PCB News - Four basic characteristics of PCB RF circuit

This paper interprets the four basic characteristics of RF circuit from four aspects: RF interface, small expected signal, large interference signal and interference of adjacent channels, and gives the important factors that need special attention in the process of PCB design.

1. Interface of RF circuit simulation

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 in the system. The fundamental frequency is used to improve the reliability of data flow and reduce the load imposed by the transmitter on the transmission medium under a specific data transmission rate. Therefore, when designing fundamental frequency circuit with PCB, a lot of signal processing engineering knowledge is required. The RF circuit of the transmitter can convert and raise the processed fundamental frequency signal to the specified channel, and inject the signal into the transmission medium. On the contrary, the RF circuit of the receiver can obtain the signal from the transmission medium, convert and reduce the frequency to the fundamental frequency.

Transmitters have two main PCB design objectives: the first is that they must transmit specific power with the least power consumption. Second, they cannot interfere with the normal operation of transceivers in adjacent channels. As far as the receiver is concerned, there are three main PCB design objectives: first, they must accurately restore the small signal; Second, they must be able to remove interference signals other than the desired channel; Finally, like transmitters, they must consume very little power.

2.Large interference signal in RF circuit simulation

The receiver must be sensitive to small signals, even when there is a large interference signal (barrier). This occurs when trying to receive a weak or long-range transmission signal, and there are powerful transmitters nearby broadcasting in adjacent channels. The interference signal may be 60 ~ 70 dB larger than the expected signal, and the reception of the normal signal can be blocked by a large amount of coverage in the input stage of the receiver, or by causing the receiver to generate too much noise in the input stage. If the receiver is driven into the nonlinear region by the interference source in the input stage, the above two problems will occur. To avoid these problems, the front end of the receiver must be very linear.

Therefore, "linearity" is also an important consideration in PCB receiver design. Since the receiver is a narrow-band circuit, the nonlinearity is counted by measuring "intermodulation distortion". This involves using two sine or cosine waves with similar frequencies and located in the central band to drive the input signal, and then measuring the product of their interactive modulation. Generally speaking, spice is a time-consuming and cost-effective simulation software, because it must perform many cyclic operations before it can get the required frequency resolution to understand the distortion.

3. Small expected signal of RF circuit simulation

The receiver must be very sensitive to detect small input signals. In general, the input power of the receiver can be as small as 1 μ V. The sensitivity of the receiver is limited by the noise generated by its input circuit. Therefore, noise is an important consideration in PCB receiver design. Moreover, the ability to predict noise with simulation tools is indispensable. Fig. 1 is a typical superheterodyne receiver. The received signal is filtered and then amplified by a low noise amplifier (LNA). The first local oscillator (LO) is then used to mix with the signal to convert the signal into intermediate frequency (if). The noise efficiency of front-end circuit mainly depends on LNA, mixer and lo. Although the noise of LNA can be found by using traditional spice noise analysis, it is useless for mixer and lo, because the noise in these blocks will be seriously affected by large LO signals.

A small input signal requires the receiver to have a great amplification function, which usually requires a gain of 120 dB. At such a high gain, any signal coupled from the output back to the input may cause problems. The important reason for using superheterodyne receiver architecture is that it can distribute the gain in several frequencies to reduce the probability of coupling. This also makes the frequency of the first LO different from that of the input signal, which can prevent the large interference signal from "polluting" the small input signal.

For different reasons, in some wireless communication systems, direct conversion or homodyne architecture can replace superheterodyne architecture. In this architecture, the RF input signal is directly converted into the fundamental frequency in a single step. Therefore, most of the gain is in the fundamental frequency, and the LO is the same as the frequency of the input signal. In this case, the influence of a small amount of coupling must be understood, and a detailed model of "stray signal path" must be established, such as coupling through substrate, coupling between package pin and bond wire, and coupling through power line.

4. Interference of adjacent channels in RF circuit simulation

Distortion also plays an important role in the transmitter. The nonlinearity generated by the transmitter in the output circuit may spread the bandwidth of the transmitted signal in adjacent channels. This phenomenon is called "spectral regrowth". Before the signal reaches the power amplifier (PA) of the transmitter, its bandwidth is limited; However, "intermodulation distortion" in PA will cause the bandwidth to increase again. If the bandwidth increases too much, the transmitter will not meet the power requirements of its adjacent channels. When transmitting digital modulation signals, in fact, spice can not be used to predict the regrowth of the spectrum. Because the transmission of about 1000 digital symbols must be simulated to obtain a representative spectrum, and it also needs to be combined with high-frequency carriers, which will make the transient analysis of spice impractical.