Many special features of RF Circuit, it is difficult to illustrate, and traditional simulation software cannot be used, such as SPICE. However, there are some other EDA software with Harmonic Balance, Shooting Method ..., which can quickly and accurately simulate RF circuits. But before learning these EDA software, you must first understand the characteristics of the RF circuit, especially to understand some of the meaning of some proprietary nouns and physical phenomena, as this is the basics of radio engineering.
Radio interface
The wireless transmitter and receiver can be conceptually divided into two parts: baseband and radio frequency. The fundamental frequency includes the frequency range of the input signal of the transmitter, and also includes the frequency range of the output signal of the receiver. The frequency band of the fundamental frequency determines the basic rate of data in the system. The base frequency is used to improve the reliability of the data stream, and under a particular data transfer rate, reduce the load applied to the transport medium (Transmission Medium). Therefore, when the PCB designs the fundamental frequency circuit, a large amount of signal processing is required. The radio frequency circuit of the transmitter can convert the processed fundamental frequency signal to the specified channel, and inject this signal into the transmission medium. Conversely, the radio frequency circuit of the receiver can acquire signals in the transmission medium, conversion, and reduce frequency.
The transmitter has two major PCB design objectives: the first is that they must transmit specific power when consuming to minimize power. The second is that they cannot interfere with the normal operation of transceivers within adjacent channels. In terms of receiver, there are three major PCB design objectives: First, they must accurately reduce small signals; second, they must remove interference signals outside the expected channel; the last point is the same as the transmitter, they consume the power required Small.
Small expected signal
The receiver must be very sensitive to detect a small input signal. In general, the input power of the receiver can be small to 1 μV. The sensitivity of the receiver is limited by the noise generated by its input circuit. Therefore, the noise is an important consideration when the PCB design receiver is designed. Moreover, it is indispensable with the ability to predict noise in simulation tools. The drawings are a typical superheterodyne receiver. The received signal is first filtered, and the input signal is enlarged by a low noise amplifier (LNA). The first local oscillator (LO) is then mixed with this signal to convert this signal to an intermediate frequency (IF). The noise effect of the Front-end circuit depends primarily on the LNA, mixer (Mixer) and LO. While using conventional SPICE noise analysis, the noise of LNA can be found, but for mixers and LO, it is useless because noise in these blocks will seriously affect the LO signal.
Small input signals require that the receiver must have great enlargement, usually require 120 dB as high gains. Under such a high gain, any signal from which to return to the input of the input is possible. An important reason for using an ultra-unity 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 are different from the frequency of the input signal, and the large interference signal "contamination" to a small input signal.
For different reasons, in some wireless communication systems, direct conversion (Homodyne) architectures can replace the ultra-horizontal architecture. In this architecture, the radio frequency input signal is directly converted to a base frequency under a single step, and therefore, most of the gains are in the base frequency, and the LO is the same as the frequency of the input signal. In this case, a small amount of coupling must be understood, and the detailed model of "striped signal path" is established, such as the coupling of the substrate, package foot and welding wires. (Bondwire) coupling between and passing through the power cord.
Large interference signal
The receiver must be very sensitive to small signals, even if there is a large interference signal (blocking material) exists. This happens at an attempt to receive a weak or long-distance transmit signal, and there is a powerful transmitter near the adjacent channel. The interference signal may be more than 60 to 70 dB of the expected signal, and can be overwise in a large amount of noise at the input phase of the receiver, or cause excessive noise amount in the input phase to generate excessive noise to block the reception of the normal signal. If the receiver is driven in the input phase, the two problems described above will occur during the input phase. To avoid these issues, the front end of the receiver must be very linear.
Therefore, "linear" is also an important consideration when the PCB design receiver is designed. Since the receiver is a narrowband circuit, the nonlinearity is statistically statistically determined. This involves driving the input signal with two frequencies, and in the central band (IN BAN) sine wave or cosine wave, and then measure the product of its interaction modulation. Generally speaking, SPICE is a time-consuming emulation software because it must perform many cyclic calculations to get the required frequency resolution to understand the distortion.
Interference of adjacent channels
Distortion also plays an important role in the transmitter. The transmitter is non-linear generated by the output circuit, which may cause the bandwidth of the transfer signal to spread in the adjacent channel. This phenomenon is called "Spectral RegroWTH". Before the signal reaches the power amplifier (PA) of the transmitter, its bandwidth is limited; but "intercalation distortion" within the Pa will result in frequency width again. If the bandwidth increases too much, the transmitter will not conform to the power requirements of its adjacent channels. When transmitting a digital modulation signal, actually, it is unable to predict the reappearance of the spectrum with SPICE. Because about 1000 digital symbols (Symbol) must be simulated to seek representative spectrum, and also need to bind to high frequencies, which will make Spice's transient analysis unreal.
Our other product:
