A Single-chip RF-IF Transceiver

A Single-chip RF-IF Transceiver

BethelTronix Inc. (BTI)
Cerritos, CA

Digital cordless telephone design is now considerably easier due to the introduction of the model BT1072B single-chip RF-IF transmitter-receiver (transceiver). This device is the state of the art in highly integrated wireless communications ICs.

A typical RF receiver (RX) is built from several standard functional blocks regardless of whether the technology used is discrete, small-scale integration (SSI) or medium-scale integration (MSI). An RF RX subsystem includes an antenna, a low noise amplifier (LNA), a downconversion mixer, one or more bandpass filters (BPF), an LO and/or a VCO, and an IF buffer amplifier. An RF transmitter (TX) subsystem includes an antenna, an RF power amplifier (PA), an upconversion mixer, an LO and/or a VCO, and one or more BPFs.

Since an RF transceiver usually needs to be able to share one antenna between the RX and TX sections, an RF duplexer or RF antenna RX/TX switch is often used. This configuration improves the efficiency of input signal coupling to the receiver section and reduces the power loss of the RF output path, increasing the total amount of power that gets coupled from the PA to the antenna.

In the past, each of these functional blocks was built using discrete transistors and a large collection of passive components. More recently, standard off-the-shelf SSI chips such as LNAs, mixers, VCOs and PAs are available. These SSI chips allow an increase in the level of integration, which reduces the number of passive components and total component count in RF transceivers. The next-generation systems demand even more improvement. The best approach to satisfy this latest demand is to increase the level of integration to MSI and incorporate all the necessary stages for a complete RF transceiver onto one IC. The result would be a reduction in the total component count, assembly cost, production line testing and production line adjustment. Furthermore, since reliability is related directly to the total number of system components and interconnections, reliability is increased significantly.

The BT1072B IC, a member of the TRFIC^TM family, is a complete RF-IF transceiver. All of the functions necessary to build the complete RF front end, IF amplifier, I/Q modulator and filter are contained on one chip.

Applications

A potential application for the BT1072B is in 900 MHz digital cordless telephone designs, including the European CT-2+ cordless telephone standard. An example of such a design is shown in Figure 1 . The RF and IF frequencies shown in the block diagram and referenced in this article are included only as an example of one possible typical application. The RF and IF frequencies needed for the specific product design are selected by the system design engineer.

Fig. 1: A typical CT-2 cordless telephone application.

Five ICs are required for this complete design, including the serial electronically erasable programmable read-only memory (EEPROM). This number represents a chip count reduction of more than three times over previous-generation designs. The two main chips are the BT1072B RF-IF transceiver and the AM79C411/412 PHOX^TM CT-2 telephone controller baseband chip. The additional support chips required consist of a dual phase-locked loop IC and a data framing chip. An RF duplexer or RX/TX switch is needed between the BT1072B and the antenna and TX BPF. The interface between the telephone controller and the BT1072B requires no additional support circuitry such as logic, level translators or buffers. The complete design is virtually identical for the remote handset (shown) and a base station unit although it would be up to the original equipment manufacturer to determine if a keypad and/or handset would be included on the base station.

The Receive Section

The receive section includes an LNA, which has an input impedance of 50 W, a noise figure (NF) of 2.8 dB, a gain of 17 dB, an input third-order intercept point (IP3) of -10 dBm, a 1 dB compression point of -20 dBm and an input SWR of 1.3. The output of the LNA is fed to an on-chip filter, which reduces the amplitude of any frequencies that are not within the desired receiver channel. The filter's output is then fed into the downconversion mixer, to an IF BPF and to the IF buffer amplifier. The combination of the mixer-buffer has a gain of 10 dB, IP3 of -5 dBm, input 1 dB compression point of -15 dBm and NF of 10 dB. This mixer is also fed by the on-chip RF VCO. The mixer's output is the sum and the difference of the LO's frequency and whatever signal is passed through from the on-chip filter. This mixer output is then limited by an IF BPF whose center frequency is selected for the desired IF range. The output of the IF BPF is then sent to a final IF buffer amplifier whose output impedance is 330 ohms.

The Transmitter Section

An I/Q modulator, filter, upconversion mixer and IF VCO are included on chip. The I/Q input frequency is 72 kHz, with a 1 V peak to peak (p-p) input swing, a DC level of one-half VDD (VDD = 2.7 to 4.8 V) and a filtered output differential signal amplitude of 100 mV p-p. The image and clock output harmonic spurious tones are -45 dBc, with the image spurious tone level at -85 dBc and a noise level of -145 dBm/Hz.

The output from the I/Q modulator/mixer is fed through the on-chip filter to the TX upconversion mixer. The TX upconversion mixer has a conversion gain of 6 dB, IP3 of 10 dBm, 1 dB compression point of 0 dBm and NF of 10 dB. The output of this mixer travels to an on-chip TX BPF and then to the on-chip TX RF PA. The PA is capable of driving at two power levels. At the low and high power settings, the output power is -10 dBm and +13 dBm, respectively. The RF PA stage has a gain of 18 dB, output impedance of 50 W, output 1 dB compression point of +10 dBm and output IP3 of +20 dBm.

The On-chip VCOs

The transceiver has two on-chip VCOs. The IF VCO runs at 80 MHz and has a closed-loop phase noise at 100 kHz offset of -90 dBc/Hz. The RF VCO runs from 810 to 1010 MHz and has a closed-loop phase noise of -100 and Ð120 dBc/Hz at 100 kHz and 1 MHz offset, respectively. The LO-to-TX RF leakage is less than -35 dBm, and the LO-to-RX RF leakage (before any filter) is less than -50 dBm.

Conclusion

The BT1072B RF-IF transceiver represents the most cost-effective RF-IF front-end solution for 900 MHz digital cordless telephone designs and for similar applications such as the European CT-2 cordless telephone. No other design approach offers fewer total components, lower assembly and test costs, or higher reliability.

Acknowledgment

The AM79C412 PHOX chip, a versatile microcontroller that has been optimized for use as a baseband communications controller, is a product of Advanced Micro Devices, Sunnyvale, CA. The device has many uses, including for applications such as digital cordless telephones and CT-2 European digital cordless phones.

BethelTronix Inc. (BTI),
Cerritos, CA (310) 407-0500.