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Radio frequency (RF) PCB design , published in the current theoretically have a lot of uncertainty , often described as a " black art ." Typically, the following circuit for the microwave band ( including low frequency and low-frequency digital circuits ) , in complete control of all types of design principles is the premise of careful planning to ensure the success of one-off designs. For microwave and high frequency bands above the PC class digital circuits , you will need two to three versions of the PCB circuit can ensure quality . For more than a microwave RF circuits band , are often more versions of PCB design and continuous improvement , and is subject to the availability considerable experience . It can be seen electric RF design challenges .
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Interference between digital and analog circuit modules circuit module
If the analog circuits ( RF ) and digital circuits work alone , may their work well. However, once the two on the same board , using the same power to work together, the whole system is likely to be unstable. This is mainly because the digital signal frequently between ground and the positive power supply (> 3 V) to swing , and particularly short period , often nanoseconds . Due to the large amplitude and a short switching time , so that the digital signal contains a large and independent of the switching frequency of the high frequency components . The analog section , transmitted from the wireless device radio tuning circuit of the signal receiving portion is generally less than 1μV. Therefore, the difference between the digital signal and the RF signal will reach 120dB. Obviously, if the digital signal is not so well separated RF signal , the weak RF signals may be damaged , so that the wireless device performance will deteriorate , or even does not work .
Power supply noise
RF circuits for power supply noise is quite sensitive , especially for glitches and other high-frequency voltage harmonics . The microcontroller will suddenly sucked each internal clock cycle, most of the current in a short time , due to the micro- controller using modern CMOS process. Therefore, assuming a microcontroller running at 1MHz internal clock frequency , it will extract this frequency current from the power supply. If you do not take proper supply decoupling , will cause the power line voltage glitches. If these voltage spikes reach the RF portion of the circuit power supply pin , the work could lead to serious failure .
Unreasonable ground
If the ground is mishandled RF circuit may produce some strange phenomena . For the design of digital circuits , even if there is no ground plane , most digital circuit function also performed well . In the RF band , even a very short ground effect will be the same as the inductor . Roughly calculated inductance per millimeter length is about 1 nH, 433 MHz when 10 mm PCB circuit inductance of about 27Ω. If the ground layer is not used , the longer will be the majority of the ground , the circuit will not have a characteristic design .
Antenna radiation interference to other parts of the analog circuit
In the design of the PCB , there are usually other analog circuitry board . For example, many circuit has an analog / digital converter (ADC) or a digital / analog converter (DAC). Radio frequency signal emitted by the transmitter antenna may arrive analog inputs of the ADC . Because any circuit lines are likely to make or receive RF signals, such as an antenna . If the ADC input processing unreasonable , RF signals may self-excitation in the ADC input ESD diodes , causing ADC bias.