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

RF circuit simulation of the RF interface

Wireless transmitter and receiver in the concept, can be divided into two parts of the base frequency and radio frequency. The fundamental frequency contains the frequency range of the input signal of the transmitter and also the frequency range of the output signal of the receiver. The bandwidth of the fundamental frequency determines the basic rate at which the data can flow in the system. The base frequency is used to improve the reliability of the data stream and to reduce the load applied by the transmitter to the transmission medium at a specific data transfer rate. Therefore, PCB design base frequency circuit, requires a lot of signal processing engineering knowledge. 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 receiver's RF circuit can be obtained from the transmission medium signal, and conversion, down to base frequency.

The transmitter has two main PCB design goals: the first is that they must, as far as possible, consume a specific power in the case of minimal power consumption. The second is that they can not interfere with the normal operation of transceivers within adjacent channels. In terms of receivers, there are three main PCB design goals: first, they must accurately reduce the small signal; second, they must be able to remove the desired channel outside the interference signal; the last point with the transmitter, they consume power must Very small

RF circuit simulation of the large interference signal

The receiver must be sensitive to small signals, even if there is a large interfering signal (obstruction) present. This situation occurs when attempting to receive a weak or long-range launch signal, and there is a strong transmitter in the vicinity of the adjacent channel broadcast. The interference signal may be 60 to 70 dB greater than the expected signal, and the reception of the normal signal may be blocked by a large amount of coverage in the input phase of the receiver or by causing the receiver to generate excessive noise at the input stage. If the receiver is driven by the source of the disturbance into the non-linear region at the input stage, the above two problems will occur. To avoid these problems, the front end of the receiver must be very linear.

Thus, "linearity" is also an important consideration when PCB design receivers. Since the receiver is a narrow-band circuit, the non-linearity is measured by measuring "intermodulation distortion". This involves the use of two sine waves or cosine waves that are close to each other and are located in the center band to drive the input signal and then measure the product of its intermodulation. In general, SPICE is a time-consuming cost of simulation software, because it must perform many times after the cycle of operation, in order to get the required frequency resolution to understand the situation of distortion.

RF circuit simulation of the small expected signal

The receiver must be sensitive to detecting a small input signal. In general, the input power of the receiver can be as small as 1 μV. The sensitivity of the receiver is limited by the noise generated by its input circuit. Therefore, noise is an important consideration when PCB design receivers. Moreover, the ability to use simulation tools to predict noise is indispensable. Figure 1 is a typical superheterodyne receiver. The received signal is filtered and the input signal is amplified with a low noise amplifier (LNA). The signal is then mixed with this signal using the first local oscillator (LO) to convert the signal to intermediate frequency (IF). The noise performance of the front-end circuit depends primarily on the LNA, the mixer, and the LO. Although the use of traditional SPICE noise analysis, you can find the LNA noise, but for the mixer and LO, it is useless, because the noise in these blocks, will be a large LO signal seriously affected.

Small input signals require the receiver to have a large amplification function, which typically requires a high gain of 120 dB. At such a high gain, any signal from the output side that couples back to the input may cause problems. An important reason for using a superheterodyne receiver architecture is that it distributes the gain in several frequencies to reduce the probability of coupling. This also makes the frequency of the first LO different from the frequency of the input signal, which can prevent the large interfering signal from "contaminating" the small input signal.

For different reasons, in some wireless communication systems, direct conversion (direct conversion) or homodyne (homodyne) architecture can replace the superheterodyne architecture. In this architecture, the RF input signal is directly converted to a fundamental frequency under a single step, so that most of the gain is in the fundamental frequency, and the LO is the same frequency as the input signal. In this case, it is necessary to understand the influence of a small amount of coupling and to establish a detailed model of the "stray signal path", such as coupling through the substrate, encapsulating the pin and the wire (bondwire) between the coupling, and through the power line coupling.

Interference of adjacent channel in RF circuit simulation

Distortion also plays an important role in the launcher. The non-linearity of the transmitter in the output circuit may cause the bandwidth of the transmitted signal to be spread over the adjacent channels. This phenomenon is called "spectral regrowth". The bandwidth is limited before the signal arrives at the transmitter's power amplifier (PA); but the "crosstalk" within the PA causes 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 a digital modulation signal, in fact, SPICE can not be used to predict the re-growth of the spectrum. Because there are about 1000 digital symbols (symbol) of the transmission must be simulated to obtain a representative spectrum, and also need to combine high-frequency carrier, which will make SPICE transient analysis becomes impractical.