Abstract: nRF902 is a single-chip RF transmitter chip, which contains the frequency synthesizer, power amplifier, crystal oscillator and modulator and other circuits, the ability to send digital signals. nRF902 using FSK modulation, can work in 868MHz of the ISM band. The paper gives nRF902 structure, principles, characteristics and application circuit.
1 Overview
nRF902 is a monolithic transmitter chip operating frequency range of 862 ~ 870MHz ISM band. The transmitter by a fully integrated frequency synthesizer, power amplifier, crystal oscillator and modulator composition. As nRF902 a crystal oscillator and a stable frequency synthesizer, therefore, very low frequency drift completely comparable to SAW resonator-based solutions. nRF902 output power and frequency offset can be programmed via external resistors. Supply voltage range of 2.4 ~ 3.6V, output power is 10dBm, the current consumption of only 9mA. Standby mode supply current is only 10nA. FSK modulation with data rate 50kbits / s. Therefore, the chip is suitable for alarm, automatic meter reading, home automation, remote control, wireless digital communications applications.
2 pin function and structure of the principle
nRF902 using SIOC-8 package, the pin function as listed in Table 1.
Pin Function Table 1 nRF902
Pin Symbol Function end
1XTAL crystal connectors / PWR-UP control
2REXT power conditioning / Clock mode / ASK modulator input word
3XO8 reference clock output (clock frequency of 1 / 8)
4VDD supply voltage (+3 V)
5DIN digital data input
6ANT2 antenna
7ANT1 antenna
8VSS ground (0V)
Figure 1 shows nRF902 internal structure can be seen from the figure: The chip includes frequency synthesizer, power amplifier, crystal oscillator and the modulator and other circuits.
The antenna output by nRF902 balanced RF signal can be output to the antenna, while the pin must be connected to the power DC-channel VDD, VDD power supply RF choke or via the center of the antenna loop access. ANT1/ANT2 output load impedance between 200 ~ 700O. If you need to 10dBm output power, load impedance of 400O should be used.
Modulation of the capacitance of the crystal by pulling to complete. To achieve the required frequency offset, the characteristics of the crystal should be met: the parallel resonant frequency fp should be equal to the emission center frequency divided by 64, the parallel equivalent capacitance Co should be less than 7pF, crystal equivalent series resistance ESR should be less than 60O, all load capacitance, including the PCB capacitance CL should be less than 10pF. Since the frequency modulation of the load by pulling the crystal (internal varactor diode) to complete, while the external resistor R4 will change the voltage of the varactor, therefore, change the value of R4 can change the offset.
The end of bias resistor R2 connected to the power supply from the end of REXT VDD output power can be adjusted for. nRF902 operating mode can be set methods listed in Table 2.
Table 2 nPF902 work mode
Pin
Mode XTALREXTXO8DIN
Low-power mode (sleep mode) GND ---
VDDGNDVDD-clock mode
ASK mode VDDASK data VDD or GNDVDD
FSK mode VDDVDDVDD or GNDFSK data
In FSK mode, the modulated data from the DIN terminal input, which is the standard operating mode nRF902.
ASK modulation can be achieved by controlling the REXT side. When R2 is connected to VDD, the chip will launch carrier. When R2 is connected to ground when the power amplifier chip off. These two states can be used in ASK systems logic "1" and logic "0" to represent. In the ASK mode, DIN terminal must be connected to VDD.
Clock mode can be applied to the case of an external microcontroller, nRF902 can give micro-controller clock. It can be XO8 ended output reference clock, XO8 ended output clock signal frequency is the oscillator frequency of 1 / 8. If the crystal frequency is 13.567MHz, the XO8 output clock signal frequency is 1.695MHz.
In low power mode (sleep mode), the current consumption of the chip is only 10nA. In the absence of data transmission, the chip can work in low-power mode to extend battery life. Circuit from low power mode to transmit mode requires 5ms time from the clock mode to transmit mode requires 50µs time.
3 Application Circuit Design
nRF902 application circuit shown in Figure 2. In order to achieve good RF performance, PCB (PCB) design is very important. Recommended at least two layers of PCB board, including a ground plate. Design of high performance RF capacitors should be used to close near the VDD terminal to complete the DC power supply decoupling. Recommend the use of large-capacity capacitor and a small capacitor in parallel between VDD and ground methods. Supply voltage should also be filtered, respectively, from the power sent to the digital circuit. All devices in, VDD connection, VDD bypass capacitors must be as close as possible nRF902 chip. RF PCB connected with the upper floor, VSS client should be connected directly to the floor. PCB ground plane using low-level, the best way is through a hole connected to VSS. Channel digital signals and control signals and the XTAL oscillator is not close to the end. I designed the PCB board using double-sided 1.6mmFR-4, bottom layer has a continuous ground plane, coupled with the ground surface area of components, thus ensuring a good grounding. A large number of through-hole components can be connected to the surface of the ground plane in the end plate on the ground plane, and there should not be in the ground plane under the antenna.