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1、外文翻译-相位锁相环 外文翻译 THE PHASE-LOCKED LOOP PLL The PLL as an FM DemodulatorAs you have seen the VCO control voltage in a PLL depends on the deviation of the incoming frequency The PLL will produce a voltage proportional to the frequency of the incoming signal which in the case of FM is the original modul
2、ating signalFigure 13-54 shows a typical connection for the LM565 as an FM demodulator If the IF input is frequency modulated by a sinusoidal signal you get a sinusoidal signal on the output as indicated Since the imum operating frequency is 500 kHz this device must be used in double-conversion FM r
3、eceiver is one in which essentially two mixers are used to first convert the RF to a 107 MHz IF and then convert this to a 455 kHz IFThe free-running frequency of the VCO is adjusted to approximately 455 kHz which is the center of the modulated IF range C can be any value but R should be in the rang
4、e from 2k to 20k The input van be directly coupled as long as there is no dc voltage difference between pins 2 and 3The VCO is connected to the phase detector by an external wire between pins 4 and 5Determine the values for RC and C for the LM565 in Figure 13-54 for a free-running frequency of 455 k
5、Hz and a capture range of 10kHzThe dc supply voltages are 6VSolution Use Equation 13-5 to calculate CChoose R 47k COMMUNICATIONS CIRCUITSThe lock range and capture range must be determined before C can be calculated The lock range isUse Equation 13-7 to calculate CThereforePractice Exercise What can
6、 you do to increase the capture range from 10 kHz to 15 kHz 13-8 REVIEW QUESTIONS 1 List the three basic components in a phase-locked loop 2 What is another circuit used in some PLLs other than the three listed in Question 1 3 What is the basic function of a PLL 4 What is the difference between the
7、lock range and the capture range lf a PLL 5 Basically how does a PLL track the incoming frequency A SYSTEM APPLICATION The DCE data communications equipment system introduced at the opening of this chapter includes an FSK frequency shift keying modem modulatordemodulator FSK is one method for modula
8、ting digital data for transmission over voice phone lines and is basically a form of frequency modulation In this system application the focus is on the low-speed modulatordemodulator modem board which is implemented with a VCO for transmitting FSK signals and a PLL for receiving FSK signalsAfter co
9、mpleting this section you should be able to Apply what you have learned in this chapter to a system applicationDescribe how a VCO and a PLL can be used in a communications system Discuss how FSK is used to send digital information over phone lines Translate between a printed circuit board and a sche
10、matic Analyze the modem circuitry Troubleshoot some common problems A SYSTEM APPLICATIONA Brief Description of the SystemThe FSK modem interfaces a computer with the telephone network so that digital data which are incompatible with the standard phone system because of bandwidth limitations can be t
11、ransmitted and received over regular phone lines thus allowing computers to communicate with each other Figure 13-55 shows a diagram of a simple data communications system in which a modem at each end of the phone line provides interfacing for a computerA data communications system The modem DCE con
12、sists of three basic functional blocks as shown in Figure 13-56the FSK modem circuits the phone line interface circuits and the timing and control circuits The dual polarity power supply is not shown Although the focus lf this system application is the FSK modem board we will briefly look at each of
13、 the other parts to give you a basic idea of the overall system functionBasic block diagram of a modem COMMUNICATIONS CIRCUITSThe Phone Line Interface The main purposes of this circuitry are to couple the phone line to the modem by proper impedance matching to provide necessary filtering and to acco
14、mmodate full-duplex transmission of data Full-duplex means essentially that information can be going both ways on a single phone line at the same time This allows a computer connected to a modem to be sending data and receiving data simultaneously without the transmitted data interfering with the re
15、ceived data Full-duplexing is implemented by assigning the transmitted data one bandwidth and the received data another separate bandwidth within the 300 Hz to 3 kHz overall bandwidth of the phone networkTiming and Control One basic function of the timing and control circuits is to determine the pro
16、per mode of operation for the modem The two modes are the originate mode and the answer mode Another function is to provide a standard interface such as RS-232C with the DTE computer The RS-232C standard requires certain defined command and control signals data signals and voltage levels for each si
