From the radio frequency interface, small desired signal, large interference signal, adjacent channel interference Interpretation four basic characteristics of the RF circuit article four, and gives an important factor in the PCB design process requires special attention.

First, the RF circuit simulation of the RF interface

Wireless transmitters and receivers on the concept can be divided into two parts baseband and RF. The output signal of the frequency range of the frequency range of the fundamental frequency of the input signal includes a transmitter, and also contains the receiver. Baseband bandwidth determines the basic rate data in the system can flow. The fundamental frequency is used to improve the reliability of the data stream, and under the specific data transmission rate, the transmitter is applied to reduce the transmission medium (transmission medium) load. Therefore, the PCB design baseband circuitry, signal processing requires a lot of engineering knowledge. RF transmitter circuit can already processed baseband signal conversion, frequency up to the specified channel, and inject this signal to the transmission medium. Conversely, the receiver circuit can be achieved from the RF signal transmission medium, and converting, down to baseband.

There are two main transmitter PCB design goals: The first is that they must be kept at a minimum power consumption, the transmit particular power. The second is that they can not interfere with the normal operation of the transceiver within the adjacent channel. On the receiver, there are three main PCB design goals: First, they must be accurately restore the small signal; second, they must be able to remove unwanted signals outside the desired channel; Finally, as with the transmitter, they consume power must small.

Second, the RF circuit simulation of large interference signal

The receiver must be very sensitive to small signals, even if there is a large interfering signal (barrier) exists. This occurs when the attempt to receive a weak or remote transmission signal, and the vicinity thereof have a strong broadcast transmitters in adjacent channels. Interfering signal may be larger than expected 60 ~ 70 dB signal, and can be in the receiver input stage of the way with plenty of coverage, or the receiver produce excessive amount of noise at the input stage, by blocking the normal signal reception. If the receiver input stage is driven into nonlinear interference source region, said that two problems will occur. To avoid these problems, the front end of the receiver must be very linear.

Therefore, the "linear" PCB also an important consideration when designing receivers. Since the receiver is narrow band circuits, nonlinear measure is "crosstalk (intermodulation distortion)" to statistics. This involves the use of two frequencies are similar, and in the center of the band (in band) sine or cosine wave to drive the input signal, and then measuring the intermodulation product. Generally speaking, SPICE is a time-consuming and cost-simulation software, because it must be performed after many cycles in operation, in order to obtain the desired frequency resolution, the situation in order to understand the distortion.

Third, small RF circuit simulation of the desired signal

The receiver must be sensitive to detect small input signal. In general, the receiver input power can be as small as 1 μV. Receiver sensitivity is limited by its noise generated by the input circuit. Therefore, noise is an important consideration when the receiver PCB design. Moreover, the ability to simulate tool to predict noise is essential. One drawing of a typical superheterodyne (superheterodyne) receiver. The received signal is first filtered, and then to a low noise amplifier (LNA) amplifies the input signal. Then use the first local oscillator (LO) signal is mixed with this, so that this signal is converted into an intermediate frequency (IF). The front (front-end) noise depends primarily on the effectiveness of the circuit LNA, mixer (mixer) and LO. Although the use of traditional SPICE noise analysis, you can find the LNA noise, but for the mixer and LO is concerned, it is useless, because the noise in these blocks, it will be seriously affected a large LO signal.

Small input signals require the receiver must have a great zoom function, usually requires 120 dB such a high gain. In such a high-gain, any coupling from output terminal (couple) back to the input signals could cause problems. Important reasons for using a superheterodyne receiver architecture is that it can be distributed in several frequencies gain in order to reduce the chances of coupling. It also makes a first LO frequency and the frequency of the input signal is different from a large interfering signals can prevent "pollution" to small input signals.

Because of different reasons, some wireless communication systems, direct conversion (direct conversion) or within the differential (homodyne) architecture can replace superheterodyne architecture. In this architecture, the RF input signal is in a single step directly into baseband, therefore, most of the gain in the fundamental frequency, and the same frequency of the LO input signal. In this case, you must understand a few coupling influence, and must establish a "spurious signal path (stray signal path)" detailed model, for example: the coupling through the substrate (substrate), the package pins and wire coupling, and through the power line coupling (bondwire) between.

Fourth, the interference RF circuit simulation of adjacent channel

Transmitter distortion also plays an important role. Transmitter nonlinear output circuit generated by the bandwidth of the transmitted signal may cause spread to adjacent channels. This phenomenon is called "re-growth of the spectrum (spectral regrowth)". Before the signal reaches the transmitter power amplifier (PA), which is limited to the bandwidth; however, in the PA's "crosstalk" will lead to bandwidth increase again. If too much bandwidth increases, the transmitter will not meet the power requirements of its neighboring channels. When the transmission of digital modulation signals, in fact, it is impossible to predict the spectrum and then grow with SPICE. Since about 1000 the number sign (symbol) of transmission jobs must be simulated in order to achieve a representative of the spectrum, and also requires a combination of high-frequency carrier, which will make SPICE transient analysis impractical.

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