Abstract: RF2903 is the high performance which, the high integration rate ultra low price special-purpose wide frequency receive chip RF Corporation produces, it has the work frequency band width, the working voltage to be low, disposition nimble, applicable scope broad and so on characteristics, very suitable to apply in DS-SS and the FH-SS wireless communication product design domain. In the article introduced the RF2903 main feature and the pin function, and have carried on the analysis to it in direct sequence spread spectrum’s orthogonal demodulation as well as in the frequency-hopping communications system’s FSK demodulation application, simultaneously has given the corresponding application electric circuit.
Key word: Receiver; Wide frequency correspondence; Frequency-hopping; QPSK/FSK; RF2903
RF2903 is a RF Corporation’s section of high performance wide frequency receive chip, it puts in advance by the radio frequency front end with the mixing, the intermediate frequency variable gain controls and demodulates the function three parts to be composed. Regarding the 200MHz following intermediate frequency signal, 
its gain may achieve 90dB, this gain may realize by an analogue voltage. RF2903 contains a received signal intensity indicating circuit, may carry on the energy level check, because in RF2903 has two symmetrical mixers, thus is very advantageous to the orthogonal demodulation. Certainly, it may also serve as FM/FSK and the AM demodulation, but when serves as the AM examination, needs an exterior examination electric circuit.
The RF2903 wide frequency receive chip may widely use in the wide frequency communications system, the double model-like mold/number receiver, the POS terminal, handhold in communications system as well as 2.4GHz systems and so on ISM wide-band receiver.
1 RF2903 main feature and function
The RF2903 main feature is as follows:
* may work under the 3~6V voltage;
* has FM, PM and the orthogonal demodulation function;
* has the linear simulation gain control as well as the RSSI intensity directive function;
* the intermediate frequency scope is 10~200MHz, the radio frequency scope is 150~1000MHz;
* the normal work temperature range is: - 40~ 85℃;
* the translation gain may reach above 94dB;
* the seal form is SSOP-24.
2 pin functions and internal circuit principle
The RF2903 internal schematic diagram and pin arrangement as shown in Figure 1, Table 1 arranges in order is the RF2903 various pins function. May see by Figure 1, front end the RF2903 radio frequency has three radio frequency amplifiers and under the frequency changer, simultaneously contains has three to have the proportional gain control and the received signal intensity instruction (RSSI) the function unit electric circuit, in addition, the RF2903 interior also has an orthogonal demodulator.
Table 1 RF2903 pin function
| Base pin serial number | Base pin definition | Functional description |
| 1 | ILO | I channel demodulation mixer LO input |
| 2 | QLO | Q channel demodulation mixer LO input |
| 3 | VCC5 | Ith, Q two channel demodulation mixer power source |
| 4 | VCC4 | IF3 amplifier’s power source |
| 5 | IF3 | IF2 outputs the /IF3 input |
| 6th, 7 | GND | Ground terminal |
| 8 | IF2 | IF1 outputs the /IF2 input |
| 9 | VCC3 | IF2 amplifier’s power source |
| 10 | VCC2 | IF1 amplifier’s power source |
| 11th, 12 | IF IN-, IF IN | IF1 amplifier’s differential input |
| 13th, 14 | MIXOUT , MIXOUT- | RF mixer output |
| 15th, 16 | LO , LO- | RF mixer differential input |
| 17 | B | RF input foot |
| 18 | E | RF triode’s emitter electrode |
| 19 | GND | Ground terminal |
| 20 | BG OUT | Band gap reference voltage output |
| 21 | GC | Simulation gain control foot |
| 22 | RSSI | Accepts the signal strength instruction |
| 23 | Q OUT | Q channel demodulation mixer baseband output |
| 24 | I OUT | I channel demodulation mixer baseband output |
front end 2.1 radio frequency radio frequency amplifications and under frequency conversion
Front end the RF2903 radio frequency’s radio frequency amplification and under the frequency conversion uses the differential input form, they have the low chirp enlargement function. If does not use the differential input form, may an input end hanging or arrives at its company power source Vcc. In order to enable the system design to have the flexibility, front end the radio frequency may select the different design method. In order to guarantee that RF2903 can receive the signal reliably in its receiving sensitivity scope, LNA and the mixer should transfer to their biggest gain and the smallest noise factor, this may use its periphery triode to add on the inductance and the electric capacity realizes. In order to enhance system’s sensitivity, may low noise amplifier (LNA), like this, simultaneously also be possible to enhance overall system’s noise factor in addition.
