【正文】 ting a 0 and a negative voltage representing a R8 is for adjusting the high level of the control voltage and R11 is for adjusting the low level for the purpose of finetuning the frequency. Transistor Q5 provides for originate/answer mode frequency selection by changing the value of the frequencyselection 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 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 data back to the originating DTE.The function of the PLL is to accept ining FSKmodulated 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 originatemode data from the other modem. The 741 opamp is connected as a parator that changes the data levels from the PLL to dualpolarity format for patibility with the RS232C interface.Now, so that you can take a closer look at the FSK modem board, let’s take it out of the system and put it on the troubleshooter’s bench.TROUBLESHOOTER’S BENCH■ ACTIVITY 1 Relate the PC Board to the SchematicLocate and identify each ponent and each input/output pin on the PC board in Figure 1361 using the schematic in Figure 1360. Verify that the board and the schematic agree. If the PC board and the schematic do not agree, indicate the problem. ■ COMMUNICATIONS CIRCUITS■ ACTIVITY 2 Analyze the CIRCUITSFor this application, the freerunning frequencies of both the PLL and the VCO circuits are determined by the formula in Equation (135).Step 1: Verify that the freerunning frequency for the PLL OC is approximately 1070 Hz in the originate mode and approximately 1270 Hz in the answer mode.Step 2: repeat Step 1 for the VCO.Step 3: Determine the approximate minimum and maximum output voltages for the 741 parator.Step 4: Determine the maximum highlevel voltage on pin 9 of the VCO.Step 5: If a 300 Hz square wave that varies from +5 V to –5 V is applied to the data from the DTE input, what should you observe on pin 4 of the VCO?Step 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 1270 Hz or 2225 Hz frequency if the transfer function of the VCO is 50 Hz/V? ■ 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 ponent Use the results of Activity 2 as appropriate.■ 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 interface.2. The LM565 properly demodulates 1070 Hz and 1270 Hz FSK data but does not properly demodulate 2025 Hz and 2225 Hz data.3. The VCO produces no FSK output.4. 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 DTE.139 REVIEW QUESTIONS1. The originate/answer input to the modem is low. In what mode is the system?2. What is the purpose of diode D1 in the FSK modem circuit?3. The VCO is transmitting 1070 Hz and 1270 Hz FSK signals. To what frequencies does the PLL respond from another modem?4. If the VCO is transmitting a constant 2225 Hz tone, what does this correspond to in terms of digital data? I what mode is the modem? ■ GLOSSARY■SUMMARY ■In amplitude modulation (AM), the amplitude of a higherfrequency carrier signal is varied by a lower–frequency 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 (intermediate frequency) amplifier, an AM detector, and audio and power amplifiers.■The IF in a standard AM receiver is 455 kHz.■The AGC (automatic gain control) in a receiver tends to keep the signal strength constant within the receiver to pensate for variations in the received signal.■In frequency modulation (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 deemphasis network. The IF is MHz.■A fourquadrant linear multiplier can handle any bination of voltage polarities on its inputs.■Amplitude modulation is basically a multiplication process.■The multiplication of sinusoidal signals produces sum and difference frequencies.■The output spectrum of a balanced modulator includes upperside and lowerside frequencies, but no carrier frequency.■The output spectrum of a standard amplitude modulator includes upperside and lowerside frequencies and the carrier frequency.■A linear multiplier is used as the mixer in receiver systems.■A mixer converts the RF signal down to the IF signal. The radio frequency varies over the AM or FM band. The intermediate frequency is constant.■One type of AM demodulator consists of a multiplier followed by a lowpass filter.■The audio and power amplifiers boost the output of the detector or discriminator and drive the speaker.■A voltagecontrolled oscillator (VCO) produces an output frequency that can be varied bb a control voltage. Its operation is based on a variable reactance.■A VCO is a ba