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25-2 An amplitude modulator using an NPN bipolar transistor.
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410 Wireless Transmitters and Receivers
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25-3 Spectral display of a typical AM voice
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communications signal.
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sideband (LSB); the RF from the carrier frequency to +3 kHz represents the upper sideband (USB). The bandwidth is the difference between the maximum and minimum sideband frequencies, in this case 6 kHz. In an AM signal, the bandwidth is twice the highest audio modulating frequency. In the example of Fig. 25-3, all the voice energy is at or below 3 kHz, so the signal bandwidth is 6 kHz. This is typical of a communications signal. In standard AM broadcasting, the AF energy is spread over a wider bandwidth, nominally 10 kHz.
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Single Sideband In AM, most of the RF signal power is consumed by the carrier alone; the two sidebands are mirrorimage duplicates. This is inefficient, and is also unnecessarily redundant! If the carrier and one of the sidebands is eliminated, these shortcomings can be overcome. That makes the signal stronger for a given amount of RF power, or allows the use of lower RF power in a given communications scenario. Another bonus is the fact that the bandwidth is reduced to less than half that of an AM signal modulated with the same data, so more than twice as many signals can fit into a specific range, or band, of frequencies. When the carrier is removed from an AM signal along with one of the sidebands, the remaining RF energy has a spectral display resembling Fig. 25-4. This is single sideband (SSB) transmission. Either the LSB or the USB alone can be used, with equally good results.
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Modulation 411
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25-4 Spectral display of a typical SSB voice
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communications signal. In this example, the carrier and the USB energy are eliminated, leaving only the LSB energy.
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An SSB signal can be obtained with a balanced modulator, which is an amplitude modulator/ amplifier using two transistors with the inputs in push-pull and the outputs in parallel (Fig. 25-5). This cancels the carrier wave in the output, leaving only LSB and USB energy. The result is a double sideband suppressed carrier (DSBSC) signal, often called simply double sideband (DSB). At some stage following the balanced modulator, one of the sidebands is removed from the DSB signal by a bandpass filter to obtain an SSB signal. Figure 25-6 is a block diagram of a simple SSB transmitter. The balanced modulator is placed in a low-power section of the transmitter. The RF amplifiers that follow any type of amplitude modulator, including a balanced modulator, must all be linear to avoid distortion and unnecessary spreading of signal bandwidth ( splatter ). They generally work in class A, except for the PA, which works in class AB or class B.
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Frequency and Phase Modulation In frequency modulation (FM), the instantaneous amplitude of a signal remains constant, and the instantaneous frequency is varied instead. A nonlinear PA such as a class C amplifier can be used in an FM transmitter without causing signal distortion, because the amplitude does not fluctuate.
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25-5 A balanced modulator using two NPN bipolar transistors. The inputs are in
push-pull, but the outputs are in parallel.
25-6 Block diagram of a basic SSB transmitter.
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Frequency modulation can be obtained by applying the audio signal to a varactor in a tuned oscillator. An example of this scheme, known as reactance modulation, is shown in Fig. 25-7. The varying voltage across the varactor causes its capacitance to change in accordance with the audio waveform. The changing capacitance results in variation of the resonant frequency of the inductance-capacitance (LC) tuned circuit, causing small fluctuations in the frequency generated by the oscillator. Another way to get FM is to modulate the phase of the oscillator signal. This causes small variations in the frequency, because any instantaneous phase change shows up as an instantaneous frequency change (and vice versa). When phase modulation is used, the audio signal must be processed, adjusting the frequency response of the audio amplifiers. Otherwise the signal will sound unnatural when it is received. Deviation is the maximum extent to which the instantaneous-carrier frequency differs from the unmodulated-carrier frequency. For most FM voice communications transmitters, the deviation is standardized at 5 kHz. This is known as narrowband FM (NBFM). The bandwidth of an NBFM signal is comparable to that of an AM signal containing the same modulating information. In FM hi-fi music broadcasting, and in some other applications, the deviation is much greater than 5 kHz. This is called wideband FM (WBFM). The deviation obtainable by means of direct FM is greater, for a given oscillator frequency, than the deviation that can be obtained by means of phase modulation. The deviation of a signal can be increased by a frequency multiplier. When an FM signal is passed through a frequency multiplier, the deviation is multiplied along with the carrier frequency. The deviation in an FM signal should be equal to the highest modulating audio frequency if optimum fidelity is to be obtained. Thus, 5 kHz is more than enough for voice. For music, a deviation of 15 kHz or even 20 kHz is required for good reproduction when the signal is received.
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