A cell phone is basically nothing more than a very sophisticated two-way radio. The RF section is critical. But, it continues to become a smaller percentage of the entire phone as RF chip manufacturers place greater amounts of circuitry on-chip and as the processing and control functions become larger and more complex with multiple air interfaces and networking capabilities. Today, the RF chips have essentially become a commodity. Many manufacturers have similar competing lines. Virtually all of them use biCMOS circuitry, and many employ SiGe bipolar devices.
Recently, Motorola designers also licensed Atmel to make their RF chips to ensure a sufficient supply to this demanding market. Motorola has even developed a SiGe:C process that integrates the silicon and germanium with carbon to produce heterojunction bipolar transistors (HBTs) into their biCMOS RF chips. The SiGe:C process extends the transistor's fT to 50 GHz and the fMAX to 90 GHz.
Newer chips typically contain multiple signal paths to accommodate the multiband operation required for 2.5G and 3G phones. A typical 3G phone permits operation in the 800- to 900-MHz band and either the 1800- or 1900-MHz bands, or perhaps even both. Two separate signal paths can normally be used, one for 800 MHz and the other for 1800/1900 MHz, if external filters can be switched in to differentiate between the 1800- and 1900-GHz ranges.
Cell phones need power amplifiers (PAs) too. Most PAs are HEMT or HBT bipolar devices made with GaAs. Some LDMOS PAs, however, are used specifically for higher-power applications in base stations.
The trend is to use direct-conversion zero-IF (ZIF) receiver ICs. An example is the Othello chip set from Analog Devices Inc. It consists of the AD6523 direct-conversion receiver and the AD6524 frequency synthesizer chip. These are designed for use in a dual-band GSM (900-/
1800-MHz) phone. Analog Devices claims that this chip set will reduce the overall cost and size of the circuitry from 30% to 50% over current 2G designs. The chip is implemented in 0.6-µm biCMOS.
The AD6523 consists of a low-noise amplifier (LNA) that drives a variable-gain amplifier (VGA) and a received signal-strength indicator (RSSI) circuit. The output drives a pair of mixers which are driven by quadrature local-oscillator signals from the separate synthesizer chip. External low-pass filters reproduce the baseband signal that goes to ADCs. Contained in the transmit section are the baseband DACs and their filters. The processed baseband signals drive upconverters to produce the RF signal with modulation. Furthermore, a SAW filter feeds an RF VGA for feedback power control. Plus, an external PA boosts the signal to the final transmit power level.
The companion AD 6524 synthesizer chip is a fractional-N design using a single crystal oscillator. It generates four output signals which are usable in DCS 1800 and PCS 1900 phones. The Othello chip set is designed to support both GPRS and EDGE protocols for the coming high-data-rate services.
Analog Devices has further announced a new ZIF chip with Mitsubishi for 3G W-CDMA applications. In addition to the I/Q mixers, the chip has an overall gain of 95 dB with gain variable in 1-dB increments. It contains on-chip low-pass filters and the RSSI circuit as well.
Another receiver approach is Philips' near-zero-IF GSM transceiver chip. The UA3535HL doesn't translate the signal directly to baseband, but it generates an IF of 100 kHz, which is very low compared to the signal frequency. An IF at 100 kHz is far easier to filter than a higher frequency. An on-chip low-pass filter eliminates the higher IF. An integrated channel bandpass filter provides the desired IF selectivity. The chip supports the 900-, 1800-, and 1900-MHz frequency ranges.
The transmit section of the UAA3535HL is traditional with I/Q mixers to upconvert the baseband digital to a transmit IF. A second circuit then provides the modulation and mixing to the transmit frequency. Designed to be controlled by external PLL circuitry, VCOs for the receiver and transmitter sections are on-chip. Three power-up input pins are featured on the chip too. They let various parts of the circuitry be powered down during idle times. The RXON, TXON, and SYN pins allow the receiver, transmitter, and synthesizer circuits to be turned on separately or in any combination. Additionally, the UAA3535HL is designed to work with Philips' OneC-GPRS chip to implement the GPRS protocol. EDGE-protocol capability is expected in the future.
Although ZIF is the trend on the receiver side of the RF chain, the trend on the transmitter side is toward PAs with greater linearity, especially in the CDMA chip sets. Because CDMA requires very wide bandwidths, the PAs must be far more linear than those used in TDMA and analog designs. While such amplifiers are far less power-efficient than those employed in other chip sets, the greater linearity is essential to reduce the intermodulation (IM) products to a level that's acceptable by the standards.
Furthermore, the CDMA chip sets require precise power control. The power of a cell-phone transmitter is controlled directly through a closed-loop process with the basestation. It ensures that the basestation receives sufficient power, but also a minimal power level that minimizes interference and reduces the noise floor in CDMA receivers. In the new CDMA 3G phones, power is controlled in 0.5- or 0.25-dB increments.
An example of an RF chip set optimized for CDMA is Conexant's CX74001 and CX74002 products. These chip sets support any of the existing or future W-CDMA standards. The CX74001 is a dual-band receive subsystem made in a biCMOS process. It features two signal paths, including an LNA, mixers, VGAs, and I/Q demodulators, as well as two receive VCOs.
The CX74002 is a SiGe dual-band transmit subsystem. The I/Q mixers upconvert the baseband signal to RF and provide the necessary modulation. Dual VHF and UHF PLLs are incorporated, eliminating the need for an external PLL. The final RF upconverter provides sufficient output to drive the external PAs. Other products in the Conexant series include the CX74004 SiGe dual-band LNA/downconverter and the CX74005 bipolar VGA/I/Q demodulator. All of these new chips were designed to minimize power consumption by up to 20% over existing competitive devices, thereby decreasing power drain and increasing talk/idle time. Plus, Conexant makes TDMA/analog chip sets. All chips support GRPS and EDGE protocols for data transmission.
Mike Civiello, director of marketing for the Wireless Transmitter Solutions Div. of Motorola, indicates that switches are an unexpected RF need in 2.5G and 3G phones. Most multiband/multimode designs require the use of RF switches. Transmit/receive (T/R) switches are common in 1800-/1900-MHz phones, but most new designs need multiple switches for band switching and other circuit-switching functions. These switches select the proper input/output pins on the transceiver chips and route the external filters. Motorola is developing a line of GaAs pHEMT bipolar switches for these applications.
The key to a successful 3G phone is processing power. All new 3G phones will have far more powerful DSP chips and may incorporate several DSP chips to accomplish this. The new 3G phones will have more computing power than the average PC of today. More and more functions are being pushed into DSP as a way to reduce the parts count and simplify the design.
Of course, the ideal cell phone is a software radio, where virtually all of the processing functions are performed in software. The ideal software radio receiver consists of an LNA and SAW filter whose output goes directly to an ADC. All downconversion, demodulation, and other processing is carried out by the DSP. The transmitter section is simply a DSP upconverter that feeds the external PA.