Wireless transmitter circuit diagram of MICRF102

　　The wireless transmitting circuit is shown in the figure, with MICRF 102 as the core. MICRF102 is a single-chip UHF ASK transmitter launched by Micrel. It is packaged in SOP(M)-8. The chip contains: a reference oscillator, phase detector, frequency divider, band-pass filter, and voltage-controlled oscillator. The circuit composed of synthesizer, transmit bias control, RF power amplifier, antenna tuning control and varactor diode is a true "data input-wireless output" monolithic wireless transmitting device. The UHF synthesizer produces carrier frequency and quadrature signal outputs. The input phase signal (I) is used to drive the RF power amplifier. Antenna tuning quadrature signals (Q) are used to compare antenna signal phases. The antenna tuning control part detects the phase of the transmitted signal in the antenna channel and controls the capacitance of the varactor diode to tune the antenna and realize automatic tuning of the antenna. The power amplifier output is controlled by the transmit bias control unit. ASK/OOK modulation, provides low power consumption mode, data transfer rate is 20 kb/s.

　　Issues that should be noted during use are: (1) REFOSC (pin 4) is the reference oscillator terminal. Connect the crystal oscillator to ground, or use AC coupling to input a clock pulse with a peak-to-peak value of 0.5 V. The transmit frequency is 32 times the reference oscillator frequency: reference oscillation frequency × 32 = transmit frequency. If an external clock signal is used, AC coupling must be used, and the peak-to-peak amplitude of the input signal is 200~500 mV. (2) MICRF102 uses differential output to drive the antenna load. The power amplifier output stage contains a varactor diode that is automatically tuned to the antenna's inductance to ensure resonance at the transmit frequency. The inductance of a typical PCB wire antenna is related to the size of the loop, the width of the antenna wire, the thickness of the PCB copper wire, and the location of the ground plate. When designing, the capacitance value of the varactor diode is generally selected to be 6.5 pF. The antenna inductance L is calculated by the formula L=1/(4π2f2C). (3) The output power of the power amplifier is related to the voltage on the PC terminal (pin 1). During normal operation, the voltage on this pin is set between 0.2~0.4 V. As the voltage on the PC terminal increases, the output power increases; however, if the voltage on the PC terminal exceeds 0.4 V, the power amplifier is current limited and the output power no longer increases. Reducing the voltage on the PC side reduces power consumption and also reduces the RF output power. (4) The STBY terminal (pin 5) is the standby mode control. Connect to VDD for transmitting mode, and connect to VSS for standby mode. (5) The MICRF 102 chip is sensitive to power supply ripple, and correct power supply bypass is necessary. Generally, 3 capacitors of 4.7μF, 0.1μF, and 100 pF are used in parallel between VDD and VSS.