Maxim's Lowest Noise, Highest Linearity, 2GHz, Zero-IF I/Q Modulator Enables Four-Carrier WCDMA Base-Station Transmitters
SUNNYVALE, CA-August 15, 2005-Maxim Integrated Products (NASDAQ: MXIM) introduces the MAX2022, the industry's lowest noise, highest linearity, 2GHz, Zero-IF I/Q modulator designed specifically for wireless infrastructure applications. The MAX2022 is the latest addition to Maxim's growing base-station portfolio, which includes complete RF-to-bits solutions for virtually every wireless standard.
Ideally suited for 2.5G/3G base-station transceivers, the MAX2022 delivers over 23dBm and 51dBm of OIP3 and OIP2 performance, respectively, while maintaining an incredibly low output-noise density of less than -173.2dBm/Hz. Uncalibrated sideband suppression is rated at 45.7dBc, and the unnulled carrier feedthrough is limited to -40.4dBm. The MAX2022 also delivers up to -20.8dBm of output power, with an exceptionally low -0.005dB/°C variation over -40°C to +85°C. Output power flatness is also impressive at 0.32dB. Together, these features enable true four-carrier WCDMA/UMTS direct conversion architectures, with -27dBm/carrier power and 67dB adjacent channel leakage ratio (ACLR) performance.
The MAX2022 is ideal for 1500MHz to 2500MHz WCDMA, DCS/PCS/EDGE, cdma2000®, and WiMAX(s) wireless infrastructure applications where high linearity and low noise figure are critical for maintaining high levels of ACLR and Error Vector Magnitude (EVM) transmitter performance. In the case of WCDMA, the MAX2022 can deliver peak ACLR levels up to 71dB, 68dB, and 67dB for 1-carrier, 2-carrier, and 4-carrier operation, respectively. (See Figure 1 for a complete depiction of ACLR, alternate channel leakage ratio (Alt CLR), and output noise as a function of carrier power.) The MAX2022 also yields a 3dB advantage in OP1dB linearity over the closest competing solution, and it offers an impressive 15dB advantage in output noise performance. (See Figure 2.)
In addition, the superb output noise performance eliminates the need for an expensive transmit mask filter immediately following the modulator. The MAX2022 is the only 4-carrier modulator on the market to do this without the use of supplemental baseband I/Q buffer amplifiers. The elimination of this transmit filter also simplifies the design of digital predistortion architectures because the system's bandwidth will not be limited by the filter's mask.
As a fully integrated SiGe ZIF modulator/demodulator, the MAX2022 integrates two state-of-the-art matched mixer cores for modulating in-phase and quadrature signals, three LO mixer amplifier drivers, and an LO quadrature splitter. On-chip baluns are also integrated to allow for single-ended RF and LO connections. (See Figure 3.) As an added feature, the baseband inputs have been matched to allow direct interfacing to the transmit DAC, thereby eliminating the need for costly I/Q buffer amplifiers. Through all this integration, the total board space used by the upconverting modulator is reduced by a factor of 4x; the discrete part count is reduced by 60%. More importantly, the MAX2022's ZIF architecture eliminates a complete IF transmit stage, thus resulting in further reductions in board size and lineup cost. Four IF amplifiers, two upconversion mixers, two LO buffer amplifiers, two filters, and a complete synthesizer (VCO + PLL) are effectively replaced by this single ZIF modulator.
As part of its commitment to providing RF-to-bits solutions for the wireless infrastructure market, Maxim is now offering a complete UMTS/WCDMA transmitter reference design that capitalizes on the breakthrough technology provided by the MAX2022. Figure 4 illustrates Maxim's transmitter solution which utilizes the recently introduced MAX5895 interpolating DAC, the MAX2057 RF VGA, the MAX2015 RF power detector, and the MAX2016 gain and VSWR detector. Actual measurements from the reference design are given below. As shown, a targeted ACLR goal of 65dB is met for the 4-carrier case, with -6dBm/carrier output power and an exceptionally low noise floor of only -146dBm/Hz. The corresponding ACLR plots are provided in Figures 5a, 5b, and 5c.
| Number of Carriers | POUT per Carrier (dBm) | ACLR (dB) | Noise Floor (dBm/Hz) |
| 1 | 0 | 73 | -146 |
| 2 | -3 | 68 | -146 |
| 4 | -6 | 66 | -146 |
The MAX2022 comes in a compact 6mm x 6mm, 36-pin thin QFN. Lead-free packaging is also available. Prices start at $8.95 (1000-up, FOB USA).
Background White PaperCost Pressures Within the Wireless Infrastructure Market
Since the advent of cellular, designers have continually wrestled with two seemingly incompatible goals, namely to reduce the cost of their BTS hardware while maintaining (or even improving) overall radio performance. Recent competitive pressures within the industry are pushing base-station cost reduction efforts to the forefront of design, and engineers are now looking for dramatic ways to reduce the cost of BTS transceiver hardware.
