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Highly Integrated IF ASICs for CDMA Cellular/PCS Telephones

This month's cover features an IC developed to simplify code-division multiple access cellular transceiver design

February 1, 1998
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Highly Integrated IF ASICs for CDMA Cellular/PCS Telephones

RF Micro Devices
Greensboro, NC

The first generation of cellular and personal communications service (PCS) telephones based on code-division multiple access (CDMA) relied on limited integration of RF and IF functions. ICs were designed to handle automatic gain control (AGC) amplification, upconversion and power amplification in the transmit chain. In the receive chain, ICs were developed to handle low noise amplification and downconversion, and receive AGC amplification. Baseband application-specific ICs (ASIC) were developed separately and included modulation, demodulation, analog/digital (A/D) conversion and all other digital processing circuitry. The next generation of telephones followed rapidly with new ideas for integration and chip partitioning. Telephone designers chose to remove quadrature modulation and demodulation functions from the baseband ASICs and place them in the analog RF/IF chips to alleviate mixed-signal isolation issues and allow for lower power CMOS designs. In an attempt to lower chip count and save valuable telephone board real estate, telephone designers moved to integrate the transmit AGC amplifier with the transmit upconverter.

To accommodate this new CDMA architecture that pushes toward higher levels of integration, three new ICs have been developed. The models RF9958 CDMA/FM transmit modulator/AGC/upconverter, RF9957 CDMA/

FM receive AGC/demodulator and RF2667 CDMA/FM receive AGC/demodulator are monolithic ICs fabricated in an advanced bipolar silicon process. All three of these low cost ICs operate from a 3 V power supply and are provided in small QSOP plastic packages.

The Transmit IC

The model RF9958 transmit IC integrates three functions into a single QSOP-28 package: quadrature modulation, variable-gain amplification and upconversion, as shown in Figure 1 . In a transmit system, the IC receives baseband information from a mixed-signal or digital ASIC in the form of in-phase/quadriphase (I/Q) data. The RF9958 ASIC provides differential input ports that are driven with the I/Q waveforms riding on a DC reference of 0.6 V DC. Single-ended operation also can be accommodated if the I/Q data are driven into the ISIG and QSIG pins with the IREF and QREF pins coupled to ground capacitively. In both cases, the 0.6 V DC reference is applied to all four pins.

The LO signal onto which the I/Q data are modulated also can be provided to the part differentially or single ended. In general, differential drive is recommended for ideal operation because it provides symmetrical swings on chip that produce better phase accuracy. However, single-ended drive will operate adequately. To split the incoming LO signal into two signals that are I/Q, the model RF9958 utilizes a divide-by-two flip-flop circuit.

Consequently, the LO signal must be twice the desired carrier frequency. In an RF transceiver, it is common to use a VCO that operates at twice the carrier frequency so that VCO leakage into other system components does not appear as co-channel interference. The flip-flop circuitry can operate up to 360 MHz, which translates to a carrier of 180 MHz. If a higher frequency is desired, the flip-flop circuitry can be adapted to operate at higher speeds with minimal effort. Under normal operation, the outputs of the flip-flop circuitry are mixed with the I/Q data and sent to the AGC amplifier. The model RF9958 ASIC features an option that bypasses the I/Q mixers, passing the flip-flop outputs directly to the AGC amplifier. This option is useful for dual-mode (CDMA/FM) telephone designers who prefer to perform FM by modulating a VCO such that the output of the VCO is connected to the LO input of the model RF9958. In this instance, the modulation and VCO frequencies are operated at twice the desired frequency. Pin 1 (MODE) on the part is the mode select switch that chooses either CDMA- or FM-mode operation. A logic high selects the CDMA mode while logic low selects the FM mode.

In the IS-98 CDMA cellular system, the cellular base station sends control signals to the mobile telephone, directing the transmit AGC amplifier to increment or decrement its gain in 1 dB steps. As the mobile unit strays further from the base station, the transmit AGC amplifier is directed to increase its gain and, hence, its output power. The opposite occurs as the mobile unit approaches the base station. The transmit AGC amplifier in the RF9958 ASIC has an 88 dB gain range and is controlled by an external DC voltage that typically is generated by a digital ASIC chip on the telephone board. With a nominal 300 mV p-p I/Q input signal, the AGC amplifier outputs over a range of –91 to –3 dBm into a 200 W differential load. In FM mode, the AGC amplifier drives +2.5 dBm into the same load. The gain is controlled by varying a DC voltage on pin 27 (GC). Using a 37 kW series resistor, the specified voltage range is from 0.5 to 2.5 V DC. Over the majority of the DC control voltage range, the gain adjusts with good linearity and the AGC amplifier exhibits good temperature tracking. The RF9958 IC's output power (MODOUT) vs. gain control voltage (GC) for Vcc = 3 V at 130 MHz is shown in Figure 2 . Typically, the output of the AGC amplifier is connected to an IF bandpass filter, which, in turn, is connected to an upconverter.

The upconverter on the IC is designed specifically around IS-98 specifications, but its good performance allows it to be used in a variety of applications. The unit has a typical power gain of 0.5 dB (source impedance = 200 W , load impedance = 50 W ), an output third-order intercept point (IP3) of +14 dBm and a noise figure of 15 dB. It also features a double-balanced mixer that provides LO and IF rejection at the RF output. The IF port is differential with a 200 W balanced input impedance. The LO port is symmetrical and can be driven differentially or single ended. Since most 900 MHz VCOs have single-ended outputs, the input impedance of the upconverter LO port is designed to be 40 W single ended into either pin 19 (LO2+) or pin 20 (LO2–).

