So far all mainstream transmitters for WCDMA are of the direct upconversion type. This architecture is versatile but requires calibration of the imbalance in its quadrature branches and DC offset at its inputs, and it is vulnerable to mixer noise. We believe it consumes more power and chip area than is warranted, and propose the polar transmitter as an alternative. Although it has been widely discussed as the ideal transmitter for EDGE, the polar architecture has yet to make significant inroads there, let alone into wideband CDMA. We will describe new circuits that enable a compact, largely self-calibrated polar transmitter, whose lower power and chip area put it ahead of state-of-the-art direct upconversion transmitters [1-3].Polar's challenges lie in finding a reliable method of wideband phase modulation (PM), and in suppressing unequal delay in the two separate paths that the PM and AM waveforms will take before they unite in the pre-PA driver. Whereas EDGE requires a PM bandwidth of about 800kHz and can tolerate AM/PM delay mismatch on the order of tens of nanoseconds, WCDMA needs at least an 8MHz bandwidth with path delay mismatch less than 2ns. The solution we describe takes care of bandwidth and unequal delay with minimum intervention.PM is transferred via a fractional-N phase-locked loop (PLL) on to a voltage-controlled oscillator (VCO). The PLL bandwidth is usually a small fraction of the reference frequency, otherwise spurs and fractional noise will leak into the output: in our circuit it is between 200 and 300kHz. Then to impose an 8MHz-wide modulation into this 300kHz-wide loop, one must resort to either pre-emphasis or two-point injection. Pre-emphasis is sensitive to variations in loop gain, and at large modulation bandwidths can push PLL components to the limits of their dynamic range. On the other hand two-point injection is very sensitive to the VCO gain (k VCO ), which changes across process, supply voltage, temperature, and operating frequency. Since in either method the oscillator is being modulated at rates well beyond the PLL bandwidth, feedback is unable to correct for VCO nonlinearity, and distortion will appear at high modulation frequencies.But if a linear VCO with known and reproducible k VCO were made available, twopoint injection becomes feasible. Then by adding to it a fairly simple AM path and a power-efficient PA driver, a wideband polar modulator may be realized. What differential delay remains between the two paths must also be predictable, and more importantly, should be self-calibrated. All-digital PLL-based transmitters for GSM/EDGE are based on this method [4], but they are sensitive to the oscillator gain and their output is usually cluttered by spurious tones. Also, extensive calibration and signal processing make them power-hungry; it is not clear whether they can reach WCDMA bandwidths. We show that our approach which strikes a balance between analog and digital calibration is simpler, more robust, and consumes less power.First we consider VCO linearization. A model-based...
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