PCB Technical - RF interface and RF circuit for PCB board design

In PCB design, many special characteristics of RF circuits are difficult to explain in a few short sentences, nor can they be analyzed using traditional simulation software, such as SPICE. However, there are some EDA software on the market that have complex algorithms such as harmonic balance, shooting method, etc., which can simulate radio frequency circuits quickly and accurately. But before learning these EDA software, you must first understand the characteristics of radio frequency circuits, especially the meaning of some proper terms and physical phenomena, because this is the basic knowledge of radio frequency engineering.

RF interface

The wireless transmitter and receiver are conceptually divided into two parts: base frequency and 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 fundamental 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 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 a fundamental frequency circuit on a PCB. The radio frequency circuit of the transmitter can convert and up-convert the processed baseband signal to a designated channel, and inject this signal into the transmission medium. On the contrary, the radio frequency circuit of the receiver can obtain the signal from the transmission medium, and convert and reduce the frequency to the base frequency.

The transmitter has two main PCB design goals:

The first is that they must emit a certain amount of power while consuming the least power possible.

The second is that they cannot interfere with the normal operation of transceivers in adjacent channels. As far as the receiver is concerned, there are three main PCB design goals: First, they must accurately restore small signals;

Third, they must be able to remove interfering signals outside the desired channel; lastly, like transmitters, they must consume very little power.

Small expectation signal

The receiver must detect small input signals very sensitively. 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 in the PCB design of the receiver. Moreover, the ability to predict noise with simulation tools is indispensable. The received signal is filtered first, and then the input signal is amplified by a low noise amplifier (LNA). Then use the first local oscillator (LO) to mix with this signal to convert this signal into an intermediate frequency (IF). The noise performance of the front-end circuit mainly depends on the LNA, mixer and LO. Although the traditional SPICE noise analysis can find the noise of the LNA, it is useless for the mixer and the LO, because the noise in these blocks will be severely affected by the large LO signal.

The small input signal requires the receiver to have a great amplification function, usually a gain of 120 dB is required. With such a high gain, any signal coupled from the output terminal back to the input terminal may cause problems. The important reason for using the superheterodyne receiver architecture is that it can distribute the gain in several frequencies to reduce the chance of coupling. This also makes the frequency of the first LO differ from the frequency of the input signal, which can prevent large interference signals from being "contaminated" to small input signals.

For different reasons, in some wireless communication systems, direct conversion or homodyne architecture can replace superheterodyne architecture. In this architecture, the RF input signal is directly converted into the fundamental frequency in a single step. Therefore, most of the gain is in the fundamental frequency, and the frequency of the LO and the input signal is the same. 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) between the coupling, and the coupling through the power line.

Big interference signal

The receiver must be very sensitive to small signals, even when there are large interference signals (obstructions). This situation occurs when trying to receive a weak or long-distance transmission signal, and a powerful transmitter nearby is broadcasting in an adjacent channel. The interfering signal may be 60~70 dB larger than the expected signal, and it can be used in a large amount of coverage during the input stage of the receiver, or the receiver can generate excessive noise during the input stage to block the reception of normal signals. If the receiver is driven into a non-linear region by the interference source during 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, the non-linearity is measured by measuring "intermodulation distortion". This involves using two sine waves or cosine waves with similar frequencies and located in the center band to drive the input signal, and then measuring the product of its intermodulation. Generally speaking, SPICE is a time-consuming and cost-intensive simulation software, because it has to perform many cycles to obtain the required frequency resolution to understand the distortion.

PCB adjacent channel interference

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