demonstration, we apply the technique to realize a wideband filter-less power upconverter for possible future multi-standard radio applications in CMOS.