At the RF output of the upconverter, an impedance transformer is required to present a 50 W output impedance. This transformation can be accomplished with two components: a shunt inductor and series capacitor. The shunt inductor actually serves two purposes: It acts as a choke to provide DC voltage to the upconverter, and it performs an impedance transformation. The series capacitor completes the transformation to 50 W . This L-C combination can be adjusted to conform to the system's particular frequency range.

In some cases, the quadrature modulator and AGC amplifier are needed, but an external upconverter is preferred. To accommodate an external upconverter for special applications, the model RF9958 ASIC features a power-down capability for its internal upconverter. To enable the internal upconverter, the user shorts pin 10 (BG OUT) and pin 15 (PD2). Pin 10 provides a DC reference voltage needed to bias the active circuitry in the upconverter and pin 15 is the upconverter power-down pin. To power down the internal converter, pin 15 (PD2) is grounded, which results in a 20 mA power supply current savings.

The Receive Ics
The model RF9957 receive IC performs all necessary receive IF signal
amplification and demodulates down to I/Q baseband data, as shown in Figure 3 . In a typical cellular telephone receiver, a low noise amplifier/mixer IC amplifies and then downconverts the RF input signal from the antenna to an IF. Then, the IF signal typically passes through a surface acoustic wave (SAW) bandpass filter. At this point, the IC takes the signal and outputs baseband I/Q data into an external mixed-signal IC.

The IF inputs of the RF9957 IC are designed to accommodate either differential or single-ended signals, depending on the SAW filter characteristics. In dual-mode applications where two different IF bandwidths exist, two distinct IF filters are used and, thus, two IF input ports are required on the IC. To date, most system designers have used a differential SAW filter for the CDMA path and a single-ended filter for the FM path. The CDMA input port (pins 4 and 5) and the FM input port (pins 8 and 9) are selectable with pin 14 (IN SEL). A logic high selects the CDMA input while a logic low selects FM.

The CDMA input port has a 2.4 kW differential input impedance. An external resistor can be placed across pins 4 and 5 to lower the port's input impedance. The FM input port has a 1.2 kW single-ended input impedance. The AGC amplifier comprises four variable-gain stages (the last three are common to both the CDMA and FM signal paths). Therefore, when selecting between the two modes, the user is actually choosing which input amplifier stage is being activated.

The AGC amplifier has a typical gain range of 105 dB. The typical cascade power gain of the AGC amplifier and demodulator is from –55 to +50 dB, as shown in Figure 4 for Vcc = 3 V at 85 MHz. Since the power gain specification is referenced to a 500 W source and a 5000 W load, the cascade voltage gain is from –45 to +60 dB. The cascade noise figure and IP3 performance of the RF9957 IC allow for easy integration into an IS-98 CDMA cellular telephone, as shown in Figure 5 . At the output of the amplifier, two pins are provided to allow for bandpass filtering. A parallel tank circuit provides filtering at the receive IF and a necessary DC pull-up to the power supply for the AGC output stage.

The output of the AGC amplifier connects internally to the input of the demodulator. The IF signal enters the demodulator and mixes with an LO signal. The output of the mixer then is I/Q baseband data. The LO input port (pins 12 and 13) can be driven differentially or single ended, but a differential drive produces better phase performance. The input impedance is 400 W single ended and

800 W balanced. The LO circuitry in the model RF9957 IC is similar to the circuitry in the model RF9958 in that it uses a divide-by-two flip-flop architecture to achieve phase quadrature, implying that the LO frequency must be twice the IF to demodulate down to baseband properly. The flip-flop circuitry limits the maximum allowable IF and, in this design, the flip-flop circuitry can handle up to approximately 500 MHz (which translates to an IF of 250 MHz).

The output of the demodulator is separated into two ports: I (pins 21 and 22) and Q (pins 15 and 16). Both outputs are differential and have 2 kW balanced output impedances. The power gain specifications for the IC were written for a 5 kW load on each of the I and Q output ports. If additional voltage gain is desired, the load impedance must be increased. To save power supply current, the RF9957 IC incorporates a power-down feature for the Q mixer. When the IN SEL pin is at logic low (FM mode), the Q mixer shuts down and only the I mixer continues to operate. In CDMA mode, both mixers are in operation. Typically, the I and Q output ports drive a mixed-signal IC, which contains A/D converters and digital lowpass filters.

To allow greater flexibility, the model RF2667 IC is offered as a slight variation of the model RF9957 IC. In the RF2667 IC, the Q mixer power-down feature has been eliminated so that when the FM mode is selected, both mixers continue to operate. This design change was made to accommodate system architectures that require both I and Q channels to decode FM data. A second difference is in the I and Q maximum output voltage swing. The RF2667 IC allows up to a 2.4 V p-p balanced typical output swing before 1 dB compression, whereas the RF9957 IC allows 500 mV p-p. This change allows the model RF2667 IC to mate with A/D converters that require higher input voltage levels. Another performance variation is frequency range; the model RF2667 IC operates up to an IF of at least 300 MHz.

Conclusion
Three highly integrated ICs have been developed to simplify CDMA cellular transceiver design. The model RF9958 IC offers quadrature modulation, transmit AGC amplification and upconversion in a single QSOP-28 plastic package. The models RF9957 and RF2667 ICs feature receive AGC amplification and quadrature demodulation in a single QSOP-24 plastic package. All of these ICs ship in production volumes and are available now.

RF Micro Devices,
Greensboro, NC (910) 664-1233.

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