三上复习短文1.doc

17、While the complex frequencys imaginary part () helps describe a response to AC signals, the real part () helps describe a circuits transient response. Looking at Figure 2.2b, we can therefore say something about the RC low-pass filters response as compared to that of the integrator. The low-pass filters transient response is more stable, because its pole is in the negative-real half of the complex plane. That is, the low-pass filter makes a decaying-exponential response to a step-function input; the integrator makes an infinite response. For the low-pass filter, pole positions further down the axis mean a higher, a shorter time constant, and therefore a quicker transient response. Conversely, a pole closer to the j axis causes a longer transient response. 18、Now lets examine the second-order functions frequency response and see how it varies with Q. As before, Figure 2.4a shows the function as a curved surface, depicted in the three-dimensional space formed by the complex plane and a vertical magnitude vector. Q =0.707, and you can see immediately that the response is a low-pass filter.The effect of increasing the Q is to move the poles in a circular path toward the axis. Figure 2.4b shows the case where Q = 2. Because the poles are closer to the axis, they have a greater effect on the frequency response, causing a peak at the high end of the passband.19、There is also an effect on the filters transient response. Because the poles negative-real part is smaller, an input step function will cause ringing at the filter output. Lower values of Q result in less ringing, because the damping is greater. On the other hand, if Q becomes infinite, the poles reach the axis, causing an infinite frequency response (instability and continuous oscillation) at. In the LCR circuit in Figure 2.3a, this condition would be impossible unless R=0. For filters that contain amplifiers, however, the condition is possible and must be considered in the design process.20、A second-order filter provides the variablesand Q, which allow us to place poles wherever we want in the complex plane. These poles must, however, occur as complex conjugate pairs, in which the real parts are equal and the imaginary parts have opposite signs. This flexibility in pole placement is a powerful tool and one that makes the second-order stage a useful component in many switched-capacitor filters. As in the first-order case, the second-order low-pass transfer function tends to zero as frequency tends to infinity. The second-order function decreases twice as fast, however, because of the factor in the denominator. The result is a double zero at infinity.21、The truth table is used as the beginning point in designing or analyzing a logic circuit. The sequential listing makes it easy to recognize if any input combinations were missed. It is made up either from the problem specifications or by sequential testing of an assembled circuit. A logic equation can be formulated from the truth table and a logic circuit can be developed from the equation. Truth tables are used in defining the basic AND, OR and NOT functions below.22、The logic AND function is defined as follows. An AND function will have a logic 1 output if and only if all of its input variables are in the logic 1 state. In True/False terminology, this says that the output will be true only if all of the inputs are true, and the output will be false if any of the inputs are false.The logic OR function, more precisely called the inclusive OR function, is defined by the following statement. An OR function will have a logic 1 output if any or all of its inputs have a logic 1 state, and a logic 0 output if and only if all of its inputs are logic 0.The logic NOT function is defined by the following statement. The output of a NOT function will be a logic 1 when its input is a logic 0 and the output will be a logic 0 when its input is a logic 1. The logic NOT function can have only one input.23、The flip-flop A flip-flop is basically a bi-state circuit in which either a 0 or 1 state can resides. Because of its simplicity, the flip-flop is extremely fast. As a basic element, the flip-flop is used in digital circuits and ICs. A flip-flop will lose its state when the supply voltage is removed. Therefore, it is volatile.The register A register is a set of flip-flops in parallel. Typically a register is 8, 16, 32 or 64 bits wide. Often a register is used to hold data, address pointers, etc. A register is volatile and very fast just like the flip-flop.24、SRAM (Static Random Access Memory) An SRAM is an array of addressable flip-flops. The array can be configured as such that the data comes out in single bit, 4-bit, 8bit, and etc. format. SRAM is simple, fast and volatile just like the flip-flop, its basic memory cell SRAM can be found on microcontroller boards (either on or off the CPU chip), where the amount of memory required is small and it will not payoff to build the extra interface circuitry for DRAMs. In addition, SRAM is often used as cache because of its high speed.25、A special case of SRAM memory is Content Addressable Memory (CAM). In this technology, the memory consists of an array of flip-flops, in which each row is connected to a data comparator. The memory is addressed by presenting data to it (not an address!). All comparators will then check simultaneously if their corresponding RAM register holds the same data. The CAM will respond with the address of the row (register) corresponding to the original data. The main application for this technology is fast look up tables. These are often used in network routers.26、DRAM memory is, just like SRAM memory constructed as an array of memory cells. A major difference between SRAM and DRAM, however, lies in the addressing technique. With an SRAM, an address needs to be presented and the chip will respond with presenting the data of the memory cell at the output, or accepting the data at the input and write it into the addressed cell. With DRAM technology, this simple approach is impossible since addressing a row of data without rewriting it will destruct all data in the row because of the dynamic nature.27、DC machines are characterized by their versatility. By means of various combinations of shunt, series, and separately excited field windings they can be designed to display a wide variety of volt-ampere or speed-torque characteristics for both dynamic and steady-state operation. Because of the ease with which they can be controlled, systems of DC machines are often used in applications requiring a wide range of motor speeds or precise control of motor output.28、As we know, the AC voltage generated in each rotating armature coil is converted to DC in the external armature terminals by means of a rotating commutator and stationary brushes to which the armature leads are connected. The commutator-brush combination forms a mechanical rectifier, resulting in a DC armature voltage as well as an armature m.m.f. wave which is fixed, in space. The brushes are located so that commutation occurs when the coil sides are in the neutral zone, midway between the field poles. The axis of the armature m.m.f. wave then is 90 electrical degrees from the axis of the field poles, i.e., in the quadrature axis.29、The outstanding advantages of DC machines arise from the wide variety of operating characteristics which can be obtained by selection of the method of excitation of the field windings. The field windings may be separately excited from an external DC source, or they may be self-excited; i.e., the machine may supply its own excitation. The method of excitation profoundly influences not only the steady-state characteristics, but also the dynamic behavior of the machine in control systems.30、An outstanding advantage of the shunt motor is ease of speed control. With a rheostat in the shunt-field circuit, the field current and flux per pole can be varie