CHAPTER 6
Radio Transmitters
1. A radio transmitter generates the carrier signal, provides power amplification, and applies modulation.
2. The simplest transmitter is an oscillator that is keyed off and on to produce cw Morse code.
3. Most transmitters consist of a crystal oscillator used to generate an accurate and stable carrier frequency, a buffer amplifier that isolates the carrier oscillator from its load, one or more driver amplifiers to increase the RF power level. and a final power amplifier that applies the signal to the antenna.
4. In AM transmitters, class C amplifiers are used to increase the RF power level. The final stage is modulated.
5. Frequency modulation transmitters use class C amplifiers to increase the RF power level.
6. Single-sideband transmitters generate the carrier and modulation at a low frequency and then translate it up with a mixer, Linear amplifiers are used to increase the power level.
7. In a class A amplifier, collector current flows continuously, The output is directly proportional to the input; therefore, it is a linear amplifier.
8. Class B amplifiers are biased at cutoff so that collector current flows for 1808 of the input cycle.
9. Class B amplifiers are usually connected in a push-pull circuit where one transistor amplifies each half of the input signal
10. Class C amplifiers are biased beyond cutoff. Current flows for 908 to 1808 of the input cycle.
11. Collector current pulses in a class C amplifier are converted into a continuous sine wave by a resonant circuit.
12. The collector current pulses in a class C amplifier contain many harmonics which are filtered out by the tuned output circuit.
13. A class C amplifier can be used as a frequency multiplier by connecting a resonant circuit tuned to some integer multiple of the input frequency in the output.
14. RF amplifiers may oscillate because of feedback from internal transistor capacitance. This can be eliminated, or prevented, by neutralization, a process that cancels the feedback with out-of-phase feedback.
15. Frequency multipliers can be cascaded to produce higher output frequencies.
16. In an FM transmitter, the frequency multipliers increase the deviation as well as the carrier frequency.
17. Linear amplifiers operating class A or B are used to increase the power level of Iow level AM and SSB signals.
18. Impedance-matching networks are used to interconnect RF amplifiers and to couple power to the antenna to ensure the optimum transfer of power.
19. Common impedance-matching circuits and components include LC L networks, LC pi networks, LC T networks, transformers, and baluns. The pi and T networks are preferred since Q can be controlled.
20. Maximum power transfer occurs when the load impedance equals the generator source impedance.
21. Radio-frequency transformers are typically constructed with doughnut-shaped powdered-iron cores called toroids.
22. The impedance-matching ability of a transformer is determined by its turns ratio: (Np/Ns)2= Zp/Zs.
23. Inductors made with toroids for a given inductance are smaller, use fewer turns of wire, have a higher Q, and do not require shielding.
24. A balun is a transformer connected in a special way to transform circuits from balanced to unbalanced or vice versa and to provide impedance matching.
25. Toroid transformers and baluns are broadband devices that operate over a wide bandwidth.
26. Broadband, linear, untuned RF power amplifiers provide amplification over a broad frequency range.
27. Speech-processing circuits in a transmitter prevent overmodulation, prevent excessive signal bandwidth, and increase the average transmitted power in AM and SSB systems.
28. A voice clipper uses diodes to limit the amplitude of the audio modulating signal. A low-pass filter smoothes out any clipping distortion and prevents excessive sidebands.
29. Voice compressors use automatic-gain control (AGC) circuits to limit the audio amplitude. The gain of the circuit is inversely proportional to the audio signal amplitude.
30. In AGC circuits, a rectifier and filter convert the audio or RF into a dc voltage that controls the gain of an audio or RF amplifier to prevent overmodulation.
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