The single-chip RF device greatly facilitates the application of wireless communication in a certain range, and a complete wireless communication link can be constructed by using a suitable microcontroller and antenna combined with the transceiver. They can be integrated on a small circuit board for wireless digital audio, digital video data transmission systems, wireless remote and telemetry systems, wireless data acquisition systems, wireless networks and wireless security systems.
1 Potential contradiction between digital circuits and analog circuitsIf the analog circuit (RF) and the digital circuit (microcontroller) work separately, they may work well, but once they are placed on the same board and work together with the same power supply, the whole system is likely to be unstable. . This is mainly due to the fact that digital signals oscillate frequently between ground and positive power supplies (3 V in size), and the period is extremely short, often ns. Due to the large amplitude and the small switching time, these digital signals contain a large number of high frequency components independent of the switching frequency. In the analog part, the signal transmitted from the antenna tuning loop to the receiving portion of the wireless device is typically less than 1 μV. Therefore, the difference between the digital signal and the RF signal will reach 10-6 (120 dB). Obviously, if the digital signal and the RF signal are not well separated, the weak RF signal may be destroyed, and the performance of the wireless device may deteriorate or may not work at all.
2 Frequently Asked Questions for RF Circuits and Digital Circuits on the Same Block PCBInsufficient isolation of sensitive lines and noisy signal lines is a common problem. As mentioned above, the digital signal has a high swing and contains a large number of high frequency harmonics. If the digital signal wiring on the PCB is adjacent to a sensitive analog signal, high frequency harmonics may couple in the past. The most sensitive node of an RF device is typically a loop-locked loop of a phase-locked loop (PLL), an external voltage-controlled oscillator (VCO) inductor, a crystal reference signal, and an antenna terminal. These parts of the circuit should be handled with extreme care.
(1) Power supply noise
Since the input/output signals have a swing of a few volts, digital circuits are generally acceptable for power supply noise (less than 50 mV). Analog circuits are quite sensitive to power supply noise, especially for glitch voltages and other high frequency harmonics. Therefore, power line routing on PCB boards containing RF (or other analog) circuits must be more careful than wiring on a common digital board, and automatic routing should be avoided. It should also be noted that the microcontroller (or other digital circuit) will suddenly draw most of the current for a short period of time within each internal clock cycle, since modern microcontrollers are designed in a CMOS process. Therefore, assuming that a microcontroller is operating at an internal clock frequency of 1 MHz, it will draw (pulse) current from the supply at this frequency, which would cause a voltage spike on the power line if proper power supply decoupling is not taken. If these voltage spurs reach the power supply pins of the RF portion of the circuit, serious operation can be caused, so the analog power supply line must be separated from the digital circuit area.
(2) Unreasonable ground wire
The RF board should always be routed with a ground plane connected to the negative side of the power supply, which can cause some strange behavior if not handled properly. This may be difficult for a digital circuit designer to understand because most digital circuit functions perform well even without a ground plane. In the RF band, even a short line will act like an inductor. Roughly calculated, the inductance per mm length is approximately 1 nH, and the inductive reactance of a 10 mm PCB line at 434 MHz is approximately 27 Ω. If the ground plane is not used, most of the ground will be longer and the circuit will not guarantee design features.
(3) Radiation of the antenna to other analog parts
This is often overlooked in circuits that contain RF and other parts. In addition to the RF section, there are usually other analog circuits on the board. For example, many microcontrollers have built-in analog-to-digital converters (ADCs) for measuring analog inputs as well as battery voltage or other parameters. If the antenna of the RF transmitter is located near this PCB (or just on this PCB), the high frequency signal that is sent may reach the analog input of the ADC. Don't forget that any circuit line can emit or receive RF signals like an antenna. If the ADC input is not handled properly, the RF signal may self-excite within the ESD diode of the ADC input, causing a bias in the ADC.
3 RF circuit and digital circuit solution on the same block PCB Some general design and routing strategies in most RF applications are given below. However, it is more important to follow the wiring recommendations for RF devices in real-world applications.
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