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Four basic characteristics of PCB RF circuit(on)

Dec. 19, 2019

Here we will interpret the four basic characteristics of the RF circuit from the four aspects of the RF interface, small expected signals, large interference signals, and interference from adjacent channels, and give important factors that need special attention in the PCB design process.

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, a large amount of signal processing engineering knowledge is required when designing a fundamental frequency circuit on a PCB. 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 electric circuit board 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. The first is that they must emit as much power as possible while consuming the least amount of power. The second is that they cannot 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.

FR4 PCB

FR4 PCB

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 an excessive amount of 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 high frequency PCB. Because the receiver is a narrow-band circuit, the non-linearity is measured by measuring "intermodulation distortion." 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, SPICE 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.