Dual-band PA Applications in CDMA and WCDMA Devices
By Chen Bem Choi, Eric Garloff, Mahendra Singh & Bruce Webber, Anadigics, Inc.

Operators worldwide are upgrading their networks from 2G voice-oriented technologies to 3G and 4G technologies that enable them to offer high-speed, broadband wireless data services to consumers and business users. Wireless data services like music downloads, email and mobile internet access are increasingly popular with subscribers, and mobile broadband data revenue is helping to offset declining voice ARPU (average revenue per user). In Europe, wireless mobile data contributes 25-30% of revenue for many operators, and contributes up to 38% for some Japanese operators, according to a recent report by ABI Research (1).

Most wireless handsets are designed to operate in more than one frequency band. Multi-band capability benefits operators and subscribers by enabling the user to easily transfer from one network to another. “Roaming” in this way improves the mobile user experience by assuring that voice and data communication are instantly available wherever the user needs them. Roaming enables the operator to utilize multiple licensed bands in building their own network, and creates opportunities for revenue from subscribers who use their phones outside their home network coverage.

CDMA/EVDO and WCDMA/HSPA are the two 3G technologies that are most widely deployed. Both technologies depend on code-division multiple access (CDMA) to enable many users to efficiently utilize the same piece of RF spectrum for voice and data communication. CDMA is a method of spread-spectrum RF communication which depends on a unique “chipping code” assigned to each user’s communications to differentiate it from others in the same RF spectrum. Spectrum spreading improves the channel capacity and allows multiple users to utilize the entire carrier bandwidth. CDMA/EVDO networks utilize 1.25MHz carriers, while WCDMA/HSPA networks utilize wider, 5MHz carriers. Data rates can be further increased by higher-order modulation in the radio access network, providing better user experience with wireless data when the airlink provides adequate signal-to-noise (SNR) for low bit-error communication. A highly linear RF subsystem is required to assure that accuracy of the data transmitted desired over the radio access network.

Multiband CDMA and WCDMA phones require greater RF design effort to assure proper operation with adequate output power, linearity and efficiency in each frequency band. Designs typically include a power amplifier and a duplexer for each band of interest, and may include RF filters and switches to tie several RF inputs to a single antenna. To reduce the design complexity in multi-band phones, ANADIGICS has developed dual-band CDMA and WCDMA power amplifier modules. By combining two power amplifier modules in a single package, these innovative products reduce board area requirements and simplify routing of DC and RF signal lines, compared to designs utilizing two separate power amplifiers. This article will describe a typical application of dual-band power amplifier modules in CDMA/EVDO and WCDMA/HSPA phone design.

This article will focus on the application of ANADIGICS AWT6221 in a design for a dual-band WCDMA/HSPA phone operating in UMTS Band 2 and 5.  This innovative power amplifier has been used by leading manufacturers for high-volume devices including the RIM Blackberry 9000 (Bold) and Sierra Wireless Compass 885 modem. (2)

The AWT6221 dual-band PA dramatically reduces average current consumption by 75%, with up to 25% increase in talk-time. ANADIGICS’ HELP3™ PAs like AWT6221 use the company’s exclusive InGaP-Plus™ technology, which integrates bipolar and field-effect transistor (FET) devices on the same InGaP GaAs die. Through selectable bias modes, the HELP3™ PAs achieve optimal efficiency across the low-range and mid-range output power levels. The intelligent bias circuit of AWT6221 reduces current consumption to 8mA at low power levels - the world’s best. Through the integration of two independent PA chains, the tiny AWT6221 delivers uncompromised performance in both frequency bands and conserves printed-circuit board area.

ANADIGICS offers dual-band WCDMA/HSPA power amplifier modules for other popular combinations of frequency bands, as well as dual-band power amplifier modules for CDMA/EVDO applications. More information is available from ANADIGICS (www.anadigics.com). ANADIGICS has developed reference designs using its products to help customers to achieve their design goals faster and easier. In this article we will describe a reference design developed to demonstrate the advantages of the AWT6221.

The AWT6221 reference design was developed for a dual-band WCDMA/HSPA phone operating in UMTS Band 2 (824 – 849MHz) and Band 5 (1850 – 1910MHz). The 3GPP technical specifications for user equipment (UE) of power class 3 in these bands require performance that meets or exceeds these minimum performance levels (3):

• Maximum Output Power: +24dBm +1/-3 dBm
• Adjacent Channel Leakage Power Ratio (ACLR) at +/- 5MHz: -33dBm
• Adjacent Channel Leakage Power Ratio (ACLR) at +/- 10MHz: -43dBm
• Maximum Spurious (Harmonic) Emissions at specified frequencies:
• Between 30 – 1000MHz: -36dBm/100kHz
• Between 1 - 12.75GHz: -30dBm/1MHz
• Between 869 – 894MHz: -60dBm/3.84MHz
• Between 1930 - 1990MHz: -60dBm/3.84MHz
• Between 2100 - 2170MHz: -60dBm/3.84MHz

Most CDMA and WCDMA networks are frequency division duplex systems, in which different RF frequency bands are assigned for uplink (from mobile to base station) and downlink (from base station to mobile) to allow simultaneous RAN links. Duplexers are utilized in the mobile device to allow UL and DL to utilize a single antenna. For UMTS Band 2, we chose the Avago ACMD-7403, a miniature FBAR duplexer, for its high-Q response. For Band 5, we chose the Epcos B-7663, a low-loss SAW duplexer. The reference design also includes CP402A thin film directional power couplers from AVX to provide coupled RF power required for many top-tier 3G chipsets.

Development of the AWT6221 reference design began with measurements of S-parameters looking into the coupler and duplexer from the PA. Zero-ohm resistors were used to short the matching component pads on the board for this measurement.

S-parameters describe the response of an N-port network to voltage signals at each port. The first number in the subscript refers to the responding port, while the second number refers to the incident port. Thus, S21 means the response at port 2 due to a signal at port 1. S-parameters come in a matrix, with the number of rows and columns equal to the number of ports. Parameters along the diagonal of the S-matrix are referred to as reflection coefficients because they only refer to what happens at a single port, while off-diagonal S-parameters are referred to as transmission coefficients, because they refer to what happens from one port to another. The S-matrix for a two-port network is shown here (4):

Reflection coefficients (S11) weremeasured for both RF paths over the frequency ranges for Band 2 and Band 5. The results, displayed in Smith chart format, show the range of impedance variation that the PA “sees” at its output in the un-matched circuit.

For any power amplifier, the output impedance has a significant effect on operation, and strongly influences the power transferred to the antenna, the linearity (ACLR) and efficiency of operation. Load-pull information available from PA makers like ANADIGICS helps engineers make good trade-offs between these important characteristics. The optimum output impedance is influenced by characteristics of other components in the RF chain, such as duplexers and switches, and it is not always 50Ω. Variation in duplexer characteristics over frequency within the band of interest and over operating temperature are particular challenges in developing a good WCDMA RF design.

The next step in developing the AWT6221 reference design was to impedance match the duplexers to the antenna to minimize the impedance variation over frequency within each band. Although a simple 2-component match can, theoretically, give good performance over a narrow band of frequencies, getting good performance over wider bandwidths, and compensating for expected variation in component characteristics due to production tolerances and changes in temperature or voltage, often requires additional matching elements. A 3-component match was chosen for both bands, with results shown in Smith chart format. The matching networks greatly reduced impedance variation at the duplexer, and thereby simplified matching for the rest of the circuit.

The next step in developing the AWT6221 reference design was to match between the PA and coupler + duplexer. Again, reflection (S11) was measured for both RF paths over the frequency ranges for Band 2 and Band 5; the results were used to develop matching networks that optimized PA performance to the requirements for WCDMA operation, with minimal impedance variation across each band. As before, 3-component networks were required to achieve good performance; results are shown in Smith chart format.

The finished reference design meets all requirements for WCDMA operation in both Band 2 and 5 at room temperature. Moreover, the matching networks minimize variation over frequency, temperature and supply voltage to maintain performance within specs under all expected conditions.

The reference design discussed in this article can be a useful starting point for developing new designs for 3G handsets, datacards, wireless modems and other WCDMA/HSPA devices. In developing a new design, the engineer will need to consider the important performance requirements of his/her own design and consider changes to the matching networks discussed here to achieve optimal trade-offs in output power, linearity, efficiency and other RF parameters. Reviewing the load-pull contours for each power amplifier will help to identify the optimal output impedance matching point, which might not be 50Ω. The engineer will also need to consider the variability of each component in the RF path and choose matching networks that reduce variation in the antenna output over the anticipated use temperature, frequency and supply voltages to keep all RF parameters within acceptable levels. Finally, it will be important to control the reflectance (return loss) at the PA to avoid conditions that lead to instability or oscillation in the circuit.

ANADIGICS AWT6221 is one of a family of dual-band power amplifiers for 3G handsets and data devices. Currently available dual-band PA products are listed below, and new designs are being developed. Contact ANADIGICS to obtain more information about any of these products.

• AWT6221: WCDMA/HSPA HELP3™ dual-band PA for UMTS Bands 2 and 5
• AWT6222: WCDMA/HSPA HELP3™ dual-band PA for UMTS Bands 1 and 6
• AWT6224: WCDMA/HSPA HELP3™ dual-band PA for UMTS Bands 1 and 8
• AWT6321: CDMA/EVDO HELP2™ dual-band PA for Cell and PCS bands

Sources
“Economic Conditions Reducing Mobile End-User Spending Up to 15%” ABI Press release, May 6, 2009.
“ANADIGICS Powers Sierra Wireless’ COMPASS™ 885 USB Modem,” company press release, November 11, 2008.
3GPP Technical Specification 3GPP TS 25.101 V7.15.0 (March 2009).
http://www.microwaves101.com/encyclopedia/sparameters.cfm

Anadigics, Inc.
www.anadigics.com
TXTLINX.COM91
Email this article to a friend!