The single-chip radio frequency device greatly facilitates the application of wireless communications fields, using suitable microcontrollers and antennas and integrally constituting a complete wireless communication link in combination with this transceiver. They can be integrated on a small circuit board, applied to wireless digital audio, digital video data transmission systems, wireless remote control, and telemetry systems, wireless data acquisition systems, wireless networks, and wireless security systems.
1, potential contradiction between digital circuits and simulation circuits
If analog circuit (radio frequency) and digital circuit (microcontroller) can work well, but once the two are placed on the same piece of board, use the same power supply to work together, the entire system is likely to be unstable . This is mainly because the digital signal is flushed between the ground and the positive power supply (size 3 V), and the cycle is particularly short, often NS level. Due to a large amplitude and smaller switching time, these digital signals contain a large number of high frequency components that are independent of the switching frequency. In the analog portion, the signal transmitted from the antenna tuning loop to the wireless device receiving portion is generally less than 1 μV. Therefore, the difference between the digital signal and the radio frequency signal will reach 10-6 (120 dB). Obviously, if the digital signal is not well separated, the weak radio frequency signal may be destroyed, so that the operating performance of wireless devices will deteriorate, and even completely.
2, FAQ for RF circuits and digital circuits on the same PCB
It is often a problem that the sensitive line and noise signal line cannot be sufficiently isolated. As described 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 board is adjacent to sensitive analog signals, high-frequency harmonics may be coupled. The most sensitive node of the RF device is typically a loop filter circuit of a phase locked loop (PLL), an external voltage controlled oscillator (VCO) inductor, a crystal base signal, and an antenna terminal, and the circuits of the circuit should be particularly careful.
(1) Power supply power noise
Since the input / output signal has a few V swing, the digital circuit is generally accepted for power supply noise (less than 50 mV). The analog circuit is quite sensitive to power supply noise, especially for glitch voltages and other high-frequency harmonics. Therefore, the power cord wiring on the PCB board containing the RF (or other simulation) circuit must be more careful than the wiring on the normal digital circuit board, should be avoided. At the same time, it should also be noted that the microcontroller (or other digital circuit) will suddenly suck most of the current in a short period of time in each internal clock cycle, which is because the modern microcontroller is designed with CMOS process. Therefore, it is assumed that a microcontroller operates in an internal clock frequency of 1 MHz, which will extract (pulse) current from the power source in this frequency, and there will be a voltage burr on the power line without taking a suitable power supply decoupling. If these voltage burrs arrive at the power pins of the circuit RF part, severely cause the work failure, so it is necessary to ensure that the analog power cord is spaced from the digital circuit area.
(2) Unreasonable ground
The RF circuit board should always be equipped with a ground layer connected to the power supply negative, and if it is improperly handled, some strange phenomena may occur. This may be difficult to understand for a digital circuit designer, because even if there is no ground layer, most digital circuit functions are good. In the RF band, even a short line will work as inductance. Rough calculation, the inductance of 10 MMPCB lines is approximately 27 Ω per mm length of about 1 NH and 434 MHz. If the ground line is not used, most of the grounds will be longer, and the circuit will not guarantee design characteristics.
(3) The radiation of the antenna to other analog parts
This is often ignored in a circuit containing radio and other portions. In addition to the RF part, there are usually other analog circuits on the board. For example, many microcontroller built-in modulus converters (ADCs) are used to measure analog input and battery voltage or other parameters. If the antenna of the RF transmitter is located near this PCB (or on this PCB), the high frequency signal may arrive at the analog input of the ADC. Don't forget that any circuit lines may issue or receive RF signals as the antenna. If the ADC input process is unreasonable, the RF signal may be self-excited in the ESD diode input to the ADC, thereby causing the deviation of the ADC.
3, RF circuits and digital circuits doing solutions on the same block PCB
The following general design and wiring strategies in most RF applications are given below. However, the wiring recommendations of RF devices in practical applications are more important.
(1) A reliable ground line
When the PCB of the RF element is designed, a reliable ground layer should always be used. Its purpose is to establish an effective 0 V potential point in the circuit, so that all devices are easy to decouple. The 0 V terminal of the power supply should be connected directly to this ground layer. Due to the low impedance of the ground layer, there will be no signal coupling between the two nodes that have been decoupled. This is very important for multiple signal amplitudes on the board. On the surface-mounted PCB, all signal wirings are in the same surface of the component mounting surface, and the ground line is in the opposite side. The ideal ground layer should cover the entire PCB (except for the antenna PCB). If two floors of PCB are used, the ground line should be placed on the layer adjacent to the signal layer (such as the next layer of the element surface). Another good way is to fill the spaces of the signal wiring layer by the ground plane, which must be connected to the main line level by multiple via holes. It should be noted that since the presence of the ground point can cause the side of the inductance characteristics to change, selecting the inductive value and the inductor must be carefully considered.
