Direct-conversion transmitters appeal to designers of wireless systems for their simplicity and low cost. Unfortunately, the simple architecture does not allow the filtering of broadband noise, images and spurious components typically executed at intermediate frequencies (IFs) in a more complex superheterodyne transmitter. For designers to migrate single-carrier base stations to multicarrier architectures using direct-conversion approaches, they must use components with high output compression and low noise. What follows is an examination of the direct-conversion approach for multicarrier WCDMA and CDMA2000, with particular attention on that critical component, the in-phase/quadrature (I/Q) modulator.

In a single-carrier WCDMA system (Fig. 1), a base station typically transmits at carrier power levels to +46 dBm (40 W). The 3GPP standard requires that the power in the adjacent and alternate channels be no greater than −45 and −50 dBc, respectively (the carrier power and adjacent/alternate channel power are both measured in a 3.84-MHz bandwidth).¹ To achieve such performance, components are backed off from their maximum power levels, and predistortion techniques are typically used in the power amplifier for improved linearity. Further from the carrier, performance requirements are dominated by the noise-floor specifications (or spurious emissions).

For example, at carrier offsets to 50 MHz, noise or spurious components, measured in a 1-MHz bandwidth, can be no greater than −15 dBm. At greater offsets from the carrier, requirements are more stringent, with the worst case at 60 MHz offset from the carrier (or at the edge of the band, whichever comes first); at the 60 MHz offset, the noise floor must be no greater than −30 dBm (1-MHz bandwidth).

Figure 2 compares single-carrier (left) and multiple-carrier (right) transmitter spectra. If the same model power amplifier is used in both systems, the per-carrier must be reduced in the four-carrier system to maintain a total output-power level of +46 dBm, resulting in multiple carriers with transmit powers of +40 dBm. For both approaches, the 3GPP standard still requires adjacent- and alternate-channel power ratios of −45 and −50 dBc, respectively, and a noise floor of −30 dBm.

As the power of each carrier is reduced by 6 dB, the resulting intermodulation distortion (IMD) in adjacent channels is also reduced as the distance to the system's third-order intercept point and compression point increases. This suggests that the adjacent-channel leakage ratio (ACLR) should improve. However, because the noise floor remains relatively constant, the signal-to-noise ratio (SNR) degrades and begins to affect the ACLR (in the single-carrier case, the ACLR is dominated by distortion). Also, even though the per-carrier power is lower than for single-carrier case, the four carriers modulate each other and contribute to ACLR. The net result is that the ACLR will degrade as a particular hardware configuration is alternately driven by a single-carrier signal and by a multi-carrier signal with the same total power. For optimum performance, multicarrier systems require signal chains that have the highest possible signal-to-noise ratio.

Figure 3 shows a block diagram of a direct-conversion transmitter, with representations of the signal spectrum along the designated points in the signal chain. The dual DACs create a baseband I/Q spectrum (A), while lowpass filters following the DACs eliminate Nyquist images and noise (B). Although this noise filtering has traditionally been less critical, the emergence of low-noise I/Q modulators (with noise floors rivaling even 14- or 16-b DACs) has made noise filtering more meaningful. This suggests that the corner frequency of the filter be as close as possible to the edge of the spectrum (this will help to improve in-band noise at the antenna). However, a trade-off must be made as placing the 3-dB corner of the filter too close to the edge of the spectrum will give in-channel group delay variations and will degrade error vector magnitude (EVM).

The filtered baseband signals drive the I and Q inputs of a quadrature modulator which is also driven by a local oscillator (LO) with frequency centered at the desired output frequency. The LO is applied to the modulator's internal limiter and split into quadrature signal components. Multiplying these quadrature components together with the baseband I and Q components creates a modulated carrier centered on the LO frequency (C). Unfortunately, any unwanted DC components in the baseband I and Q signals will also be multiplied with the LO and generate LO leakage (the arrow in the center of the spectrum). The presence of this LO leakage will degrade the quality of the modulated carrrier's EVM. Because this signal component falls within the desired channel, it cannot be filtered without removing the desired signals.

The problem can be avoided by using an I/Q modulator with low LO leakage (low input offset voltages on the I and Q input ports). If the DAC and the modulator have the same DC-bias level, allowing a DC-coupled connection, it is possible to use the DAC to apply compensating offset voltages to eliminate LO leakage. However, this is only effective if the I/Q modulator's input offset voltages are stable over temperature.

Nonideal quadrature splitting of the LO and/or gain mismatch between the I and Q channels will also degrade EVM (but will not add out-of-channel spurious signals). Like LO leakage, this effect can be reduced by varying the relative amplitudes and phases of the baseband I and Q signals, although such control must also be maintained over temperature.

In a frequency-agile system, the signal chain must be designed so that carrier frequencies can be synthesized over a defined range. For example, a WCDMA base station might be designed to operate anywhere from 1930 to 1990 MHz or 2110 to 2170 MHz. The LO must tune over this range, but the modulator output cannot be filtered inside this range. Thus, post-modulator filtering can at best reduce out-of-band noise rather than in-band noise.