电子信息与通信工程专业英语lesson2-3(太原理工大学版)

1、2.3 Radio Receiver Circuit RF, the radio frequency portion of the radio spectrum is considered to extend from 3 to 1,000 MHz (1 GHz). At the opposite end of the spectrum, below the high frequency (HF) band (3-30 MHz), the AM radio band extends from 650 to 1,650 kHz. At these relatively low frequenci

2、es the effects of parasitic inductance and capacitance on circuit designs are minimal. OutlineThe superheterodyne receiver-analog system Receiver characterization 8/15/20221通信与信息工程专业英语教程电子信息与通信工程专业英语Terminologyanalog electronics 模拟电子技术digital signal processing（DSP） 数字信号处理superheterodyne（superhet） 电子

3、超外差式收音机；adj.超外差的selectivity n.选择性sensitivity n.敏感, 灵敏(度)，灵敏性preselector n.预选器，调谐器transmitter n.转送者, 传达人发报机, 话筒, 发射机, 让渡者the receive phase 接收相位harmonics n.谐波impedance match 阻抗匹配8/15/20222通信与信息工程专业英语教程电子信息与通信工程专业英语undesired signal 干扰信号the local oscillator 本机震荡器intermediate frequency（IF） 中频modulation s

4、idebands 调制边带overload characteristics 过载特性demodulator n.解调器sophisticated error 经验误差additive, white, Gaussian noise n.加性高斯白噪声power spectral density 功率谱密度megahertz n.物兆赫 (MHz)兆赫AGC abbr.自动增益控制Spurious responses (接收机)乱真信号响应8/15/20223通信与信息工程专业英语教程电子信息与通信工程专业英语 The superheterodyne, or superhet, receiver,

5、 is a well-known and almost universal receiver architecture for radio receivers. We will see that such a receiver can provide both good selectivity and sensitivity, because the noise bandwidth can be limited to the channel bandwidth without compromising the receivers ability to tune across the entir

6、e RF band. Its basic components are shown in Fig.2-3-1. IntroductionText tour1. The superheterodyne receiver-analog system 8/15/20224通信与信息工程专业英语教程电子信息与通信工程专业英语8/15/20225通信与信息工程专业英语教程电子信息与通信工程专业英语The purpose of this component is to filter out all unwanted signals lying outside the RF band containing

7、the possible channels to be detected. Unwanted signals can include signals fed from the transmitter itself, which might share a common antenna. In this case the preselector is either a diplexer, a filter that allows one frequency band to pass between the antenna and receiver and another (different)

8、band to pass between the transmitter and antenna (as, for example, in CDMA mobile systems), and/or a transmit/receive switch that turns on during the receive phase and off during the transmit phase (as, for example, in GSM systems). RF preselector8/15/20226通信与信息工程专业英语教程电子信息与通信工程专业英语The function of t

9、he preselection filter is also to suppress the undesired responses at the output that arise from incoming signals lying at spurious frequencies that the receiver is not tuned to receive . We will see that such spurious frequencies can include the image frequency as well as harmonics of the incoming

10、tuned frequency. Ideally, the preselector will also have a good impedance match in-band to avoid bandpass ripple. 8/15/20227通信与信息工程专业英语教程电子信息与通信工程专业英语 RF amplifierThe function of the input amplifier is to linearly amplify the input signal and minimize the noise added by the receiver to the signal it

11、self. It should have a good input and output match to avoid gain ripple. In addition, the input amplifier must not introduce distortion of the signal, because strong signals may be simultaneously present in adjacent (unwanted) channels, and any nonlinear distortion of the amplifier could swamp a wea

12、ker signal in the channel we are trying to detect. Of course, the RF amplifier can consist of multiple stages in order to provide the necessary gain.8/15/20228通信与信息工程专业英语教程电子信息与通信工程专业英语Interstage selectorThe preceding amplifier will provide gain to all the channels within the RF bandwidth, and its g

13、ain is likely to roll off slowly beyond it. Furthermore, the amplifier will amplify noise across the entire band, and possibly at the image frequency as well. Therefore, this (optional) component is a filter to suppress any gain of undesired signal responses at spurious frequencies, and in particula

14、r at the image frequency. It thus maintains the system noise figure by preventing image noise from entering the mixer. It also helps to minimize LO reradiation from the RF mixer port. This component should have low in-band loss.8/15/20229通信与信息工程专业英语教程电子信息与通信工程专业英语 This is a strong signal that is nor

15、mally generated by a frequency synthesizer, and is typically tuned across a bandwidth equal to the entire RF bandwidth, but offset from it, to choose any desired channel. Its function is to drive the devices within the mixer into a nonlinear regime for frequency translation (mixing). An important os

16、cillator specification is its phase noise, since any phase fluctuation on the oscillator signal is directly superimposed on the mixer output signal. Its broadband noise should also be low so as not to raise the system noise floor. It will also require a good tuning range or bandwidth, and low spurio

17、us and harmonic content. Local oscillator8/15/202210通信与信息工程专业英语教程电子信息与通信工程专业英语First mixerThis component translates all incoming signals in the RF frequency range into signals in some intermediate frequency range, depending on the local oscillator signal frequency. The mixer translates all frequencie

