heterodyning principle for cw operation is shown in figure 3-4. The circuit can be made to oscillate by increasing the amount of energy fed back to the tank circuit from the collector-output circuit (by physically moving tickler coil L2 closer to L1 using the regeneration control). This feedback overcomes losses in the base-input circuit and causes self-oscillations which are controlled by tuning capacitor C1.">
A simple, one-transistor REGENERATIVE DETECTOR circuit that uses the heterodyning principle for cw operation is shown in figure 3-4. The circuit can be made to oscillate by increasing the amount of energy fed back to the tank circuit from the collector-output circuit (by physically moving tickler coil L2 closer to L1 using the regeneration control). This feedback overcomes losses in the base-input circuit and causes self-oscillations which are controlled by tuning capacitor C1. The received signal from the antenna and the oscillating frequency are both present at the base of transistor Q1. These two frequencies are heterodyned by the nonlinearity of the transistor. The resulting beat frequencies are then rectified by the emitter-base junction and produce a beat note which is amplified in the collector-output circuit. The af currents in the collector circuit actuate the phones. The REGENERATIVE DETECTOR (figure 3-4) produces its own oscillations, heterodynes them with an incoming signal, and rectifies or detects them.
Figure 3-4. - Regenerative detector.
The regenerative detector is used to receive short-wave code signals because it is easy to adjust and has high sensitivity and good selectivity. At high frequencies, the amount of signal detuning necessary to produce an audio-beat note is a small percentage of the signal frequency and causes no trouble. The use of the regenerative detector for low-frequency code reception, however, is usually avoided. At low frequencies the detuning required to produce the proper audio-beat frequency is a considerable percentage of the signal frequency. Although this type detector may be used for AM signals, it has high distortion and is not often used.
Amplitude modulation refers to any method of modulating an electromagnetic carrier frequency by varying its amplitude in accordance with the message intelligence that is to be transmitted. This is accomplished by heterodyning the intelligence frequency with the carrier frequency. The vector summation of the carrier, sum, and difference frequencies causes the modulation envelope to vary in amplitude at the intelligence frequency, as discussed in chapter 1. In this section we will discuss several circuits that can be used to recover this intelligence from the variations in the modulation envelope.
The detection of AM signals ordinarily is accomplished by means of a diode rectifier, which may be either a vacuum tube or a semiconductor diode. The basic detector circuit is shown in its simplest form in view (A) of figure 3-5. Views (B), (C), and (D) show the circuit waveforms. The demodulator must meet three requirements: (1) It must be sensitive to the type of modulation applied at the input, (2) it must be nonlinear, and (3) it must provide filtering. Remember that the AM waveform appears like the diagram of view (B) and the amplitude variations of the peaks represent the original audio signal, but no modulating signal frequencies exist in this waveform. The waveform contains only three rf frequencies: (1) the carrier frequency, (2) the sum frequency, and (3) the difference frequency. The modulating intelligence is contained in the difference between these frequencies. The vector addition of these frequencies provides the modulation envelope which approximates the original modulating waveform. It is this modulation envelope that the DIODE DETECTORS use to reproduce the original modulating frequencies.
Figure 3-5A. - Series-diode detector and wave shapes. CIRCUIT
Figure 3-5B. - Series-diode detector and wave shapes. RF INPUT SIGNAL
Figure 3-5C. - Series-diode detector and wave shapes. RECTIFIED SIGNAL
Figure 3-5D. - Series-diode detector and wave shapes. AUDIO SIGNAL
Let's analyze the operation of the circuit shown in view (A) of figure 3-5. This circuit is the basic type of diode receiver and is known as a SERIES-DIODE DETECTOR. The circuit consists of an antenna, a tuned LC tank circuit, a semiconductor diode detector, and a headset which is bypassed by capacitor C2. The antenna receives the transmitted rf energy and feeds it to the tuned tank circuit. This tank circuit (L1 and C1) selects which rf signal will be detected. As the tank resonates at the selected frequency, the wave shape in view (B) is developed across the tank circuit. Because the semiconductor is a nonlinear device, it conducts in only one direction. This eliminates the negative portion of the rf carrier and produces the signal shown in view (C). The current in the circuit must be smoothed before the headphones can reproduce the af intelligence. This action is achieved by C2 which acts as a filter to provide an output that is proportional to the peak rf pulses. The filter offers a low impedance to rf and a relatively high impedance to af. (Filters were discussed in NEETS, Module 9, Introduction to Wave-Generation and Wave-Shaping Circuits.) This action causes C2 to develop the waveform in view (D). This varying af voltage is applied to the headset which then reproduces the original modulating frequency. This circuit is called a series-diode detector (sometimes referred to as a VOLTAGE-DIODE DETECTOR) because the semiconductor diode is in series with both the input voltage and the load impedance. Voltages in the circuit cause an output voltage to develop across the load impedance that is proportional to the input voltage peaks of the modulation envelope.
Q.9 What are the three requirements for an AM demodulator?
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