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Chapter 12 Voice Modes

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Chapter 12 Voice Modes Modulation Sources (What and How We Communicate) 12 odulation Sources”—what does that mean? An engineer might simply call this chapter Baseband. Engineers use that term to distinguish information before it’s used to modulate “M a carrier. So, this chapter covers the various kinds of information we impress on RF (audio, video, digital, remote control) before that information has been moved to some intermediate frequency (IF) or the desired RF. Baseband carries the connotation “at or near dc” because the final RF is usually much higher than the baseband frequency, yet that is misleading. For example, a baseband ATV signal usually extends up to 5 MHz, which is hardly dc. Nonetheless, compared to the operating frequencies (52 to 806 MHz for broadcast, 420 MHz and higher for amateur) it is practically dc. Here we will discuss characteristics of the information (such as bandwidth), how we prepare it for transmission, optimize the transfer and process it after reception. Nearly all of the processing discussed in this chapter can be implemented with the emerging digital- signal-processing (DSP) technology. For example, the CLOVER-II system described later in this chap- ter uses DSP to vary the modulation scheme as required by propagation conditions. Look to the Digital Signal Processing chapter for further information about DSP techniques. BANDWIDTH Whenever information is added to a carrier (we say the carrier is modulated), sidebands are produced. Sidebands are the frequency bands on both sides of a carrier resulting from the baseband signal varying some characteristic of the carrier. The modulation process creates two sidebands: the upper sideband (USB) and the lower sideband (LSB). The width of each sideband is generally equal to the highest frequency component in the baseband signal. In some modulation systems, the width of the sidebands may greatly exceed the highest baseband frequency component. The USB and LSB are mirror images of each other and carry indentical information. Some modulation systems transmit only one sideband and partially or completely suppress the other in order to conserve bandwidth. According to FCC Rules, occupied bandwidth is: The frequency bandwidth such that, below its lower and above its upper frequency limits, the mean powers radiated are each equal to 0.5 percent (–23 dB) of the total mean power radiated by a given emission. In some cases a different relative power level may be specified; for example, –26 dB (0.25%) is used to define bandwidth in §97.3(a)(8) of the FCC rules. Modulation Sources (What and How We Communicate) 12.1 Occupied bandwidth is not always easy for amateurs to determine. It can be measured on a spectrum analyzer, which is not available to most amateurs. Occupied bandwidth can also be calculated, but the calculations require an understanding of the mathematics of information theory and are not covered in this book. The FCC has defined necessary bandwidth as: For a given class of emission, the minimum value of the occupied bandwidth sufficient to ensure the transmission of information at the rate and with the quality required for the system employed, under specified conditions. 12.2 Chapter 12 Voice Modes AMPLITUDE MODULATION (AM) AM voice was the second mode used in Amateur Radio (after Morse code). AM techniques are the basis for several other modes, such as single-sideband voice and AFSK. This material was supplied by Jeff Bauer, WA1MBK. AM is a mixing process. When RF and AF signals are combined in a standard AM modulator, four output signals are generated: the original RF signal (carrier), the original AF signal, and two sidebands, whose frequencies are the sum and difference of the original RF and AF signals, and whose amplitudes are propor- tional to that of the original AF signal. The sum component is called the upper sideband (USB). It is direct: A frequency increase of the modulating AF causes a frequency increase in the RF output. The difference component is called the lower sideband (LSB), which is inverted: A frequency increase in the modulating AF produces a decrease in the output frequency. The amplitude and frequency of the carrier are unchanged by the modulation process, and the original AF signal is rejected by the RF output network. The RF envelope (sum of sidebands and carrier), as viewed on an oscilloscope, has the shape of the modulating waveform. Fig 12.1B shows the envelope of an RF signal that is 20% modulated by an AF sine wave. The envelope varies in amplitude because it is the vector sum of the carrier and the sidebands. A spectrum analyzer or selective receiver will show the carrier to be constant. The spectral photograph also shows that the bandwidth of an AM signal is twice the highest frequency component of the modulating wave. An AM signal cannot be frequency multiplied without special processing because the phase/frequency relationship of the modulating-waveform components would be severely distorted. For this reason, once an AM signal has been generated, its frequency