Design RF circuit and PCB layout considerations

RF interface for RF circuit simulation


Wireless transmitters and receivers can be conceptually divided into two parts, the fundamental frequency and the 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 base frequency is used to improve the reliability of the data stream and reduce the load placed on the transmission medium by the transmitter at a specific data transmission rate.


Therefore, PCB design requires a lot of signal processing engineering knowledge when designing the fundamental frequency circuit. The RF circuit of the transmitter can convert and up-convert the processed baseband signal to the specified channel, and inject this signal into the transmission medium. On the contrary, the receiver's radio frequency circuit can obtain the signal from the transmission medium, and convert and reduce the frequency to the fundamental frequency.


The transmitter has two main PCB design goals:


They must, as far as possible, emit a certain amount of power while consuming the least amount of power.


They must not interfere with the normal operation of transceivers in adjacent channels.


In terms of receivers, there are three main PCB design goals: first, they must accurately reproduce small signals; second, they must be able to remove interfering signals outside the desired channel; and finally, like transmitters, they must consume power Very small.

Large interference signals in RF circuit simulation


The receiver must be sensitive to small signals, even in the presence of large interfering signals (blockers). This situation occurs when trying to receive a weak or long-range transmitted signal, and a powerful transmitter nearby broadcasts on an adjacent channel. The interference signal may be 60 ~ 70 dB larger than the expected signal, and it can block the reception of normal signals with a large amount of coverage during the input stage of the receiver, or make the receiver generate excessive noise during the input stage. If the receiver is in the input stage and is driven into a non-linear region by the interference source, the two problems mentioned above will occur. To avoid these problems, the front end of the receiver must be very linear.


Therefore, "linearity" is also an important consideration when designing a receiver for a PCB. Because the receiver is a narrow-frequency circuit, the non-linearity is measured by measuring "inte rmodulati on distorTI on". This involves driving the input signal with two sine or cosine waves that are close in frequency and located in the center band, and then measuring the product of their intermodulation. Generally speaking, SPI CE is a kind of time-consuming and cost-intensive simulation software, because it must perform many loop operations to obtain the required frequency resolution to understand the distortion situation.


Small Expected Signal for RF Circuit Simulation


The receiver must be sensitive to small input signals. Generally speaking, the input power of the receiver can be as small as 1 μV. The receiver's sensitivity is limited by the noise generated by its input circuit. Therefore, noise is an important consideration when designing a receiver for a PCB. Moreover, the ability to predict noise with simulation tools is essential.


Figure 1 is a typical superheterodyne receiver. The received signal is filtered and then the input signal is amplified by a low noise amplifier (LNA). This signal is then mixed with the first local oscillator (LO) to convert this signal to an intermediate frequency (IF). The noise performance of the front-end circuit mainly depends on the LNA, mixer, and LO. Although traditional SPICE noise analysis can be used to find the noise of the LNA, it is useless for the mixer and the LO, because the noise in these blocks will be seriously affected by the large LO signal. The small input signal requires the receiver to have great amplification capabilities, usually requiring a gain as high as 120 dB. With such a high gain, any signal coupled from the output back to the input can cause problems. An important reason for using a super-heterodyne receiver architecture is that it can spread the gain across several frequencies to reduce the chance of coupling. This also makes the frequency of the first LO different from the frequency of the input signal, which can prevent large interference signals from "pollution" to small input signals. For different reasons, in some wireless communication systems, direct conversion or homodyne architecture can replace super heterodyne 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 the LO is the same frequency as the input signal. In this case, the influence of a small amount of coupling must be understood, and a detailed model of the "stray signal path" must be established, such as: coupling through the substrate, package pins and bonding wires (Bondwire) coupling, and coupling through the power line.


Interference from adjacent channels in RF circuit simulation


Distortion also plays an important role in the transmitter. The non-linearity produced by the transmitter in the output circuit may spread the bandwidth of the transmitted signal among adjacent channels. This phenomenon is called "spectral regrowth." Before the signal reaches the transmitter's power amplifier (PA), its bandwidth is limited; however, 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, in fact, it is impossible to predict the further growth of the spectrum with SPICE. Because there are about 1000 digital symbol (symbol) transmission operations must be simulated to obtain a representative frequency spectrum, and also need to combine high-frequency carriers, these will make SPICE transient analysis impractical.