armow-band noise though the IF filter fom the RF amplifier and mixer plus the broadband 10 or 20 MHz generated within the IC. Some addi tional noise bandwidth filtering should therefore be provided between the Ic and the detector circuit Kg15鑫 Pad.ed alter Distortion must also be cansidered Since the F amplifiers must be capable of automatic gain control, they must have a square-law or second- order transfer characteristic(V./V.). Gain can then be changed by var ing the amplifiers bias. As long as the secondorder characteristic is me intained, no distortion will occur, assuming that the IF filters are relatively narow band, less than l octave. All harmonics and intermodulation pn ducts will then fall outside the pasaband, ao the amplifier will appear lin ear.But if any odd-order distortion is present, undesirable in-band mixing roducts and conpression will appear This type of distortion can obvious occur in the bipolar transistors normally used for the IF amplifiers, and the amoumt can be controlled through careful biasing and selection of the tran sistos used. It can also occur in some not-s0-obvious oampooents. Any quartz, ceramic, or mechanical filters involve paysical movement of tbeir emal elements and there will be symmetrical limits to this linear motion The filters themselves can therefore be a source of distortion, particularly if the applied signal leve is too high. Fenite materials commomly used in fil ters can also be source of nonlinearities. For very demanding applica tions, then, each component of the reoeiver must be carefully analyzed for its contribution to the final performance of the eceiver 偏置 大多数 2
ceramic 陶瓷 不足 demodulate 解调检波 diode 二极管 distinguishable 可分辨的 失真 派移 铁酸盐 florent 灯丝 滤波器 频率 harmonic 谐波 阻抗 不可免 互调 混频番 非线性 棋糊的 octave 倍频程 overloading 过载 板垫 通带 相 电位计 石英 辐射 resemble 像类似 射腻 灵敏度 边绿 specification 指标规格 spurious 寄生的,虚假的
超外差的 endell 趋向 发射机 atmospheric noise 大气噪声 平衡式混频器 调频立体声广播 镜象信号 incandescnet lamp 白炽灯 intermediate frequency 本地振荡器 负反馈 噪声系数 phase-locked loop 锁相环 tumed circuitS 调谐回路 双端口暑件 No′es: 1. When over 120 dB o rF gain is involved, every little bit helps. yE 及的射频增益超过120分贝,每一点都有用。 2. For each type o modulation used(i. e, AM, FM, SSB, PM),a number of different circuits exist. Some will have gain, others a loss 对于所应用的各种调制方式(如调幅、调频、单边带和脉冲调制),都有一 些不同的电路。一些可获得增益,另一些则插人损耗。 3. An otherwise good design can be useless if unwanted signal can sneak into the reoeiver at the IF frequency(s), the image frequency, at vari- ous spurious fre elated to ulation products, and hrough croee-modulatian problems.通过交调如果不期望信号不知不 觉地在中频进入接收机或镜像频率与互调分量有关的各种寄生频事那 再好的设计也没用。 4. Capture ratio:1.5dB.撸获率为1SdB即FM信号大于其它信号至 少1.5d,才能听到。 5. Sensitivity:0.5pfor10dBS+ N/N ratio.灵敏度:对于信号加噪声 功率与噪声功率比为10dB时,灵敏度为0.5pv 6. Some discrimination against unwanted mixing products can therefore be
had if all RF input levels to the mixer are kept as low as possible and the local oscillator signal kept as high as possible.如果到混频器的所 有射频输人电平都保持尽可能地低并且本振信号尽可能地高就能对一 些不期望的混频分量抑。 7. We have already seen that image has to be removed and also any group of frequencies that could themselves mix and produce an IF output. R 们已经看到,必须移去(滤除)镜像频率以及任何一组可以互相混频并产 生中频输出的频率 8. Any rapid voltage changes, bowever, will cause linear current nce the thermal time constamt of the filament will be long to hold the resistance constant for a while.然而任何快速的电压变化都 能导致电流线性变化因为灯丝的热时间常致将足够长可以暂时保持电 阻为常数。 1.2 Digital Design Digital design can be divided into two general areas. The first is the creation and production of direct hardware from available building block The second is the computer software or programming aspects, which may or may not involve the desigm of hardware items The second area employs techniques that are quite different from those of conventional hardware design and also require a substantial in vestment in special test equipment for efficient development and debug ging. The quantity of information needed is worthy of a separate book and is not covered here Marry aspects of computer technology, however, are very important in couventional hardware design. Among these are the programmable read-only memories referred to ag PROMs and the read-write memories called RAMs(random-access memories). The word firmware is commonly used for these applications The design of digital circuits differs greatly from the design of analo circuits, being more like a systems design on small scale. For the large part it consists of connecting standard building blocks without the use of
modifying passive components. This is very evident in the construction of these circuits, which often take on an even pattem as if put together by a cklayer. Many circuits do, of both analog parts, and the construction of these systems is a mintme of the two 1.2.1 Fundamentals nANd gate. A simple version is shown in Fig. 1.6 Rt D If both A and B are open or tied to a(+) voltage, a curent flon E ode. Eo is then near ground potential, and its output is called a0" orlow. "If either A or B or both are grounded, the current through R, is diverted to that ground. This provides a voltage drop of one diode at the amode of Do However, a voltage equal to two diode drop, that of Do and the transi tor, is required to tm the transistor am. With the transistor oi Eo ap- proaches the V+level and is called a"1"or"high It will be noticed that an inversion is involved in this circuit is, if the inputs are low, the output is high, and vice versa.To make circuit noninverting would require an extra stage. This is why digital cir- cuitry is based on inverting logic. The word NAND is a contraction of the phae“ INVERTNG AND