2.2 zone lines gain control and RSSI intermediate frequency amplifier
The RF2903 intermediate frequency strip is composed of three group of same zone lines gain control and the RSSI intermediate frequency amplifier. When RF2903 applies in the linear system (for instance in direct sequence spread spectrum receiver), the gain control will cause the signal level will maintain at certain level. But when it applies when the nonlinear system (for instance FM or in the frequency-hopping receiver), its gain may maintain the biggest condition.
2.3 gain controls
The control which increases to RF2903 may through adjust a pair of differential amplifier, but lets another pair of amplifier work not realize. Because intermediate frequency amplifier’s two pair of differential input to is the mutual breakover, i.e., when flows pair of electric current is big, flows to another pair of electric current to be small.
2.4 received signal intensity instruction
Through RSSI may know that the received signal the intensity, it the change which increases along with the control changes. Thus, when makes the system design may using RSSI design a feedback loop, with the aim of maintaining the output signal the stability.
2.5 orthogonal demodulators
The RF2903 orthogonal demodulator by two mixer constitutions, and may divide the actuation by intermediate frequency amplifier’s output incoming messenger. What must pay attention, its input signal must have 90° the difference, this may the simple shift circuit which through shown in Figure 2 realize. Because the LO input is with inputs triode’s base electrode to be connected directly, therefore needs outside one on to pull the resistance. The mixer output may by under first-level emitter electrode cushion, like this, it be possible the direct drive low-cut filter. The chip bringing band gap reference voltage output may use for the compensating circuit, in this process, the flat and thin piece is insensitive to the temperature and power source’s change, therefore it in 2.7V to under the 5.5V electric power supply condition may the normal work.
Figure 3
3 RF2903 in wide frequency correspondence application
At present the modulation demodulation way which uses in the wide frequency correspondence is mainly the quadrature modulation demodulation, shown in Figure 3 is RF2903 in the wide frequency system receiver’s orthogonal demodulation application electric circuit. It is noteworthy that the mixer output and the intermediate frequency input pin, what because uses is the single end filter, therefore is not the unnecessary use balanced filter’s output and the balance intermediate frequency strip input, but only uses an input and an output then. In the chart takes 14 feet the mixer the output, 11 feet take the intermediate frequency input. Certainly may also use 13 feet and 12 feet. Because has filter in the mixer out-port and the intermediate frequency input end, when therefore requests isolates highly, recommends with 11 feet and 14 feet, but does not use 12 feet and 13 feet. After test, when 900MHz, the RF2903 receiving sensitivity may reach - 100dBm.
It is well known, in the frequency-hopping system mainly uses the FSK modulation and the demodulation way, shown in Figure 4 is RF2903 in the frequency-hopping communications system receiver’s FSK demodulation application electric circuit. The figure shows, its peripheral circuit is similar with it in the orthogonal demodulation application’s peripheral circuit, is only has some changes in the signal input end. When RF2903 uses in the FSK demodulation, its gain control may adjust in a big way, because in this case, the supersonic amplification does not need it to satisfy the linear condition. What final demand explanation is, if does not use RF2903 the demodulation function, but adds a DC voltage in the LO input end, that RF2903 may also take an ordinary amplifier to use.
Figure 4
4 concluding remark
RF2903 is mainly uses in DS-SS and the FH-SS ultra low price special-purpose receive chip, it has the work frequency band width, the working voltage to be low, disposition nimble, applicable scope broad and so on characteristics. Therefore RF2903 is suitable to apply in the wireless communication domain.
