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Characteristics of PCB RF interface circuit design and RF circuits

[Guide] RF circuit has many special properties, use of conventional simulation software for analysis. There are a number of EDA software with complex harmonic balance algorithm, projection method, you can quickly and accurately simulate RF circuits. Before learning EDA software, you must first understand the characteristics of the RF circuit.

 

RF circuits (RF circuit) the number of special features, it is difficult to explain briefly a few words, you cannot use the traditional simulation software to analyze, for example, SPICE. However, there are some EDA software currently on the market with a harmonic balance (harmonic balance), the projection method (shooting method)…. Complex algorithms that can quickly and accurately simulate RF circuits. But before learning the EDA software, you must first understand the characteristics of the RF circuit, in particular, to understand the meaning of some terminology and physical phenomena , because it is the basis of knowledge of RF engineering .

RF interface

Wireless transmitter and receiver conceptually can be divided into two parts, the baseband and RF. Baseband frequency range of the frequency range of the input signal includes a transmitter, the receiver is also included in the output signal . Determines the bandwidth of the baseband data rate of the system substantially flowable. Baseband data stream is used to improve the reliability, and in a particular data transmission rate, the transmitter is applied to reduce the transmission medium (transmission medium) load. Therefore, the baseband circuit PCB design, signal processing requires a lot of engineering knowledge. RF transmitter circuit can already processed baseband signal conversion, frequency up to the specified channel, and this signal is injected into the transmission media. Instead, the RF receiver circuit to obtain the signals from the transmission medium, and convert the frequency down to baseband .

The transmitter has two main PCB design goals: First, they must be the least possible power consumption in the case of transmission of specific power. The second is that they cannot interfere with the normal operation of the transceiver within the adjacent channel. The receiver is concerned, there are three main PCB design goals: First, they must be accurately restored small signal; Second, they must be able to remove the interfering signals outside the desired channel ; final point as the transmitter , they consume power must small.

Small Signal expectations

The receiver must be very sensitive to detect the small input signals. Generally, the receiver’s input power can be as small as 1 μV. The receiver sensitivity is limited by its noise generated by the input circuit. Therefore, the noise is an important consideration when the receiver PCB design. Moreover, in order to have the ability to predict noise simulation tool is essential. One drawing of a typical super heterodyne receiver. After receiving the first signal filtering, and then to a low noise amplifier (LNA) to amplify the input signal. Then take advantage of the first local oscillator (LO) signal is mixed with this, to make this signal into an intermediate frequency (IF). The front end (front-end) noise performance of the circuit depends on the LNA, mixer (mixer) and LO. Although the use of conventional SPICE noise analysis can find LNA noise, but for the purposes of the mixer and LO, it is useless because of the noise in these blocks will be severely affected large LO signal.

Small input signal receiver is required to be of great amplification, 120 dB typically require such high gain. In such a high -gain, any custom output coupled (couple) back to the input signal are likely to cause problems. Important reasons to use a superheterodyne receiver architecture is that it can be distributed in several frequency gain in order to reduce the probability of coupling. It also makes the first LO frequency different from the frequency of the input signal; you can prevent large interfering signals “pollution “to small input signals.

For different reasons, in some wireless communication systems, direct conversion (direct conversion) or in the difference (homodyne) superheterodyne architecture structure may be substituted. In this architecture, the RF input signal is in a single step directly into the baseband, so most of the gain in the fundamental frequency, and with the same frequency LO input signal. In this case, you must understand the influence of a small amount of coupling and must establish a “spurious signal path (stray signal path) ” detailed models , such as: through the coupling of the substrate (substrate) , the package pins and wire coupling, and the coupling through the power supply line (bondwire) between .

Large interfering signals

The receiver must be very sensitive to small signal, if there is a large interfering signal (barrier) exists. This occurs when the attempt to receive a weak signal or the transmission distance, and near a strong adjacent channel transmitters in the broadcast. Interfering signals may be larger than expected 60 ~ 70 dB signal, and can be in the receiver input stage with a large covered way, or the receiver excessive amount of noise in the input stage to block the reception of normal signals. If the receiver input stage, the interference source is driven into the nonlinear region, the problem occurs that two. To avoid these problems, the front end of the receiver must be very linear.

Thus , ” linear ” is also an important consideration in PCB design when the receiver . Because the receiver is a narrow circuit, based on the measurement of nonlinear ” Intermodulation distortion (intermodulation distortion) ” to the statistics. This involves the use of two frequencies are similar, and in the center of the band (in band) sine or cosine waves to drive the input signal , and then measuring the intermodulation product . Generally speaking, SPICE is a time-consuming and cost simulation software as many times since it must perform the calculation cycle to obtain the desired frequency resolution for the case of distortion.

Interference from adjacent channels.

The transmitter distortion also plays an important role. In the output circuit of the transmitter generated by a nonlinear, may cause the bandwidth of the signal transmitted spread to adjacent channels. This phenomenon is called ” spectral re-growth (spectral regrowth) .” Before the signal reaches the transmitter power amplifier (PA), which is limited to the bandwidth; But in the PA’s “intermodulation distortion ” will result in increased bandwidth again . If too much bandwidth increases, the transmitter will be unable to meet the power requirements of its adjacent channels. When sending digital modulated signals, in fact, is unable to predict the spectrum of the re-growth using SPICE. Since about 1000 the number sign (symbol) transfer operations must be simulated in order to obtain a representative of the spectrum, and also requires a combination of high-frequency carrier, which will enable SPICE transient analysis becomes impractical.

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