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Intelligent Integration of the CDMA RF Front End

The benefits of an integrated RF front end for multiband handsets are considerable; so are the design problems. This article explains in detail how to get from here to there.

Allen Chien, Saul Espino, and Won Kyu Kim, Wireless Semiconductor Division, Avago Technologies

Today’s mobile devices are quite complex, with many RF radios packed into an ever shrinking footprint. How to pack more radio components into less space? Shrinking the individual radio components is the obvious one, while integration of the individual parts is the other.

The reasons for integration have been well documented. Besides the actual space savings, there is also improved performance and simplified design resulting in faster time-to-market. If there was a ‘holy grail’ here it would be a single plug-and-play module containing the entire RF front end that customers can just drop onto their PC boards.

An example of this ‘holy grail’ can be seen in the Wi-Fi space. Today, complete dual band 802.11 a/b/g/n Wi-Fi front ends exist in single modules as small as 4x6mm in size. These modules contain the entire RF front end including the power amplifier (PA), low-noise amplifier (LNA), filters, and switches all on a single substrate which represents a true plug and play between the RFIC and the antenna. Adoption of these modules is especially strong for the next generation of laptops being designed this year.

Integration of CDMA and UMTS RF front ends in mobile handsets is not as advanced as Wi-Fi front ends yet. One of the reasons is that the content of the UMTS RF front end is still continually varying and is region-specific. The CDMA RF front end, however, is fairly standardized in North America as dual mode/tri-band, Cell-band-plus-GPS and integrated module solutions have the potential to be as widely implemented as Wi-Fi solutions.

The two main paths for RF integration are by-band (combining PA + duplexer of a single band) or by-technology (combined PA modules and combined filter modules). Both provide the benefits of saving space and simplifying design, but the by-technology approach can also provide performance improvements through pin map optimization. Of the two choices, integration by-technology has proven to be the more common solution in today’s phones. From the component supplier point of view, fewer suppliers actually have both technologies (PA + filter) in-house, and few have the expertise to integrate them together. This article will demonstrate a tri-band CDMA RF front end solution using three highly integrated RF modules that takes the place of seven discrete components.

The discrete solution for a CDMA front end consists of seven RF components: PCS duplexer, Cell duplexer, triplexer, PCS PA, Cell PA, GPS LNA, and a GPS filter as shown in Figure 1a. When used in combination with Qualcomm’s single chip QSC 60X5 series module, the average PCB space used is ~525mm2.


Figure 1: a) Schematic of discrete CDMA RF front end solution and b) schematic of integrated CDMA RF front end solution.

Several complications arise from this solution, the first of which is line routing. Due to component placement and pin-out, there will be RF traces crossing each other, which then requires the use of the inner layers of the PCB for routing. Transitioning from microstrip lines used on the top layer of the PCB to lossier striplines in the inner PCB layers results in more total RF loss. The large area also results in longer line RF traces and more RF loss as well. Lastly, the long crossing lines increase the possibility of attracting interference from other noise sources on the PCB, resulting in degraded system performance.

Benefits of Using an Integrated Solution

In comparison, Figure 1b shows an integrated solution, with three modules replacing seven discrete components, using only ~390mm2 of PCB space for approximately 25 percent area savings. The integrated solution has fewer components to lay out, and the pinout for each module has already been optimized for the chipset pin out, which results in less design and layout time.

The ACPM-7353 is a 4x5mm dual band (PCS + Cell) power amplifier that uses Avago Technologies’ 5th generation CoolPAM technology, offering 3-mode operation with a new bypass mode option for low current consumption at low power. The ACFM-7103 is Avago’s 3rd generation (PCS + Cell + GPS) FBAR 4x7mm multiplexer. It offers a 30 percent size reduction compared to previous generations and a new option that allows the multiplexer to be used with either single or dual antennas for increased design flexibility. In addition, the low insertion loss and high degree of isolation result in excellent talk time and system sensitivity. The 3.3x2.1 mm ALM-1412 GPS LNA-Filter module integrates a pHEMT LNA and an FBAR filter together for low noise figure, high linearity and excellent out-of-band signal rejection.

The performance of this integrated solution offers superior improved talk time, sensitivity, blocker immunity (Single Tone Desense), and GPS sensitivity compared to a discrete solution. PCS sensitivity is an extremely low -108dBm. Let’s take a closer look at each of the three integrated module’s features and value propositions.

