RF2903 is a high-performance spread-spectrum receiver chip from RF. It consists of three parts: pre-amplifier and mixing, IF variable gain control and demodulation. For IF signals below 200MHz, 
Its gain can reach 90dB, which can be achieved by an analog voltage. RF2903 includes a received signal strength indicator circuit for energy level detection. Since RF2903 has two symmetrical mixers, it is advantageous for quadrature demodulation. Of course, it can also be used as FM/FSK and AM demodulation, but when used for AM detection, an external detection circuit is required.
The RF2903 spread spectrum receiver chip can be widely used in systems such as spread spectrum communication systems, dual mode analog/digital receivers, POS terminals, handheld communication systems, and 2.4 GHz ISM wideband receivers.
1 The main features and functions of RF2903
The main features of RF2903 are as follows:
â— It can work under the voltage of 3~6V;
â— With FM, PM and quadrature demodulation functions;
â—With linear analog gain control and RSSI intensity indication function;
â— The intermediate frequency range is 10 to 200 MHz, and the radio frequency range is 150 to 1000 MHz.
◠Normal operating temperature range is -40 to +85 °C;
â— Conversion gain can reach more than 94dB;
â— The package form is SSOP-24.
2-pin function and internal circuit principle
The internal schematic diagram and pinout of RF2903 are shown in Figure 1. Table 1 lists the functions of each pin of RF2903. As can be seen from Figure 1, the RF front end of the RF2903 has three RF amplifiers and downconverters. It also includes three unit circuits with linear gain control and Received Signal Strength Indication (RSSI). In addition, the RF2903 has an internal quadrature. Demodulator.
Table 1 Pin Functions of RF2903
| Pin number | Pin definition | Function description |
| 1 | ILO | I channel demodulation mixer LO input |
| 2 | QLO | Q channel demodulation mixer LO input |
| 3 | VCC5 | I, Q two-channel demodulation mixer power supply |
| 4 | VCC4 | IF3 amplifier power supply |
| 5 | IF3 | IF2 output / IF3 input |
| 6,7 | GND | Ground terminal |
| 8 | IF2 | IF1 output / IF2 input |
| 9 | VCC3 | IF2 amplifier power supply |
| 10 | VCC2 | IF1 amplifier power supply |
| 11,12 | IF IN-, IF IN+ | Differential input for IF1 amplifier |
| 13,14 | MIXOUT+, MIXOUT- | RF mixer output |
| 15,16 | LO+, LO- | Differential input for RF mixer |
| 17 | B | RF input pin |
| 18 | E | Emitter of RF Transistor |
| 19 | GND | Ground terminal |
| 20 | BG OUT | Bandgap reference voltage output |
| twenty one | GC | Analog gain control pin |
| twenty two | RSSI | Accept signal strength indication |
| twenty three | Q OUT | Q channel demodulation mixer baseband output |
| twenty four | I OUT | I channel demodulation mixer baseband output |
2.1 RF amplification and down conversion of the RF front end
The RF amplification and downconversion of the RF2903 RF front-end uses differential input, all with low noise amplification. If you are not using a differential input form, you can either drop one of the inputs or connect it to the power supply Vcc. In order to make the system design flexible, the RF front end can be designed differently. To ensure that the RF2903 can reliably receive signals within its receiver sensitivity range, the LNA and mixer should be tuned to their maximum gain and minimum noise figure, which can be achieved with its peripheral transistors plus inductors and capacitors. In order to increase the sensitivity of the system, a low noise amplifier (LNA) can be added, which also increases the noise figure of the entire system. 
2.2 IF amplifier with linear gain control and RSSI
The IF portion of the RF2903 consists of three identical sets of IF amplifiers with linear gain control and RSSI. When RF2903 is applied to a linear system (such as a direct sequence spread spectrum receiver), gain control will keep the signal level at a certain level. When applied to a nonlinear system (such as an FM or frequency hopping receiver), its gain can be kept at its maximum.
2.3 gain control
Control of the gain of the RF2903 can be achieved by adjusting a pair of differential amplifiers while leaving the other pair of amplifiers inoperative. Because the two pairs of differential input pairs of the IF amplifier are mutually conductive, that is, when the current flowing to a pair is large, the current flowing to the other pair is smaller.
2.4 Received signal strength indication
The intensity of the received signal can be known by the RSSI, which varies with the control gain. In this way, RSSI can be used to design a feedback loop when designing the system to keep the output signal stable.
2.5 Quadrature Demodulator
The RF2903 quadrature demodulator consists of two mixers and can be driven differentially by the output of the IF amplifier. It must be noted that its input signal must have a phase difference of 90°, which can be achieved by the simple phase shift circuit shown in Figure 2. Since the LO input is directly connected to the base of the input transistor, an external pull-up resistor is required. The mixer output can be buffered by the emitter of the next stage so that it can directly drive the low-impedance filter. The chip's own bandgap reference voltage output can be used to calibrate the circuit. In this process, the chip is insensitive to temperature and power supply variations, so it can operate normally from 2.7V to 5.5V.
image 3
3 RF2903 application in spread spectrum communication
At present, the modulation and demodulation methods used in spread spectrum communication are mainly quadrature modulation and demodulation. Figure 3 shows the orthogonal demodulation application circuit of RF2903 in the spread spectrum system receiver. It is worth noting that the mixer output and the IF input pin are used because of the single-ended filter, so it is not necessary to use the output of the balanced filter and the input of the balanced IF section, but only one input and one output. In the figure, 14 feet are used as the output of the mixer, and 11 is used as the intermediate frequency input. Of course, you can also use 13 feet and 12 feet. Since there is a filter at the output of the mixer and at the IF input, when high isolation is required, it is recommended to use 11 and 14 feet instead of 12 and 13 feet. After testing, the receiving sensitivity of RF2903 can reach -100dBm at 900MHz.
It is well known that FSK modulation and demodulation methods are mainly used in frequency hopping systems. Figure 4 shows the FSK demodulation application circuit of RF2903 in a frequency hopping communication system receiver. As can be seen, its peripheral circuitry is similar to its peripheral circuitry in quadrature demodulation applications, with some changes at the signal input. When RF2903 is used for FSK demodulation, its gain control can be adjusted to the maximum, because in this case, IF amplification does not require it to satisfy linear conditions. Finally, it should be noted that if the RF2903's demodulation function is not used, but a DC voltage is applied to the LO input, the RF2903 can also be used as a normal amplifier.
Figure 4
4 Conclusion
RF2903 is an ultra-low-cost dedicated receiving chip mainly used for DS-SS and FH-SS. It has the characteristics of wide working frequency band, low working voltage, flexible configuration and wide application range. Therefore, the RF2903 is very suitable for applications in the field of wireless communications.
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