How to design RF circuits?

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How to design RF circuits? [Copy link]

Wireless transmitters and receivers can be conceptually divided into two parts: baseband and RF. The baseband 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 baseband determines the basic rate at which data can flow in the system. The baseband is used to improve the reliability of the data flow and reduce the load imposed by the transmitter on the transmission medium under a specific data transmission rate. Therefore, a lot of signal processing engineering knowledge is required when designing the baseband circuit on the PCB. The RF circuit of the transmitter can convert and up-convert the processed baseband signal to a specified channel and inject this signal into the transmission medium. Conversely, the RF circuit of the receiver can obtain the signal from the transmission medium and convert and down-convert it to the baseband.
Transmitters have two main PCB design goals: the first is that they must transmit a specific power while consuming the least power as possible. The second is that they cannot interfere with the normal operation of transceivers in adjacent channels. For receivers, there are three main PCB design goals: first, they must accurately reproduce small signals; second, they must be able to reject interfering signals outside the desired channel; and finally, like transmitters, they must consume very little power.　

Large interference signal in RF circuit simulation

The receiver must be sensitive to small signals even in the presence of large interfering signals (obstructions). This situation occurs when trying to receive a weak or distant transmitter signal while there is a powerful transmitter broadcasting in an adjacent channel nearby. The interfering signal may be 60-70 dB larger than the desired signal and can block the reception of the normal signal by flooding the receiver input stage or by causing the receiver to generate excessive noise at the input stage. If the receiver is driven into a nonlinear region at the input stage by the interferer, 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 on a PCB. Since the receiver is a narrowband circuit, nonlinearity is measured by measuring "intermodulation distortion". This involves driving the input signal with two sine or cosine waves of similar frequency and located in the center band, and then measuring the product of their mutual modulation. Generally speaking, SPICE is a time-consuming and costly simulation software because it must execute many cycles of calculations to obtain the required frequency resolution to understand the distortion situation.

RF circuit simulation of small desired signals

The receiver must be sensitive enough to detect small input signals. Generally speaking, the input power of the receiver can be as low 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 designing a receiver on a PCB. Moreover, the ability to predict noise with simulation tools is essential. Figure 1 shows a typical superheterodyne receiver. The received signal is first filtered and then amplified by a low noise amplifier (LNA). The signal is then mixed with the first local oscillator (LO) to convert it to an intermediate frequency (IF). The noise performance of the front-end circuit mainly depends on the LNA, mixer, and LO. Although the noise of the LNA can be found using traditional SPICE noise analysis, it is useless for the mixer and LO because the noise in these blocks will be severely affected by the large LO signal.

Small input signals require a lot of receiver amplification, often with gains as high as 120 dB. At such high gains, any signal that couples back from the output to the input can be a problem. An important reason to use a superheterodyne receiver architecture is that it spreads the gain over several frequencies, reducing the chance of coupling. This also allows the first LO to be at a different frequency than the input signal, preventing large interfering signals from "contaminating" the small input signal.
For different reasons, direct conversion or homodyne architectures can replace superheterodyne architectures in some wireless communication systems. In this architecture, the RF input signal is converted directly to baseband in a single step, so most of the gain is at baseband and the LO is at the same frequency as the input signal. In this case, the impact of small amounts of coupling must be understood, and detailed models of "stray signal paths" must be built, such as coupling through the substrate, coupling between package pins and bondwires, and coupling through power lines.

Adjacent channel interference in RF circuit simulation

Distortion also plays a role in transmitters. Nonlinearities in the transmitter output circuitry can cause the bandwidth of the transmitted signal to spread across adjacent channels. This phenomenon is called "spectral regrowth." Before the signal reaches the transmitter's power amplifier (PA), its bandwidth is limited; but "intermodulation distortion" within the PA causes 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 digitally modulated signals, it is actually impossible to predict spectral regrowth using SPICE. This is because the transmission of approximately 1,000 digital symbols must be simulated to obtain a representative spectrum, and this requires the combination of high-frequency carriers, which makes SPICE transient analysis impractical.

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