Cost reduction targets of 30% to 40% are commonplace with each new-generation design. On the radio front, there are two key ways to significantly reduce cost: the first—and most effective—approach is to increase the capacity of the radio lineup by employing multicarrier architectures (i.e., increase the ratio of carriers/radio). The second approach seeks to reduce the radio's overall component count. The MAX2022 achieves both objectives through its state-of-the-art, high performance, zero-IF transmit architecture.
Zero-IF Architectural Opportunities and Challenges
Zero-IF techniques are by no means new. Virtually all of today's mature handset designs employ Zero-IF (ZIF) architectures due to their simplicity and cost effectiveness. On the base-station front, however, ZIF architectures have been used sparingly due to their performance limitations. ZIF transmitters could historically support single-carrier cdmaOne™ and cdma2000 operation with adequate transmit mask performance. Unfortunately, the mask requirements for wide bandwidth applications, including WCMDA and multicarrier cdma2000, were virtually impossible to meet with acceptable margins.
The mask refers to the transmitter's spectral profile, and it is primarily defined by the lineup's cascaded ACLR performance. The 4-carrier WCDMA/UMTS designs are particularly daunting as ACLR limits of 65dB are often targeted for power levels on the order of -6dBm/carrier. Superheterodyne architectures traditionally predominated due to their inherent superiority in ACLR performance. These designs, however, were noticeably inferior in terms of component count, board space, and power consumption.
Two variations of the superheterodyne architecture are, nonetheless, currently found in base-station transmitter designs. (See Figures 6a and 6b.) The first approach utilizes high-IF injection coupled with a single upconversion from IF to RF.
While the single upconversion is obviously attractive from the standpoint of component count and layout, that architecture requires an exceptionally high-performance DAC. The advantages of this system are thus nullified by the complexity and cost of the data converter. Consequently, most base-station superheterodyne transmitters utilize a second, alternative approach: a double upconversion configuration which decreases the complexity and cost of the DAC, but leads to additional component count, layout space, and cost.
Until the MAX2022, designers had to tolerate these architectural shortcomings in order to meet their performance and cost targets.
MAX2022 Device Enablers
The MAX2022 was specifically defined to address the linearity and noise limitations that historically plagued ZIF designs. At the core of the design are two of Maxim's propriety SiGe FET mixers, key blocks originally developed as part of the MAX2039/MAX2041/MAX2043 and the MAX9994/MAX9995/MAX9996 families of high-linearity up-/down-converters. The LO drive circuits from these devices were also leveraged to provide an incredibly low-output phase noise of only -163.5dBc/Hz. For POUT levels <-27dBm/carrier, the resulting output noise approaches the thermal limit at -173.2dBm/Hz. When combined in a classic quadrature configuration (Figure 2), these cores yield a cascaded noise floor which eclipses the closest competitors by as much as 15dB. (See Figures 1 and 3.) The P1dB linearity of the device is also superior by 3dB. Consequently, the MAX2022 holds an 18dB advantage in dynamic range over all competing modulators.
MAX2022 Application Enablers
The MAX2022 is ideal for wireless infrastructure applications spanning the 1500MHz to 2500MHz frequency range, including systems which address the WCDMA/UMTS, DCS/PCS/EDGE, cdma2000, and WiMAX standards.
One key feature of this device is its superb output-noise performance which eliminates the need for a transmit mask filter immediately following the modulator. The MAX2022 design is the only 4-carrier modulator available that can accomplish this feat without using supplemental baseband I/Q buffer amplifiers. Eliminating this transmit filter enables, and inherently simplifies, the design of ZIF digital predistortion architectures because the system's bandwidth will not be limited by the filter's mask. Predistortion algorithms essentially compensate for 3rd and 5th (and often higher) order intermodulation (IM) distortion products. The compensating algorithms must, therefore, generate complex signals with bandwidths that are 3x to 5x greater than the carrier bandwidth. The mask filter required by competing modulator designs will obviously compromise the predistortion algorithm as the mask response, at 1x the carrier bandwidth, will reject the compensating 3rd and 5th order corrections.
Maxim Integrated Products is a leading international supplier of quality analog and mixed-signal products for applications that require real world signal processing. For more information, contact Maxim at 120 San Gabriel Dr., Sunnyvale, CA 94086. Telephone: 408-737-7600 or our URL: www.maxim-ic.com.
cdma2000 is a registered trademark of the Telecommunicatons Industry Association.
cdmaOne is a trademark of CDMA Development Group.
WiMAX is a service mark of Bandwidth.com, Inc.
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