4 basic features of PCB RF circuit

　　Below the four basic characteristics of RF circuits are explained from four aspects: RF interface, small expected signal, large interference signal and interference of adjacent channels. The important factors that need special attention in PCB design process are given.

　　 RF interface

　　In concept, wireless transmitter and receiver can be divided into two parts: fundamental frequency and RF. The fundamental frequency includes the frequency range of the input signal of the transmitter and the frequency range of the output signal of the receiver. The bandwidth of the fundamental frequency determines the basic rate at which data can flow through the system. Baseband is used to improve the reliability of the data stream and reduce the load on the transmission medium imposed by the transmitter at a specific data transfer rate. Therefore, a lot of signal processing engineering knowledge is needed when PCB design of baseband circuit. The RF circuit of the transmitter can convert the processed baseband signal, raise the frequency to the specified channel, and inject the signal into the transmission medium. In contrast, the receiver's RF circuit can take the signal from the transmitted media and convert and reduce the frequency to the fundamental frequency

　　Transmitters have two main PCB design objectives: the first is that they must emit specific power with minimal power consumption. Second, they cannot interfere with the normal operation of transceivers in adjacent channels. In terms of receivers, there are three main PCB design objectives: first, they must accurately restore small signals; Second, they must be able to remove interference outside the desired channel; Finally, like transmitters, they must consume very little power.

　　Interference signal

　　The receiver must be sensitive to small signals, even in the presence of large interference signals (barriers). This occurs when an attempt is made to receive a weak or distant transmission that is broadcast on an adjacent channel by a powerful transmitter nearby. The interference signal may be 60~70 dB larger than the expected signal, and it can block the reception of the normal signal by means of a large amount of coverage at the input stage of the receiver, or by making the receiver produce too much noise at the input stage. Both of these problems occur if the receiver is driven into a nonlinear region by an interference source during the input phase. To avoid these problems, the receiver's front end must be very linear

　　Therefore, "linearity" is also an important consideration when designing a PCB receiver. Because the receiver is a narrow-frequency circuit, the non-linearity is calculated by measuring intermodulation distortion. This involves using two sinusoidal or cosine waves of similar frequencies in the central band to drive the input signal, and then measuring the product of their intermodulation. In general, SPICE is a time-consuming and expensive simulation software because it has to perform many cycles to get the required frequency resolution to see the distortion.

　　Expected signal

　　The receiver must be sensitive to small input signals. In general, the input power of the receiver can be as small as 1μV. The sensitivity of the receiver is limited by the noise generated by its input circuit. Therefore, noise is an important consideration when designing a PCB receiver. Moreover, the ability to predict noise with simulation tools is indispensable. The received signal is filtered and then amplified with a low noise amplifier (LNA). The signal is then mixed with the first local oscillator (LO) to convert the signal to intermediate frequency (IF). The noise efficiency of the front-end circuit depends mainly on LNA, mixer, and LO. Although the LNA noise can be detected using the traditional SPICE noise analysis, it is useless for the mixer and LO because the noise in these areas is severely affected by a large LO signal.

　　Small input signals require the receiver to be extremely amplified, usually with a gain of 120 dB. At such a high gain, any signal coupled from the output back to the input is likely to cause problems. The important reason for using the superheterodyne receiver architecture is that it can distribute the gain over several frequencies to reduce the probability of coupling. This also makes the frequency of the first LO different from that of the input signal, preventing the large interference signal from "contaminating" the small input signal.

　　For different reasons, the superheterodyne architecture can be replaced by direct conversion or homodyne architecture in some wireless communication systems. In this architecture, the RF input signal is converted directly to the baseband in a single step, so most of the gain is in the baseband, and LO is the same frequency as the input signal. In this case, the impact of a small amount of coupling must be understood, and a detailed model of the stray signal Path, such as coupling through the Substrate, coupling between the encapsulation pins and the bondwire, and coupling through the power cord, must be established.

　　Interference from adjacent channels

　　Distortion also plays an important role in the emitter. The nonlinearity of the transmitter in the output circuit may cause the bandwidth of the transmitted signal to be spread over adjacent channels. This phenomenon is called spectral regrowth. The bandwidth of the signal is limited until it reaches the power amplifier (PA) of the transmitter. But "intermodulation distortion" in PA causes bandwidth to increase again. If the bandwidth is increased too much, the transmitter will not be able to meet the power requirements of its adjacent channels. In practice, SPICE cannot be used to predict spectral growth when transmitting digital modulation signals. Since about 1,000 symbol transmissions had to be simulated to get a representative spectrum and combined with high-frequency carriers, this would make SPICE's transient analysis impractical.

　　Due to the RF circuit is distributed parameter circuit, in the actual work of the circuit is easy to produce skin effect and the coupling effect, so in the actual PCB design, will disturb the radiation found in the circuit is difficult to control, such as: mutual interference between digital circuit and analog circuit, power supply noise, ground interference problems such as unreasonable. Because of this, how to balance the advantages and disadvantages and find a suitable compromise point in the design process of PCB, so as to reduce these interferences as much as possible, or even avoid the interference of some circuits, is the key to the success or failure of RF circuit PCB design. 

　　For the whole RF circuit, RF units of different modules should be isolated with cavities, especially between sensitive circuits and strong radiation sources. In high-power multistage amplifiers, the isolation between stages should also be ensured. After the whole circuit tributary is placed, then it’s the treatment of the shielded cavity. The treatment of the shielded cavity has the following matters needing attention.

　　The whole shielded cavity is made into regular shape as far as possible to facilitate mold casting. For each of the shielded cavity as far as possible to make a rectangle, avoid square shielded cavity

The Angle of shielded cavity adopts arc, the shielded metal cavity generally adopts casting molding, the arc corner is convenient for casting molding. See Figure 12.

Figure 12 cavity

　　The periphery of the shielded cavity is sealed, and the line of the interface into the cavity generally adopts strip line or microstrip line, while the different modules inside the cavity adopt microstrip line, and the joint of different cavities adopts slot processing. The width of the slot is 3mm, and the microstrip line is in the middle.

　　A 3mm metallized hole is placed at the corner of the cavity to fix the shield shell, and the same metallized hole should be evenly placed on each long cavity to reinforce the supporting function.

　　Generally, the cavity is windowed to facilitate welding of the shielding shell. Generally, the thickness of the cavity is more than 2 mm. Two rows of windows are added to the cavity, and the holes are staggered, with the spacing between the same row of holes being 150mil.