17、gnalDigital Data Before we get into FSK lets briefly review digital data A detailed knowledge of binary numbers is not necessary for this system application Information is represented in digital form by 1s and 0s which are the binary digits or bits In terms of voltage waveforms a 1 is generally repr
18、esented by a high level and a 0 by a low level A stream of serial data consists if a sequence of bits as illustrated by an example in Figure 13-57 a FIGURE 13-57A serial stream of digital dataBaud Rate A low-speed modem such as the one we are focusing on sends and receives digital data at a rate of
19、300 bitss or 300 baud5For example if we have an alternating sequence of 1s and 0s highs and lows as indicated in Figure 13-57 b each bit takes 333 ms Since it takes two bits a 1 and a 0 to make up the period of this particular waveform the fundamental frequency of this format is 1667 ms 150 Hz This
20、is the imum frequency of a 300 baud data stream because normally there may be several consecutive 1s andor several consecutive zeros in a sequence thus reducing the frequency As mentioned earlier the telephone network has a 300 Hz minimum frequency response so the fundamental frequency of the 300 ba
21、ud data stream will fall outside of the telephone bandwidth This prevents sending digital data in its pure form over the phone linesTechnically bit rate and baud rate are not the same Baud rate indicates how many frequency shifts are sent per second Each frequency shift can represent more than one b
22、it thus a 14400 bitss modem actually transmits at 2400 baud SYSTEM APPLICATIONFrequency-Shift Keying FSK FSK is one method used to overcome the bandwidth limitation of the telephone so that digital data can be sent over the phone lines The basic idea of FSK is to represent 1s and 0s by two different
23、 frequencies within the telephone bandwidth By the way any frequency within the telephone bandwidth is an audible tone The standard frequencies for a full-duplex 300 baud modem in the originate mode are 1070 Hz for a 0 called a space and 1270 Hz for a 1 called a mark In the answer mode 2025 Hz is a
24、0 and 2225 Hz is a 1 The relationship of these FSK frequencies and the telephone bandwidth is illustrated in Figure 13-58Signals in both the originate and answer bands can exist at the same time on the phone line and not interfere with each other because of the frequency separationFrequencies for 30
25、0 baud full-duplex data transmissionAn example of a digital data stream converted to FSK by a modem is shown in Figure Example if FSK data COMMUNICATIONS CIRCUITSModem Circuit Operation The FSK modem circuits shown in Figure 13-60 contain an LM565 PLL and a VCO integrated circuit The VCO can be a de
26、vice such as the 4046 not covered specifically in this chapter which is a PLL device in which the VCO portion van be used by itself because all of the necessary inputs and outputs are available The VCO in the LM565cannot be used independently of the PLL because there is on input pin for the control
27、voltage The function of the VCO is to accept digital data from a DTE and provide FSK modulation The VCO is always the transmitting device The digital data come in on the control voltage input pin 9 of the VCO via a level-shifting circuit formed by Q3 and Q4This circuit is used because the data from
28、the RS-232C interface are dual polarity with a positive voltage representing a 0 and a negative voltage representing a 1Potentiometer R8 is for adjusting the high level of the control voltage and R11 is for adjusting the low level for the purpose of fine-tuning the frequency Transistor Q5 provides f
29、or originateanswer mode frequency selection by changing the value of the frequency-selection resistance from pin 11 to ground Transistors Q1 and Q2 perform a similar function for the PLL A SYSTEM APPLICATIONWhen the digital data are at high levels corresponding to logic 0s the VCO oscillates at 1070
30、 Hz in the originate mode and 2025 Hz in the answer mode When the digital data are at low levels corresponding to logic 1s the VCO oscillates at 1270 Hz in the originate mode and 2225 Hz in the answer mode An example of the originate mode is when the receiving DTE responds to a request and sends dat
31、a back to the originating DTEThe function of the PLL is to accept incoming FSK-modulated data and convert it to a digital data format for use by the DTD The PLL is always a receiving device When the modem is in the originate mode the PLL os receiving originate-mode data from the other modem The 741
32、op-amp is connected as a comparator that changes the data levels from the PLL to dual-polarity format for compatibility with the RS-232C interfaceNow so that you can take a closer look at the FSK modem board lets take it out of the system and put it on the troubleshooters benchTROUBLESHOOTERS BENCH