18、s linearly, preserving phase information within the new range of frequencies. Within some range of RF signal amplitudes, the amplitude of the output signal is also preserved on the IF. It requires low LO feed through to the RF and IF, and a large spurious-free dynamic range for the incoming RF signa

19、l. The selection of the IF frequency is important in ensuring the receiver response to unwanted spurious responses is minimized.IF frequencies from 45 to 82 MHz are common for mobile radio receivers in the 800-MHz band, and from 110 to 300 MHz for radios in the 1,800-MHz band or for both bands. 8/15

20、/202211通信与信息工程专业英语教程电子信息与通信工程专业英语IF filter This component rejects the unwanted signal components generated by the mixer and other components. Its bandwidth must be sufficiently wide to pass the modulation sidebands in the desired channel without distortion. It can be high Q because it is of fixed fr

21、equency. The blocking and overload characteristics of the receiver are often determined here in combination with the following.8/15/202212通信与信息工程专业英语教程电子信息与通信工程专业英语IF amplifierThis component should provide adequate gain to the IF signal to drive the following stages. Because it is at a fixed frequen

22、cy it can provide high gain and be well stabilized.8/15/202213通信与信息工程专业英语教程电子信息与通信工程专业英语Second mixer, local oscillator, and second filter Sometimes known as the second IF strip, these components mirror the functions of the first IF strip, but across a different frequency band and with different pass

23、band characteristics. The power handling capability (intercept point) is generally higher. The purpose of a second IF strip will unfold later in this chapter, but as a general rule, the first IF strip is designed to receive the entire RF passband and to reject spurious frequencies and the image freq

24、uency in particular, while the function of the second IF strip is to narrow in and select the desired channel from the entire passband, thereby providing additional selectivity to the receiver. For commercial AM radios, 455 kHz is a fairly standard second IF frequency, while mobile phone receivers t

25、ypically use frequencies around 10 MHz. 8/15/202214通信与信息工程专业英语教程电子信息与通信工程专业英语Demodulator This component extracts the modulated signal from the IF signal and converts it to baseband. In the case of an analog system, this information will be either AM or FM; for a digital system, it will typically be

26、symbols having multiamplitude levels that are later decoded. (A symbol is the way a bit or combinations of bits are coded in the waveform.)8/15/202215通信与信息工程专业英语教程电子信息与通信工程专业英语Baseband amplifierThis provides output power to derive drive the relevant output device, which could typically be a speaker,

27、 fax output, or video screen.8/15/202216通信与信息工程专业英语教程电子信息与通信工程专业英语2. Receiver characterizationCommunications channelA result that can be derived from the above is that for a given channel with some maximum information throughput, the power (or more specificallv, the signal-to-noise ratio at the dete

28、ctor) needed to transmit that information through it can be traded off for spectral occupancy or bandwidth according toThus if the spectral occupancy can be increased, the signal-to-noise ratio SNR can be lower for the same information transfer rate. 8/15/202217通信与信息工程专业英语教程电子信息与通信工程专业英语Receiver noi

29、seAlthough we sometimes refer to noise voltage and noise current per hertz, in RF systems we typically measure power into a fixed reference impedance level (usually). In this case, the fundamental result is noise power where K is Boltzmanns constant and T is the temperature in Kelvin. Clearly, the l

30、arger the bandwidth, the greater the noise power. It is for this reason that the final IF filter needs to be as narrow as possible, in order to minimize the noise power just prior to demodulation and detection. This final IF filter determines the overall noise bandwidth of the entire receiver since

31、it will be the most narrowband component in the entire chain prior to detection. 8/15/202218通信与信息工程专业英语教程电子信息与通信工程专业英语Receiver sensitivityThis minimum input signal strength needed to produce a good quality output signal is referred to as the receiver sensitivity. However, just as there are many defi

32、nitions of what constitutes acceptable quality, there are equally numerous definitions of sensitivity. Most audiophiles believe that a signal-to-noise ratio of at least 10 dB is necessary for acceptable sound quality, so that an output signal-to-noise ratio of 10 dB is used to measure the system sen

33、sitivity. 8/15/202219通信与信息工程专业英语教程电子信息与通信工程专业英语System nonlinearityThe components in any system ultimately reach a point at which they become nonlinear， essentially a component is nonlinear when its output amplitude or phase is no longer linearly proportional to its input amplitude or phase. In most

34、changes active devices, this results predominantly from amplitude-induced in the device transconductance, but the variation of device resistances and capacitances with voltage amplitude also contributes to nonlinear behavior. Gross nonlinearity arises from cutoff and saturation effects that occur as

35、 the device exceeds the limits of its normal active momentarily. As a result, we observe effects such as gain distortion and cross modulation such as AM-to-PM conversion. 8/15/202220通信与信息工程专业英语教程电子信息与通信工程专业英语Receiver dynamic rangeIn order to increase the useful range of a receiver, the gain of a sys

36、tem can be automatically controlled in order to decrease the gain when strong signals cause overload or distortion. However, the impact of doing this is not always obvious and will require some care in considering the signal-to-noise ratio and making a number of trade-offs. For instance, consider AG

37、C applied to an LNA. AGC is typically applied in an analog receiver. As the desired signal increases in input power, the gain of the system is reduced in order to minimize distortion. The gain of the output stages is ideally reduced first to avoid raising the system noise figure and reducing sensitivity, and as ear