Demodulator chip RF2667 with receiving AGC and its application

Abstract: RF2667 is a CDMA / FM demodulator chip with receiving AGC produced by RF Micro Device. The chip integrates a complete intermediate frequency automatic gain control (AGC) amplifier and quadrature demodulator, which can be used in dual-mode CDMA / FM cellular mobile communication systems and PCS systems. The article introduces the principle, characteristics and typical application circuit of RF2667.

1 Overview

RF Micro Device has integrated a complete intermediate frequency automatic gain control (AGC) amplifier and quadrature demodulator in its chip RF2667. This makes RF2667 applicable to dual-mode CDMA / FM cellular mobile communication systems and PCS systems. This chip can provide a gain control range of 100dB when amplifying the received IF signal, and can demodulate the intermediate frequency signal into I and Q channel signals. The key specifications of the chip (such as noise figure and IP3, etc.) are compatible with CDMA cellular mobile communication system standards IS-98 and J-STD-018. It is manufactured using an advanced silicon bipolar process (FT = 15GHz). At the same time, the standard small 24-pin SSOP package is used. The main features of RF2667 are as follows:

* Similar to RF9957, but it has a higher I / O output voltage than RF9957;

* Support dual mode operation (CDMA and FM);

* With digital control low power consumption mode;

* Power supply is 2.7 ～ 3.3V;

* Power supply is 2.7 ～ 3.3V;

* With orthogonal local oscillator frequency divider;

* The internal IF AGC amplifier has a gain control range of 100dB.

RF2667 demodulator chip can be widely used in CDMA / FM cellular communication system, CDMA PCS system, GSM / DCS system, TDMA system, spread spectrum cordless telephone, wireless local loop system, high-speed data modem and general digital with its excellent characteristics Receivers and other systems.

2 Internal structure

RF2667 contains input selection circuit, gain control circuit, signal demodulation circuit and time slot reference. The input selection circuit is used to select CDMA input signal or FM input signal. The function of the automatic gain control circuit is to perform AGC amplification on the CDMA or FM signal received by the device so that the demodulator can demodulate it better. AGC amplification of the received signal is a major feature of the RF2667 chip. Figure 1 shows the internal block diagram of RF2667.

3 pin function and application circuit

Figure 2 shows the typical application circuit of RF2667. The pin arrangement of RF2667 is also shown in the figure, and the power of each pin is now described as follows:

1 foot (VCC1): Power supply terminal of local oscillator trigger, frequency divider and limiting amplifier. This terminal is used in parallel with pins 2 and 3. And should connect the bypass circuit of 0.01μF. On the PCB, the connection between this pin and the bypass capacitor should be as short as possible, and the ground terminal of the bypass capacitor should be directly connected to the ground plane.

2 feet (VCC2): Band gap voltage source, gain control bias circuit and AGC 2, 3, 4 power supply terminal.

3 feet (VCC3): Power end of AGC input stage of FM and CDMA.

4, 5 feet (CDMA IN +, CDMA IN-): CDMA balanced input. A DC bias is added inside this terminal. In single-ended input applications, one of the pins is used as an input and the other end is AC coupled to ground. The input impedance of the balanced input mode is 2.4kΩ, and the input impedance of the single-ended input mode is 1.2kΩ.

6, 7, 17, 20 feet (GND): ground terminal. For best performance, the connection between these endpoints and ground should be as short as possible.

8, 9 feet (FM IN +, FM IN-): FM balanced input terminal. DC bias is also added inside this terminal. In single-ended input applications, one of the pins is used as an input and the other end is AC coupled to ground. The input impedance of this pin in balanced input mode is 2.4kΩ, and the input impedance in single-ended input mode is 1.2kΩ.

10 feet (BGOUT): Band gap voltage reference. This voltage is maintained over the entire temperature range and power supply range, and is used to provide bias for internal circuits. A bypass capacitor of 0.01μF should be connected externally. The connection between this pin and the bypass capacitor should be as short as possible, and the ground terminal of the bypass capacitor should be directly connected to the ground plane.

11 feet (DEC): AGC decoupling end. This pin also needs to connect a 0.01μF bypass capacitor externally. And the connection between this pin and the bypass capacitor should be as short as possible, and the ground terminal of the bypass capacitor should also be directly connected to the ground plane.

Figure 2 Typical application circuit of RF2667 in CDMA system

12, 13 feet (LO-, LO +): LO balanced input. The content of this end is added with DC bias. In single-ended input applications, one of the pins is used as an input and the other end is AC coupled to ground. Since the signal frequency of pins 12 and 13 will be internally divided by 2, the carrier frequency of the modulator is half of the applied frequency. The input impedance of these two ends in the balanced input mode is 800Ω, and the input impedance of the single-ended input mode is 400Ω. Although the LO terminal can be operated with a single-ended input mode or a balanced input mode, the balanced input mode can obtain the best gain and corresponding balance.

14 feet (INSEL): CDMA and FM mode selection input, for data input. When the end is logic high, the CDMA mode is selected, and when the end logic is low, the FM mode is selected. The impedance of this pin is 30kΩ.

15, 16 feet (QOUT-, QOUT +): Q mixer balanced baseband output. Because DC bias is added inside the terminal, DC blocking capacitors need to be added externally. This output is active in both CDMA mode and FM mode. However, in a single-ended configuration, since one of the two outputs is unused, the output voltage will be reduced by half. The impedance of this port at the single-ended output is 1.2kΩ, and the impedance at the balanced output is 2.4kΩ.

18, 19 feet (FL-, FL +): balanced AGC output / demodulator input.

21, 22 feet (IOUT +, IOUT-): I mixer balanced baseband output. A DC bias is added inside this terminal, and a DC blocking capacitor needs to be added externally during use. This output is active in both CDMA mode and FM mode. In a single-ended configuration, one of the two outputs is unused and the output voltage will be reduced by half. The single-ended output impedance is 1.2kΩ and the balanced output impedance is 2.4kΩ.

Pin 23 (GC): Analog gain control of the AGC amplifier. The effective control voltage range is 0.5 ～ 2.5VDC. The gain range of AGC is 95dB.

24 feet (PD): Low power consumption control. When this terminal is logic 1, all circuits work normally; when this terminal is logic 0, all circuits are shut down.

4 The main performance parameters of RF2667

The main electrical performance parameters of the RF2667 chip are shown in Table 1.

Table 1 The main performance parameters of the chip

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