33、ACTIVITY 1 Relate the PC Board to the SchematicLocate and identify each component and each inputoutput pin on the PC board in Figure 13-61 using the schematic in Figure 13-60 Verify that the board and the schematic agree If the PC board and the schematic do not agree indicate the problem COMMUNICATI
34、ONS CIRCUITS ACTIVITY 2 Analyze the CIRCUITSFor this application the free-running frequencies of both the PLL and the VCO circuits are determined by the formula in Equation 13-5 Step 1 Verify that the free-running frequency for the PLL OC is approximately 1070 Hz in the originate mode and approximat
35、ely 1270 Hz in the answer modeStep 2 repeat Step 1 for the VCOStep 3 Determine the approximate minimum and imum output voltages for the 741 comparatorStep 4 Determine the imum high-level voltage on pin 9 of the VCOStep 5 If a 300 Hz square wave that varies from 5 V to 5 V is applied to the data from
36、 the DTE input what should you observe on pin 4 of the VCOStep 6 When the data from the DTE are low pin 9 of the VCO is at approximately 0V At this level the VCO oscillates at 1070 Hz or 2025 Hz When the data from the DTE go high to what value should the voltage at pin 9 be adjusted to produce a 127
37、0 Hz or 2225 Hz frequency if the transfer function of the VCO is 50 HzV ACTIVITY 3 Write a Technical Report Describe the overall operation of the FSK modem board Specify how each circuit works and what its purpose is Identify the function of each component Use the results of Activity 2 as appropriat
38、e ACTIVITY 4 Troubleshoot the System for Each of the Following Problems By Stating the probable Cause or Causes1 There is no demodulated data output voltage when there are verified FSK data from the phone line interface2 The LM565 properly demodulates 1070 Hz and 1270 Hz FSK data but does not proper
39、ly demodulate 2025 Hz and 2225 Hz data3 The VCO produces no FSK output4 The VCO produces a continuous 1070 Hz tone in the originate mode and a continuous 2025 Hz tone in the answer mode when there are proper data from the DTE13-9 REVIEW QUESTIONS1 The originateanswer input to the modem is low In wha
40、t mode is the system2 What is the purpose of diode D1 in the FSK modem circuit3 The VCO is transmitting 1070 Hz and 1270 Hz FSK signals To what frequencies does the PLL respond from another modem4 If the VCO is transmitting a constant 2225 Hz tone what does this correspond to in terms of digital dat
41、a I what mode is the modem GLOSSARYSUMMARY In amplitude modulation AM the amplitude of a higher-frequency carrier signal is varied by a lowerfrequency modulating signal usually an audio signal A basic superheterodyne AM receiver consists of an RF amplifier not always a mixer a local oscillator an IF
42、 intermediate frequency amplifier an AM detector and audio and power amplifiersThe IF in a standard AM receiver is 455 kHzThe AGC automatic gain control in a receiver tends to keep the signal strength constant within the receiver to compensate for variations in the received signalIn frequency modula
43、tion FM the frequency of a carrier signal is varied by a modulating signal A superheterodyne FM receiver is basically the same as an AM receiver except that it requires a limiter to keep the IF amplitude constant a different kind of detector or discriminator and a de-emphasis network The IF is 107 M
44、HzA four-quadrant linear multiplier can handle any combination of voltage polarities on its inputsAmplitude modulation is basically a multiplication processThe multiplication of sinusoidal signals produces sum and difference frequenciesThe output spectrum of a balanced modulator includes upper-side
45、and lower-side frequencies but no carrier frequencyThe output spectrum of a standard amplitude modulator includes upper-side and lower-side frequencies and the carrier frequencyA linear multiplier is used as the mixer in receiver systemsA mixer converts the RF signal down to the IF signal The radio
46、frequency varies over the AM or FM band The intermediate frequency is constantOne type of AM demodulator consists of a multiplier followed by a low-pass filterThe audio and power amplifiers boost the output of the detector or discriminator and drive the speakerA voltage-controlled oscillator VCO pro
47、duces an output frequency that can be varied bb a control voltage Its operation is based on a variable reactanceA VCO is a basic frequency modulator when the modulating signal is applied to the control voltage inputA phase-locked loop PLL is a feedback circuit consisting of a phase detector a low-pa
48、ss filter a VCO and sometimes an amplifierThe purpose of a PLL is to lock onto and track incoming frequenciesA linear multiplier can be used as a phase detectorA modem is a modulatordemodulatorDTE stands for digital terminal equipmentDCE stands for digital communications equipment GLOSSARY Key terms are in color All terms are included in the end-of-book glossaryAmplitude modula