can be changed only by heterodyning. All of the information in an AM signal is contained in the sidebands, but two-thirds of the RF power is in the carrier. If the carrier is suppressed in the transmitter and reinserted (in the proper phase) in the receiver, signifi- cant advantages accrue. When the reinserted carrier is strong compared to the incoming double-sideband signal (DSB), exalted carrier reception is achieved, and distortion from selective fading is reduced (A) greatly. A refinement called synchronous detection uses a PLL to reject interference. Suppressing the carrier also eliminates the heterodyne in- terference common with adja- cent AM signals. More impor- tant, eliminating the carrier increases overall transmitter (B) efficiency. Transmitter power requirements are reduced by Fig 12.1—Electronic displays of AM signals in the frequency and time domains. A shows an unmodulated carrier or single-tone SSB 66%, and the remaining 34% signal. B shows a full-carrier AM signal modulated 20% with a sine has a light duty cycle. wave. Modulation Sources (What and How We Communicate) 12.3 Mathematics of AM AM can happen at low levels (as in a driver or predriver stages of a transmitter) or high levels, in the final output stage. The numbers are consistent for both methods. For example, to 100% modulate a 10 W RF carrier, 5 W of clean AF is required from the modulator. Good engineering calls for a 25% overdesign; thus 6.25 W of AF allows plenty of system headroom. The circuitry can then “loaf along” at the 100% level. Overmodulation Overmodulation occurs when more audio is impressed on a carrier than is needed for 100% modula- tion. It is also known as flattopping. Overmodulation causes distortion of the information conveyed and produces splatter (spurious emissions) on adjacent frequencies. Splatter interferes with others sharing our already-crowded bands: Prevent overmodulation at all times. Years ago amateurs used ingenious ways to detect and prevent or control overmodulation. Nowadays with solid-state, large-scale integration (LSI) chips and microprocessor control, bullet-proof overmodulation prevention can be designed into transmitters. The most familiar method is called ALC, for automatic level control. Although modern use of full-carrier AM on the amateur bands is very limited, there is a core group of AM aficionados experimenting with pulse duration modulation (PDM). This system was pioneered in AM broadcast transmitters. This form of high-level modulation differs from conventional AM in that the PDM modulator operates in switching mode, with audio information contained during on-pulses. The audio amplitude is therefore determined by the duty cycle of the modulator or switching tube. Those interested in further reading on the topic should look in William Orr’s (W6SAI) Radio Handbook (published by Howard W. Sams and Co) and the AM Press/Exchange (for contact information, see the Address List in the References chapter). Balanced Modulators The carrier can be suppressed or nearly eliminated by using a balanced modulator or an extremely sharp filter. Contemporary amateur transmitters often use both methods. The basic principle of any balanced modulator is to introduce the carrier in such a way that only the sidebands will appear in the output. The balanced-modulator circuit chosen by a builder depends on constructional considerations, cost and the active devices to be employed. In any balanced-modulator circuit, there is (theoretically) no output when no audio signal is applied. When audio is applied, the balance is upset, and one branch conducts more than the other. Since any modulation process is the same as “mixing,” sum and difference frequencies (sidebands) are generated. The modulator is not balanced for the sidebands, and they appear in the output. SINGLE-SIDEBAND (SSB) A further improvement in communications effectiveness can be obtained by transmitting only one of the sidebands. When the proper receiver bandwidth is used, a single-sideband (SSB) signal will show an effective gain of up to 9 dB over an AM signal of the same peak power. Because the redundant information is eliminated, the required bandwidth of an SSB signal is half that of a comparable AM or DSB emission. Unlike DSB, the phase of the local carrier generated in the receiver is unimportant. Table 12.1 shows the qualities of a good SSB signal. SSB Generation: The Filter Method If the DSB signal from the balanced modulator is applied to a narrow band-pass filter, one of the 12.4 Chapter 12 sidebands can be greatly at- Table 12.1 tenuated. Because a filter can- Guidelines for Amateur SSB Signal Quality not have infinitely steep skirts, Parameter Suggested Standard the response of the filter must Carrier suppression At least 40-dB below PEP begin to roll off within about Opposite-sideband suppression At least 40-dB below PEP 300 Hz of the phantom carrier Hum and noise At least 40-dB below PEP Third-order intermodulation distortion At least 30-dB below PEP to obtain adequate suppression Higher-order intermodulation distortion At least 35-dB below PEP of the unwanted sideband.
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