How to design RF circuits and their PCB layout

This article interprets the four basic characteristics of RF circuits from four aspects: RF interface, small desired signal, large interference signal, and interference from adjacent channels, and gives important factors that need special attention in the PCB design process.

RF interface of RF circuit simulation

Conceptually, wireless transmitters and receivers can be 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, when designing the baseband circuit on the PCB, a lot of signal processing engineering knowledge is required. 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. 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 possible.

Second, they must not interfere with the normal operation of transceivers in adjacent channels. For receivers, there are three main PCB design goals: first, they must accurately restore small signals; second, they must be able to remove interference signals outside the desired channel; and finally, like transmitters, they must consume very little power.

Large Interference Signals in RF Circuit Simulation

Receivers must be sensitive to small signals even when large interference signals (blockers) are present. This situation occurs when trying to receive a weak or distant transmission signal while there is a powerful transmitter nearby broadcasting in an adjacent channel. Interference signals may be 60~70 dB larger than the desired signal and can block the reception of normal signals by flooding the input stage of the receiver or by causing the receiver to generate excessive noise at the input stage. If the receiver is driven into a nonlinear region by the interference source at the input stage, 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 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 perform many cycles of calculations before it can get the required frequency resolution to understand the distortion.

RF Circuit Simulation of Small Desired Signals

The receiver must be very sensitive to small input signals. Generally speaking, 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 designing a receiver on a PCB. Moreover, the ability to predict noise with simulation tools is indispensable. Figure 1 is a typical superheterodyne receiver. The received signal is first filtered and then amplified by a low noise amplifier (LNA). Then the first local oscillator (LO) is used to mix with this signal to convert it to an intermediate frequency (IF). The noise performance of the front-end circuit mainly depends on the LNA, mixer and LO. While traditional SPICE noise analysis can be used to find the noise of the LNA, it is useless for the mixer and LO because the noise in these blocks is heavily influenced by the large LO signal.

Small input signals require the receiver to have a lot of amplification, usually as high as 120 dB.

At such high gain, any signal that couples back from the output to the input can cause problems. An important reason for using a superheterodyne receiver architecture is that it spreads the gain over several frequencies to reduce the chance of coupling. This also makes the first LO frequency different from the input signal frequency, which prevents large interfering signals from “contaminating” the small input signal.

For different reasons, in some wireless communication systems, the direct conversion or homodyne architecture can replace the superheterodyne architecture. In this architecture, the RF input signal is directly converted to baseband in a single step, so most of the gain is at baseband and the LO is 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 established, 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 an important role in transmitters. The nonlinearity generated by the transmitter in the output circuit may cause the bandwidth of the transmitted signal to be 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” in 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 use SPICE to predict spectral regrowth. Because the transmission of about 1,000 digital symbols must be simulated to obtain a representative spectrum, and a high-frequency carrier must be combined, these will make SPICE transient analysis impractical.