PCB Technical - Characteristics of RF PCB circuit

ipcb introduces the four basic characteristics of RF PCB circuit from four aspects: RF interface, small expected signal, large interference signal and interference from adjacent channels, and gives the important factors needing special attention in PCB design process.

RF interface of RF PCB 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 fundamental frequency determines the basic rate of data flowing in the system. The fundamental frequency is used to improve the reliability of the data stream and reduce the load imposed on the transmission medium by the transmitter under a specific data transmission rate. Therefore, a lot of signal processing engineering knowledge is needed when designing the fundamental frequency circuit of 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: they must transmit specific power with as little power as possible. 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 signals outside the desired channel; and, like transmitters, they must consume very little power.

Large interference signal in RF PCB circuit simulation

The receiver must be sensitive to small signals, even in the presence of large interference signals (obstructions). This occurs when an attempt is made to receive a weak or long-range transmission signal, and a strong transmitter nearby is broadcasting on the adjacent channel. The interference signal may be 60 ~ 70 dB larger than the expected signal, and it can block the normal signal reception by means of a large amount of coverage in the input phase of the receiver, or make the receiver generate too much noise in the input phase. If the receiver is driven into a nonlinear region by an 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 consideration in PCB receiver design. Since the receiver is a narrow-band circuit, the nonlinearity is calculated by measuring "intermodulation distortion". This involves using two sinusoidal or cosine waves with similar frequencies and located in the in band to drive the input signal, and then measuring the product of its interactive modulation. Generally speaking, spice is a time-consuming and cost-effective simulation software, because it has to perform many cycles before it can obtain the required frequency resolution to understand the distortion.

RF PCB circuit

Small expected signal in RF PCB circuit simulation

The receiver must be sensitive to small input signals. Generally speaking, the receiver can input a small power of 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. Figure 1 is a typical superheterodyne receiver. The received signal is filtered and then amplified by low noise amplifier (LNA). The signal is then mixed with a local oscillator (LO) to convert the signal into an 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 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.

The small input signal requires the receiver to have a large amplification function, which usually requires a gain of 120 dB. At such a high gain, any signal from the coupling back to the input can cause problems. The important reason for using 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 each Lo different from that of the input signal, which can prevent the large interference signal from "polluting" to 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, so most of the gain is in the fundamental frequency, and lo is the same frequency as the input signal. In this case, the influence of a small amount of coupling must be understood, and the detailed model of "stray signal path" must be established, such as coupling through substrate, coupling between package pin and bonding wire, and coupling through power line.

Interference of adjacent channels in RF PCB circuit simulation

Distortion also plays an important role in the transmitter. The nonlinearity of transmitter in the output circuit may make the bandwidth of transmitted signal spread in adjacent channels. This phenomenon is called "spectrum growth.". Before the signal reaches the power amplifier (PA) of the transmitter, its bandwidth is limited; however, "intermodulation distortion" within the PA will cause the bandwidth to increase again. If the bandwidth increases too much, the transmitter will not be able to meet the power requirements of its adjacent channels. When transmitting digital modulation signals, spice can not be used to predict the spectrum regrowth. Because about 1000 symbols must be simulated to obtain a representative spectrum, and also need to combine high frequency carrier, these will make spice transient analysis impractical.