RF circuits (RF circuit) the number of special features , it is difficult to explain briefly a few words , you can not 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) ... and so on 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 .
Wireless transmitter and receiver conceptually can be divided into two parts, the baseband and RF . The output signal of the frequency range of the frequency range of the input baseband signal includes a transmitter , and also contains the receiver. Determines the bandwidth of the baseband data rate of the system substantially flowable . The fundamental frequency is used to improve the reliability of the data stream , 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 . The RF transmitter circuit can already processed baseband signal conversion , frequency up to a specified channel , and the injection of the signal to the transmission medium . Instead , the receiver circuit to obtain the RF signal from the transmission medium , and convert into a baseband frequency down .
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 can not 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 unwanted signals outside the desired channel ; final point as the transmitter , they consume power must small.
Small desired signal
The receiver must be very sensitive to detect small input signal. 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 superheterodyne (superheterodyne) receiver. The received signal is first filtered , and then to a low noise amplifier (LNA) to amplify 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 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 requires a receiver must have a great zoom function usually requires such a high gain 120 dB . 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 frequency of the first LO frequency and the input signal type , the interference signal can prevent large "pollution" of the small input signal.
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 directly converted in a single step into a base frequency , and therefore , most of the gain in the fundamental frequency and the LO frequency is the same as the 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: the coupling through the substrate (substrate) , the package pins and wire coupling, and the coupling through the power supply line (bondwire) between .
Large interference 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 . The interference signal may be large than expected 60 ~ 70 dB signal and the receiver may be a large number of input stage cover means or the receiver excessive amount of noise in the input stage , to block the normal signal reception . 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.
Therefore, the " linear" PCB is also an important consideration when designing receivers . 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 , so that the bandwidth of the transmission signal may be spread to adjacent channels . This phenomenon is called " spectral regrowth (spectral regrowth)". Before the signal reaches the transmitter power amplifier (PA), with which the bandwidth is limited ; but in the PA " crosstalk " causes the bandwidth increases again . If too much bandwidth increases , the transmitter will be unable to meet the power requirements of its adjacent channels. When sending the digital modulation signal , in fact, can not be predicted spectral re-growth with the 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.
RF PCB technology
With the development of communication technology , use of hand-held radio frequency circuit technology more widely , such as: radio pager , mobile phone , a wireless PDA, wherein the RF circuit performance directly affects the quality of the product . A most important feature of these products is smaller handheld , meaning the density miniaturized components is large, which makes the components ( including SMD, SMC, die , etc. ) interfere with each other very prominent . Electromagnetic interference signal if handled improperly , could cause the entire electrical system does not work, therefore , how to prevent and suppress electromagnetic interference, improve electromagnetic compatibility, has become a very important when designing RF pcb issue . The same circuit , different PCB design structure , its performance will vary greatly.