Handset phone use models show that high power is not needed the majority of the time in urban areas where dense base station coverage exists. To achieve higher efficiencies, Avago’s CoolPAM technology uses an architecture that can bypass various power stages in the PA when necessary to reduce current consumption at low output powers. The key feature of the CoolPAM technology is that the active stage bypass is implemented without using any RF switches, which are bulky and expensive.

The fifth generation CoolPAM technology has three operation modes: bypass, mid-power and high-power. In bypass mode the driver and PA stages of the module are inactive and only the Active Bypass Network is used. With this quiescent currents of ~4 mA can be achieved. In mid-power mode only the driver stage of the amplifier is active. While in high-power mode all stages of the PA are used. The reduced current consumption results in 20 minutes of improved talk time. Moreover, the ACPM-7353 dual-band 4x5mm PA offers 20 percent space savings vs. using discrete components and reduces component count from two to one.

Avago demo board
Figure 2: Photograph of a CDMA RF front end demo board
populated with the ACPM-7353, ACFM-7103 and
ALM-1412 modules.

Integrating the PCS duplexer, Cell band duplexer, GPS filter, and triplexer together, Avago’s ACFM-7103 multiplexer reduces component count and saves space. Insertion loss improvement is one of the main benefits of the multiplexer. The transmit (Tx) IL has a direct relationship to battery life, while the receive (Rx) IL has a direct impact on receiver sensitivity. In the discrete solution, the triplexer contains high-pass, low-pass and band-pass filters plus a GPS SAW filter. When used with discrete duplexers, the loss of the triplexer, discrete duplexer and line loss all cascade together.

The FBAR multiplexer uses an impedance matching network to link the filters together, resulting in less RF loss compared to the triplexer solution. This type of matching network can be implemented with FBAR filters because of the FBAR filters’ superior Q values and out-of-band rejection compared to SAW filters. The multiplexer’s typical PCS Tx IL is 1.5 dB (3.1 dB max) and can reduce power consumption by 50-70 mA. The excellent typical Rx IL for PCS (1.6 dB typical) and Cell band (1.4 dB typical) can improve receiver sensitivity by 0.7-1 dB compared to the discrete solution. In addition, all the phase matching to the triplexer has already been done resulting in a simplified design.

The integrated LNA-FBAR filter module is used in conjunction with the GPS pre-LNA filter in the multiplexer. The EpHEMT LNA allows for single-supply operation down to very low voltages while maintaining the highly desirable properties of traditional pHEMTs such as low noise figures, high linearity and gain. The FBAR filter provides low IL for excellent noise figures and superior out of band blocking to prevent jammers from compressing the GPS receiver chain.

These integrated solutions were assembled onto a single demo board (Figure 2) and characterized. Notice that the pinout of all the components has been designed to match to chipset I/Os so no inner layer traces are needed, nor do any of the RF lines cross each other. For PA layout, bypass caps were placed next to the PA to minimize noise. The Vdd choke was placed close to GPS LNA module to minimize noise and output matching. Placement of the GPS LNA module should be as close as possible to the multiplexer for better system noise performance. Placement of the multiplexer was also done to minimize Tx and Rx line lengths, and orthogonal trace routing from the multiplexer was implemented, which is essential for best pin-to-pin isolation. In general the shortest possible PCS and Cell Rx trace length to the IC is desired to maximize receiver sensitivity.

Performance Results

On the transmit side, current consumption is the key metric that determines battery life or talk time for the RF front end. Current consumption testing was done at 4V, which is the ‘full battery’ condition. Adjacent-channel power ratio (ACPR) testing is done at 3.4V, which is the worst case ‘low battery’ condition. For the PCS chain, 4 mA Icc was demonstrated at -10 dBm antenna power, 10 mA at +0 dBm antenna power, and 391 mA at +2 4dBm full output power. These current consumption levels will provide excellent talk time. Similarly for the Cell band, low current consumption of 340 mA was demonstrated in high-power mode. ACPR1 for PCS and cell band are both better than 50dBc at antenna power of +24dBm. A summary of typical performance is listed in Table 1.