(2) Shorten the connection distance from the ground layer
All connection to the ground layer must be as short as possible, and the grounding via should be placed at the pads of the (or very close) element. Never let the two signals share a grounded via, which can cause crosstalk between the two pads due to the via connection impedance.
(3) RF decoupling
Decoupling capacitors should be placed as close as possible to the pins, and each of the pins that need to be decoupled should be decoupled. With high quality ceramic capacitors, the dielectric type is preferably "NPO", "X7R" can work better in most applications. The ideal selection capacitance value should make it tandem resonance equal to the signal frequency. For example, when 434 MHz, the 100 P F capacitance of the SMD mount will work well. When this frequency, the capacitance is about 4 Ω, and the sensing of the via is also in the same range. The series capacitance and via are formed to form a notch filter for the signal frequency, which enables effective decoupling. At 868 MHz, the 33 P F capacitor is an ideal choice. In addition to the RF decoupling small value, a large capacitor should also be placed on the power line to decouple the low frequency, and a 2. 2 μF ceramic or 10 μF tantalum capacitance can be selected.
(4) Star wiring of the power supply
Star wiring is a well-known technique in analog circuit design (as shown in Figure 1). The star wiring - the module on the circuit board has a power line from a public power supply point. In this case, the star wiring means that the digital portion and the RF portion of the circuit should have its own power line, which should be decoupled at close to the IC. This is an effective way to separate from the digital portion and the power supply noise from the RF. If a module with severe noise is placed on the same circuit board, the inductor (magnetic bead) or the small resistance resistor (10 Ω) can be connected in series between the power supply line and the module, and must be used for tantalum capacitor of at least 10 μF. The power supply of the module is decoupled. Such modules such as RS 232 drivers or switching power supply regulators.
(5) Rationalize PCB layout reasonably
To reduce interference from the noise module and the peripheral simulation, the layout of each circuit module is important on the board. A sensitive module (RF portion and antenna) should always be away from the noise module (microcontroller and RS 232 driver) to avoid interference.
(6) Effect of shielding RF signals on other simulation portions
As described above, the RF signal causes interference to other sensitive analog circuit modules such as ADCs when transmitting. Most problems occur at lower operating bands (such as 27 MHz) and high power output levels. The use of RF decoupling capacitance (100P f) is connected to the ground decoupling sensitive point is a good design habit.
(7) Special considerations in the board ring antenna
The antenna can be made as a whole on the PCB. Contrast the traditional whip antenna, not only space and production costs, but also more securely reliable. In the convention, the ring antenna (LOOP Antenna) is designed to relatively narrow bandwidth, which helps to suppress unwanted strong signals to avoid interfering with the receiver. It should be noted that the annular antenna (all other antennas) may receive noise coupled by the nearby noise signal line. It interferes with the receiver or may affect the modulation of the transmitter. Therefore, there must be a digital signal line near the antenna, and it is recommended to keep free space around the antenna. Any object close to the antenna will constitute a part of the tuning network, which leads to the tuning of the antenna to deviate from the expected frequency point, so that the transmission and receiving radiation range (distance) is reduced. For all all kinds of antennas, you must pay attention to this fact that the outer casing (peripheral packaging) of the board may affect the antenna tuning. At the same time, attention should be taken to remove the ground level of the antenna area, otherwise the antenna cannot work effectively.
(8) connection of board
If the RF board is connected to an external digital circuit with a cable, twisted wires should be used. Each signal line must be twisted with the GND line (DIN / GND, DOUT / GND, CS / GND, PWR _UP / GND). Remember to connect the RF board and the digital application board for the GND line of twisted wire cable, the cable length should be as short as possible. The line powered by the RF board must also be twisted with GND twisted (VDD / GND).
in conclusion
Rapidly developed radio frequency integrated circuits to engage in wireless digital audio, video data transmission systems, wireless remote control, telemetry systems, wireless data acquisition systems, wireless networks, and wireless security systems, etc., provide the largest possible. At the same time, the design of the RF circuit requires designers to have certain practical experience and engineering design capabilities. This article is the experience of the author in actual development, hoping to help many RF integrated circuit developers shorten the development cycle, avoiding unnecessary detours, saving manpower and financial resources.
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