CHAPTER 3
Amplitude Modulation Circuits
1. Amplitude modulation can be accomplished by multiplying the carrier sine wave by a gain or attenuation factor that varies in accordance with the intelligence signal.
2. Amplitude modulation can be carried out by linearly combining the carrier and intelligence signals then applying the result to a nonlinear component or circuit. A diode is an example.
3. The simplest AM circuit uses resistors to linearly mix the carrier and information signal, a diode to rectify the result, and a tuned circuit to complete the waveform.
4. Amplitude modulation can be produced by feeding the carrier to a voltage divider or other network and then varying one of the resistive elements in accordance with the modulating signal. Common variable resistive components are diodes and FETs. PIN diodes are used at UHF and microwave frequencies.
5. Low-level modulation is the process of generating 'the AM signal at low voltage and/or power levels and then using linear amplifiers to increase the power level.
6. High-level modulation is the processes of amplitude modulating the final power amplifier of a transmitter.
7. High-level modulation is accomplished with a collector (plate in vacuum tubes) modulator that varies the collector supply voltage in accordance with the modulating signal.
8. For 100 percent high-level modulation, the modulation amplifier must produce an out put wave whose peak-to-peak is 2 times the collector supply voltage.
9. For 100 percent high-level modulation, the modulation amplifier must generate an output power that is one-half of the final RF power amplifier input power (Pi =Vcc x Ic).
10. The simplest and best amplitude demodulator is the diode detector. The AM signal is rectified by a diode and then filtered by a capacitor to recover the envelope which is the original modulating information.
11. Balanced modulators are AM circuits that cancel or suppress the carrier but generate a DSB output signal that contains the upper (sum) and lower (difference) sideband frequencies.
12. A popular balanced modulator is the lattice modulator that uses a diode bridge circuit as a switch. The carrier turns the diodes off and on letting segments of the modulating signal through to produce a DSB output signal. A carrier suppression of 40 dB is possible.
13. Another widely used balanced, modulator is an integrated circuit (IC) using differential amplifiers as switches to switch the modulating signal at the carrier frequency. A popular device is the 1496 or 1596. Carrier suppression can be as high as 50 to 65 dB.
14. The most common way of generating an SSE signal is to use the filter method which incorporates a balanced modulator followed by a highly selective filter that passes either the upper or lower sideband.
15. To make both sidebands available, SSB generators use two filters, one for each sideband, or switch the carrier frequency to put the desired sideband into the fixed filter, bandpass.
16. Most SSB, filters are made with quartz crystals.
17. A quartz crystal is a frequency-determining component that acts like an LC circuit with a very high Q.
18. Crystals have series and parallel resonant modes. These can be combined into a lattice (bridge) circuit that provides extremely sharp selectivity over a desired bandwidth.
19. Ceramic filters use ceramic resonators that act like crystals but are smaller and lower in cost.
20. Mechanical bandpass filters are also used in SSE generators. These devices use multiple resonant disks that vibrate at some frequency in the 200- to 500-kHz range.
21. The phasing method of SSE generation uses two balanced modulators and 90° phase shifters for the carrier and modulating signal to produce two DSB signals that when added cause one sideband to be canceled out.
22. In phasing-type SSE generators, the accuracy of the phase shifters determines the degrees of unwanted sideband suppression.
23. Precision RC networks are normally used to produce the desired 90° phase shifts.
24. A demodulator for SSE is a mixer such as a balanced modulator that, is called a product detector. The carrier is reinserted in the receiver with a local oscillator.
25. Frequency translation or conversion is an AM process that converts a signal with any modulation to a higher (up conversion) or lower (down conversion) frequency.
26. Frequency conversion is produced by a circuit called a mixer.
27. Another name for frequency conversion is heterodyning.
28. Almost any low-level AM circuit can be used for mixing.
29. One of the most commonly used mixers is a single diode. Germanium or hot-carrier diodes are used for mixing at very high radio frequencies.
30. The inputs to a mixer are the signal to be translated fs and a sine "wave fo from a local oscillator (LO). The outputs are fo, fs, fo + fs, and fo - fs. A tuned circuit at the output selects either the slim or difference frequency while suppressing the others. Any modulation on the input appears on the output signal.
31. Bipolar and field-effect transistors can be used as mixers by operating them in the nonlinear region of their characteristics.
32. Transistor mixers offer the benefit of gain over diode mixers.
33. Balanced modulators are widely used as mixers.
34. Doubly balanced modulators and GaAs FETs are the best mixers at VHF, UHF, and microwave frequencies.
35. A single transistor can be connected to perform both the mixer and LO functions. Such a mixer is called a converter and is used primarily in AM receivers at frequencies below 30 MHz.
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