This article explains the 4 basic characteristics of RF circuits from four aspects: RF interface, small expected signal, large interference signal, and interference from adjacent channels, and gives important factors that need special attention in the PCB design process.
RF circuit simulation of the interface of RF
Wireless transmitter and receiver in the concept, can be divided into two parts of the fundamental frequency and radio frequency. The fundamental frequency contains 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 fundamental frequency is used to improve the reliability of the data flow and to reduce the load imposed by the transmitter on the transmission medium at a given data rate. Therefore, the PCB design of the fundamental frequency circuit requires extensive knowledge of signal processing engineering. The RF circuitry of the transmitter converts and upscales the processed fundamental frequency signal to a specified channel and injects this signal into the transmission medium. Conversely, the receiver’s RF circuitry acquires the signal from the transmission media and converts and downscales it to the fundamental frequency.
Transmitters have two main PCB design goals: the first is that they must transmit a specific amount of power while consuming the least amount of power possible. The second is that they can not interfere with the normal operation of the transceiver in adjacent channels. In terms of the receiver, 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; the last point is the same as the transmitter, they must consume very little power.
RF circuit simulation of large interfering signals
Receivers must be sensitive to small signals, even when large interfering signals (blockers) are present. This situation arises when trying to receive a weak or distant transmit signal with a powerful transmitter broadcasting in the adjacent channel nearby. The interfering signal may be 60 to 70 dB larger than the expected signal and can block the reception of the normal signal in the input phase of the receiver with a large amount of coverage or by causing the receiver to generate an excessive amount of noise in the input phase. Those two problems mentioned above can occur if the receiver, in the input stage, is driven into the region of nonlinearity by the source of interference. To avoid these problems, the front end of the receiver must be very linear.
Therefore, “linearity” is also an important consideration when designing the receiver PCB. As the receiver is a narrow-band circuit, so the nonlinearity is to measure the “intermodulation distortion (intermodulation distortion)” to the statistics. This involves using two sine or cosine waves of similar frequency and located in the center band (in band) to drive the input signal, and then measuring the product of its intermodulation distortion. By and large, SPICE is a time-consuming and costly simulation software because it must perform many cycles before it can obtain the desired frequency resolution to understand the distortion.
RF circuit simulation of small desired signal
The receiver must be very sensitive to detect small input signals. In general, 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 for PCB. Moreover, having the ability to predict noise with simulation tools is essential. Figure 1 is a typical superheterodyne (superheterodyne) receiver. The received signal is first filtered and then the input signal is amplified with a low-noise amplifier (LNA). The first local oscillator (LO) is then used to mix with this signal to convert this signal to intermediate frequency (IF). Front-end (front-end) circuit noise effectiveness depends mainly on the LNA, mixer (mixer) and LO. although the use of conventional SPICE noise analysis, you can look for the LNA noise, but for the mixer and LO, it is useless, because the noise in these blocks, will be a very large LO signal seriously affected.
The small input signal requires the receiver to be extremely amplified, usually requiring a gain as high as 120 dB. At such a high gain, any signal coupled from the output (couples) back to the input can create problems. The important reason for using the super outlier receiver architecture is that it allows the gain to be distributed over several frequencies to reduce the chance of coupling. This also makes the first LO frequency is different from the input signal frequency, can prevent large interference signal “pollution” to the small input signal.
For different reasons, in some wireless communication systems, direct conversion (direct conversion) or internal differential (homodyne) architecture can replace the ultra-outer differential architecture. In this architecture, the RF input signal is directly converted to the fundamental frequency in a single step, so that most of the gain is in the fundamental frequency and the LO is at the same frequency as the input signal. In this case, the impact 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, coupling between the package footprint and the solder line (bondwire), and coupling through the power line coupling.
RF Circuit Simulation of Adjacent Channel Interference
Distortion also plays an important role in the transmitter. The nonlinearity generated by the transmitter in the output circuit may cause the frequency width 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; however, “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 neighboring channels. When transmitting a digital modulation signal, it is practically impossible to predict the re-growth of the spectrum with SPICE. Because about 1000 digital symbols (symbol) of the transmission operation must be simulated to obtain a representative spectrum, and also need to combine the high frequency carrier, these will make the SPICE transient analysis becomes impractical.
Post time: Mar-31-2022