RF Modulator Enables Multicarrier Transmitters

Cellular transmitters rely on high-performance RF modulators to maintain linearity and dynamic range. With the growth of multicarrier transmitters, RF modulators must maintain a low noise floor while also delivering good high-level performance as determined by the second- and third-order intercept points. Fortunately, the MAX 2022 direct quadrature RF modulator from Maxim Integrated Products (Sunnyvale, CA) provides the performance needed from 1500 to 2500 MHz to support a wide range of modulation formats and multiple carriers with minimum distortion in efficient direct-conversion transmitters.

Until recently, digital-to-analog-converter (DAC) and direct-conversion-modulator performance was inadequate for third-generation (3G) multicarrier cellular base stations. Modern transmitter designs require low cost and flexibility. The RF modulator is particularly important since it sets the transmitter's performance limits and architecture. The MAX2022 is a practical solution because it combines high output second-order-intercept (OIP2) and output third-order-intercept (OIP3) performance along with an output noise floor approaching −174 dBm/Hz. This combination allows a single transmitter architecture to support multiple types of modulations—from cdma2000 to WCDMA to orthogonal frequency-division multiplex (OFDM)—with as many as nine carriers. The MAX2022 is fabricated with a high-frequency silicon-germanium (SiGe) process for frequency coverage of 1500 to 2500 MHz.

The modulator features a single-ended local-oscillator (LO) input (50 Ω internally matched) that accepts input LO drive levels of −3 to +3 dBm (Fig. 1). The LO is internally buffered, split by a quadrature splitter, and applied to two high-performance passive mixers. The quadrature in-phase (I) and quadrature (Q) inputs are differential with a 44-Ω input impedance. Due to the exceptional input bandwidth of greater than 1 GHz, the device can be used as either a baseband direct RF modulator or as an image-reject mixer with quadrature intermediate-frequency (IF) inputs. The quadrature inputs are designed to interface directly with current-output DACs, eliminating the need for expensive buffer amplifiers. The mixer outputs are combined and applied to a single-ended RF output, which is internally matched to 50 Ω.

The performance of an RF modulator can be evaluated by several independent parameters. For example, the output third-order-intercept point (OIP3) is +22 dBm with output power at 1-dB compression (P1dB) of +12 dBm. Because intermodulation-distortion (IMD) products between multiple carriers are dependent on the OIP3, high OIP3 ensures low IMD.

Output second-order-intercept point (OIP2) is another critical parameter for zero-IF applications. In the UMTS band, the MAX2022's OIP2 is +50 dBm. In particular, second-harmonic effects in baseband signals will produce spectral spreading in the RF output, compromising the ACLR performance. High values of OIP2 ensure low levels of ACLR distortion (Fig. 2).

The noise-floor performance of the MAX2022 is significantly enhanced by the use of passive mixers for the modulation function (Fig. 3). Passive mixers do not generate excess noise, helping the modulators to approach an output noise level of −174 dBm/Hz for typical output signal levels. For signal levels above −10 dBm, the phase noise of the LO buffers comes into play. These buffers were designed to have exceptionally low phase noise of −164 dBc/Hz.

When comparing RF modulator performance, it is useful to look at the following figure of merit: the dynamic range of the device, the difference between the maximum practical signal level as expressed by P1dB, and the noise floor. The MAX2022 has a dynamic range of 186 dB, exceeding the range of commercial integrated RF modulators.

In the PCS and UMTS bands, LO leakage levels are ¾−40 dBm. Sideband suppression is better than 45 dB for these bands. A digital predistortion control loop can further reduce these levels, driving the LO leakage to below −80 dBm and the sideband suppression to better than 60 dB. The RF passband flatness is better than 0.5 dB over a 100-MHz bandwidth, making these modulators ideal for broadband systems.

The performance levels of the MAX2022 translate well into real-world applications. Consider the problem of generating four carriers with WCDMA modulation (Fig. 4). Contemporary transmitter designs must accommodate the bandwidth of the WCDMA carriers themselves equal to 20 MHz. In addition, they must support the bandwidth that is necessary to digitally pre-distort the transmit signal in order to correct for subsequent distortion by the power amplifier. This bandwidth can exceed 100 MHz.

Figure 5 illustrates the ACLR performance for one-, two-, and four-carrier WCDMA generation in the UMTS band. Due to the wide dynamic range of the MAX2022, the exceptionally good ACLR values are maintained over a very wide output-power-level range. This broad range of usable output power is useful in system design. The noise-floor performance also is included to illustrate the total available dynamic range for a chosen ACLR performance. For example, a four-carrier WCDMA signal at −28 dBm per carrier will have an ACLR of 66 dB and an output noise floor of −173.5 dBm/Hz.

The MAX2022's interfaces have been designed to minimize the ancillary circuitry. The impedance-matched integrated LO buffers and balun allow the use of a single-ended LO interface at low LO power levels of −3 to +3 dBm. The integrated RF balun allows a single-ended RF output, which is impedance matched to 50 Ω. The baseband I and Q inputs present a differential interface with a 44-Ω internal impedance. These inputs allow a direct connection to the outputs of high-performance current-output DACs with no intervening buffer amplifiers.

Figure 6 offers a recommended DAC termination interface to the MAX2022. The 50-Ω resistors to ground terminate the DAC appropriately. In addition, the typical full-scale current of 20 mAp-p delivers up to 0 dBm to the baseband input of the MAX2022. External baseband amplifiers are not needed in this configuration.

Figure 7 shows how to use the MAX2022 to generate four WCDMA-modulated carriers with digital predistortion capability. The signal levels, noise levels, and ACLR are listed for the cascaded lineup at each stage's output. Starting with the DAC, the following is required: a part that can generate a signal with 50 MHz of bandwidth; ACLR significantly better than the target of 65 dB for this design; and a low noise and spurious floor. The MAX5895 is suggested to fulfill these requirements. The key DAC specifications are the ACLR for four-carrier operation and the noise and spurious floors. For this application, interpolating DACs are recommended to allow the DAC to operate at a high output sample rate and a relatively low input data rate. The 95-dB attenuation of the interpolation filters then becomes significant, since the lowpass filter following the DAC will not have significant attenuation of the near-in interpolation images.

Moving on to the modulator output, it is clear that the output signal level will be −28 dBm per carrier (−22 dBm total for four carriers). The ACLR will be set by the modulator's performance at 66 dB. (The DAC's performance is not a limitation here.) The noise floor, however, has degraded from the −174 dBm/Hz of the modulator alone to −170 dBm. This degradation is due to the cascaded noise level of the DAC. Here, it becomes apparent that one must choose all elements of the lineup carefully to achieve the highest level of performance.

To avoid degradation of the cascaded ACLR, the following RF amplifier must have a low noise figure and adequate OIP3. An OIP3 greater than +30 dBm is recommended for this stage if the gain is 12 dB. The output stage is chosen with a high OIP3 to avoid degrading the cascaded ACLR. The MAX2057 RF-VGA is suggested to allow the adjustment of the overall lineup's gain to set the output level at −6 dBm per carrier or 0 dBm total. The OIP3 of +37 dBm ensures that the cascaded ACLR remains at 65 dB.

This cascaded transmitter lineup's performance yields an excellent ACLR of 65 dB while maintaining a noise floor of −139 dBc/Hz relative to each individual carrier.

The MAX2022 allows both zero-IF and image-reject-mixer architectures. This aspect facilitates a very streamlined, cost-effective, and flexible transmitter architecture. Maxim Integrated Products, 120 San Gabriel Dr., Sunnyvale, CA 94086; (978) 934-7628, Internet: www.maxim-ic.com.