Interpretation of 4 basic characteristics of RF circuits

Interpretation of 4 basic characteristics of RF circuits

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

RF interface for RF circuit simulation

Conceptually, wireless transmitters and receivers can be divided into two parts: fundamental 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 at a given data 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 the designated channel, and inject this signal into the transmission medium. Conversely, the receiver's RF circuitry can take the signal from the transmission medium and convert and down-convert it to the fundamental frequency.

Transmitters have two main PCB design goals: The first is that they must emit a specific amount of power while consuming the least amount of power possible. The second is that they cannot interfere with the normal operation of transceivers in adjacent channels. As far as receivers are concerned, 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, the power they consume must be very small.

Large interference signal in RF circuit simulation

The receiver must be sensitive to small signals even in the presence of large interfering signals (blockers). This occurs when trying to pick up a weak or distant transmission while a nearby powerful transmitter is broadcasting on an adjacent channel. The interfering signal may be 60-70 dB larger than the expected signal, and can block the reception of normal signals in a manner of massive coverage at the input stage of the receiver, or by causing the receiver to generate an excessive amount of noise at the input stage. If the receiver is driven into the nonlinear region by the interferer during the input stage, 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, the nonlinearity is measured as "intermodulation distortion". This involves driving the input signal with two sine or cosine waves that are close in frequency, in band, and then measure the product of their intermodulation. In general, SPICE is a time-consuming and expensive simulation software, because it must perform many loops to obtain the required frequency resolution to understand the distortion situation.

Small Expected Signals for RF Circuit Simulation

The receiver must be very sensitive to detect small input signals. Typically, the input power to the receiver can be as small as 1 μV. The receiver's sensitivity is limited by the noise generated by its input circuitry. Therefore, noise is an important consideration when designing a receiver on a PCB. Furthermore, the ability to predict noise with simulation tools is essential. Figure 1 shows a typical superheterodyne receiver. The received signal is filtered and then amplified with a low noise amplifier (LNA). This signal is then mixed with the first local oscillator (LO) to convert the signal to an intermediate frequency (IF). The noise performance of the front-end circuit mainly depends on the LNA, mixer and LO. While using traditional SPICE noise analysis, LNA noise can be found, it is useless for mixers and LOs, because the noise in these blocks is heavily affected by the large LO signal.

The small input signal requires that the receiver must have a very large amplification function, usually requiring a gain as high as 120 dB. At such high gains, any signal coupled from the output back to the input can cause problems. An important reason for using a superheterodyne receiver architecture is that it distributes the gain over several frequencies to reduce the chance of coupling. This also makes the frequency of the first LO different from that of the input signal, preventing large interfering signals from "contaminating" the small input signal.

For different reasons, in some wireless communication systems, direct conversion or homodyne architectures can replace superheterodyne architectures. In this architecture, the RF input signal is directly converted to the fundamental frequency in a single step, so 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 "stray signal paths" such as coupling through the substrate, package pins and bond wires must be established (bondwire) coupling, and coupling through power lines.

Adjacent Channel Interference in RF Circuit Simulation

Distortion also plays an important role in transmitters. The nonlinearity created by the transmitter at the output circuit may spread the bandwidth of the transmitted signal across adjacent frequency 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 is increased too much, the transmitter will not be able to meet the power requirements of its adjacent channels. When transmitting digitally modulated signals, it is practically impossible to use SPICE to predict spectral regrowth. Because transmission operations of about 1000 digital symbols must be simulated to obtain a representative spectrum, and also need to incorporate high frequency carriers, these make SPICE transient analysis impractical.