 

 

Channel

 

 

Low

Mid

High

PCS Band

 

 

 

 

 

Current (mA)

365

368

391

 

ACPR 1- (dB)

-58

-56

-56

 

ACPR 1+ (dB)

-59

-57

-56

 

ACPR 2- (dB)

-62

-59

-61

 

ACPR 2+ (dB)

-62

-59

-61

 

Sensitivity (dBm)

 

 

 

 

Integrated Solution

-108.2

-109.4

-108.3

 

Discrete Solution

-107.2

-108.6

-107.7

 

STD (dB)

-25.7

-26.9

-25.7

Cell Band

 

 

 

 

 

Current (mA)

340

317

335

 

ACPR 1- (dB)

-53

-55

-56

 

ACPR 1+ (dB)

-53

-54

-52

 

ACPR 2- (dB)

-67

-64

-61

 

ACPR 2+ (dB)

-68

-64

-62

 

Sensitivity (dBm)

 

 

 

 

Integrated Solution

-109

-109.7

-108.6

 

Discrete Solution

-108.3

-108

-108.1

 

STD (dB)

-25.2

-24.6

-25.3

GPS Band

 

 

 

 

 

Sensitivity (dBm)

 

 

 

 

Integrated Solution

 

-157.7

 

 

Discrete Solution

 

-156.6

 


Table 1: Summary of PCS, Cell, and GPS characterization data
from integrated CDMA RF front end solution.

Sensitivity, the measure of the minimum power that a receiver can demodulate, is the key receiver specification. Any distortion added to the receive chain will lower the handset sensitivity. There are many reasons why a handset may have degraded sensitivity. The most common ones are poor TX to RX isolation, high PA noise floor, and high insertion loss in RX path. Achieving good PCB board isolation with adequate grounding, well placed vias, and orthogonal trace routing is essential to maintain good sensitivity performance. System sensitivity is also directly correlated to the RX path loss. This path loss is determined by the insertion loss of the Rx filters in the multiplexer plus the trace loss in routing to the antenna and the LNA.

The key component in this receive chain that gives superior sensitivity in this solution is the Rx insertion loss of the ACFM-7103 multiplexer (typical 1.6 dB in PCS band and typical 1.5 dB in Cell band). Table 1 shows a summary of sensitivity and STD for all channels tested. With -108 dBm sensitivity or better demonstrated in phones, the customer is expected to have 4  dB of margin for manufacturing margin. Single Tone Desense (STD) is very similar to sensitivity except for the fact that a -30d Bm Jammer is added at 1.25 MHz away from the RX channel of interest. Since sensitivity is harder to achieve in the presence of a jammer, the 3GPP2 spec is relaxed from -104 dBm to -101 dBm. Phone service providers generally look for -108 dBm min for sensitivity and -104 dBm for STD performance levels. With STD values of -104 dBm or better achieved, 3-5 dB of manufacturing margin is expected for the end customer.

The GPS path from the antenna consists of a pre-LNA filter, LNA and post-LNA filter. Typically the filter in front of the LNA is used for blocking out-of-band blockers. This helps prevent blockers such as Bluetooth and Cell band from mixing inside the LNA and impacting GPS sensitivity or gain compression of the GPS LNA from PCS/Cell Tx leakage. The GPS filter after the LNA is used for removing unwanted noise power or spurs generated from the LNA which might affect the down converter that follows. The complete GPS receive chain achieved 10.6 dB of gain with a 2.0 dB noise figure while consuming only 5 mA from a 2.7V supply. The integrated solution demonstrates up to 1 dB of improved GPS sensitivity is demonstrated versus a discrete solution using SAW filters as shown in Table 1. The improved sensitivity is primarily due to the low 0.8 dB typical IL of the FBAR pre-LNA filter.

Summary

In summary, the objective of this article was to present an example of a highly integrated tri-band dual mode (CDMA + GPS) RF front end solution using three of Avago’s highly integrated RF modules—the ACPM-7353, ACFM-7103 and ALM-1412 (dual band PA, FBAR multiplexer, and GPS LNA-Filter module)—that take the place of seven discrete components. The module pinouts are arranged to match Qualcomm’s QSC 60X5 chipset. This solution saves 25 percent of PCB space, while simultaneously improving current consumption and sensitivity. Approximately 20 minutes of talk time improvement is demonstrated in PCS/CELL bands through 50-70 mA current savings at a maximum antenna power of +24 dBm. Receiver sensitivity in the PCS and Cell bands was improved by 1.0 dB and 0.7 dB relative to the discrete solution. Finally, up to 1 dB of improved GPS sensitivity is demonstrated versus a discrete solution. The proper use of ground vias, trace routing, and component location when doing layout are critical to maintaining good system performance in handset applications.

Avago Technologies
San Jose, CA
(408) 435-7400
www.avagotech.com

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