【正文】 thus, a 14,400 bits/s modem actually transmits at 2400 baud. ■ SYSTEM APPLICATIONFrequencyShift Keying (FSK) FSK is one method used to overe 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 frequencies within the telephone bandwidth. By the way, any frequency within the telephone bandwidth is an audible tone. The standard frequencies for a fullduplex 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 0 and 2225 Hz is a 1. The relationship of these FSK frequencies and the telephone bandwidth is illustrated in Figure 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 separation.Frequencies for 300 baud, fullduplex data transmission.An 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 1360, contain an LM565 PLL and a VCO integrated circuit. The VCO can be a device 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 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 e in on the control voltage input (pin 9) of the VCO via a levelshifting circuit formed by Q3 and circuit is used because the data from the RS232C interface are dual polarity with a positive voltage representing 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