This paper will interpret the four basic characteristics of RF circuit from four aspects of RF interface, small expected signal, large interference signal and interference of adjacent channels, and give the important factors that need special attention in the process of PCB design.
RF interface of RF circuit simulation
In concept, wireless transmitter and receiver can be divided into two parts: fundamental frequency and radio frequency. 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 on the transmission medium by the transmitter under a specific data transmission rate. Therefore, a lot of engineering knowledge of signal processing is needed when designing fundamental frequency circuit on PCB. The RF circuit of the transmitter can convert 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, and convert and reduce the frequency to the fundamental frequency.
Transmitters have two main PCB design goals: first, they must transmit specific power with the least power consumption. Second, they cannot interfere with the normal operation of the transceivers in the adjacent channels. As far as receivers are concerned, there are three main PCB design objectives: first, they must accurately restore small signals; second, they must be able to remove interference signals beyond the desired channel; and last, like transmitters, they must consume very little power.
Large interference signal of RF circuit simulation
The receiver must be sensitive to small signals, even in the presence of large interfering signals (obstructions). This occurs when an attempt is made to receive a weak or long-range transmission signal with a powerful transmitter nearby broadcasting in an adjacent channel. The interference signal may be 60 ~ 70 dB larger than the expected signal, and the normal signal reception can be blocked by a large amount of coverage in the input stage of the receiver, or by causing the receiver to produce too much noise in the input stage. If the receiver is driven into the nonlinear region by the interference source in the input phase, 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 factor in PCB receiver design. Because the receiver is a narrow-band circuit, the nonlinearity is measured by “intermodulation distortion”. This involves driving the input signal with two sine or cosine waves with similar frequencies and located in the central band, and then measuring the product of their intermodulation. Generally speaking, spice is a time-consuming and cost-effective simulation software, because it has to perform many cycles to get the required frequency resolution to understand the distortion.
Small expected signal in RF circuit simulation
The receiver must be very sensitive to small input signals. Generally speaking, the input power of the receiver can be as low as 1 μ v. The sensitivity of the receiver is limited by the noise generated by its input circuit. Therefore, noise is an important factor in PCB receiver design. Moreover, it is necessary to have the ability to predict noise with simulation tools. Fig. 1 is a typical superheterodyne receiver. The received signal is filtered first, and then the input signal is amplified by low noise amplifier (LNA). The first local oscillator (LO) is then used to mix with the signal to convert the signal to intermediate frequency (if). The noise performance of front-end circuit mainly depends on LNA, mixer and lo. Although the traditional spice noise analysis can find the noise of LNA, it is useless for mixer and lo, because the noise in these blocks will be seriously affected by the large LO signal.
Small input signal requires the receiver to have a great amplification function, which usually requires a high gain of 120 dB. At such a high gain, any signal coupled from the output to the input may cause problems. An important reason for using superheterodyne receiver architecture is that it can distribute the gain in several frequencies to reduce the coupling probability. 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 to the fundamental frequency in a single step. Therefore, most of the gain is in the fundamental frequency, and 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 the coupling through the substrate, the coupling between the packaging pin and the bond wire, and the coupling through the power line.
Interference of adjacent channels in RF circuit simulation
Distortion also plays an important role in the transmitter. The nonlinearity of the transmitter in the output circuit may spread the bandwidth of the transmitted signal in the adjacent channels. This phenomenon is called “spectral regrowth”. The bandwidth of the signal is limited before it reaches the PA of the transmitter, but the “intermodulation distortion” in the PA will cause the 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. When transmitting digital modulation signals, it is impossible to predict the regrowth of spectrum with spice. Because there are about 1000 digital symbol transmission operations must be simulated to obtain the representative spectrum, and also need to combine high frequency carrier, which will make the transient analysis of spice impractical.