Custom Search
|
|
  |
|
|
DEMODULATION LEARNING OBJECTIVES Upon completion of this chapter you will be able to:
INTRODUCTION In chapters 1 and 2 you studied how to apply intelligence (modulation) to an rf-carrier wave. Carrier modulation allows the transmission of modulating frequencies without the use of transmission wire as a medium. However, for the communication process to be completed or to be useful, the intelligence must be recovered in its original form at the receiving site. The process of re-creating original modulating frequencies (intelligence) from the rf carrier is referred to as DEMODULATION or DETECTION. Each type of modulation is different and requires different techniques to recover (demodulate) the intelligence. In this chapter we will discuss ways of demodulating AM, cw, fm, phase, and pulse modulation. The circuit in which restoration is achieved is called the DETECTOR or DEMODULATOR (both of these terms are used in NEETS). The term demodulator is used because the demodulation process is considered to be the opposite of modulation. The output of an ideal detector must be an exact reproduction of the modulation existing on the rf wave. Failure to accurately recover this intelligence will result in distortion and degradation of the demodulated signal and intelligence will be lost. The distortion may be in amplitude, frequency, or phase, depending on the nature of the demodulator. A nonlinear device is required for demodulation. This nonlinear device is required to recover the modulating frequencies from the rf envelope. Solid-state detector circuits may be either a pn junction diode or the input junction of a transistor. In electron-tube circuits, either a diode or the grid or plate circuits of a triode electron tube may be used as the nonlinear device. Q.1 What is demodulation? CONTINUOUS-WAVE DEMODULATION Continuous-wave (cw) modulation consists of on-off keying of a carrier wave. To recover on-off keyed information, we need a method of detecting the presence or absence of rf oscillations. The CW DEMODULATOR detects the presence of rf oscillations and converts them into a recognizable form. Figure 3-1 illustrates the received cw in view (A), the rectified cw from a diode detector in view (B), and the dc output from a filter that can be used to control a relay or light indicator in view (C). Figure 3-1A. - Cw demodulation. RECEIVED CW
Figure 3-1B. - Cw demodulation. RECTIFIED CW FROM DETECTOR
Figure 3-1C. - Cw demodulation. OUTPUT FROM FILTER
Figure 3-2 is a, simplified circuit that could be used as a cw demodulator. The antenna receives the rf oscillations from the transmitter. The tank circuit, L and C1, acts as a frequency-selective network that is tuned to the desired rf carrier frequency. The diode rectifies the oscillations and C2 provides filtering to provide a constant dc output to control the headset. This demodulator circuit is the equivalent of a wire telegraphy circuit but it has certain disadvantages. For example, if two transmitters are very close in frequency, distinguishing which transmitting station you are receiving is often impossible without a method of fine tuning the desired frequency. Also, if the stations are within the frequency bandpass of the input tank circuit, the tank output will contain a mixture of both signals. Therefore, a method, such as HETERODYNE DETECTION, must be used which provides more than just the information on the presence or absence of a signal. Figure 3-2. - Cw demodulator.
HETERODYNE DETECTION The use of an af voltage in the detector aids the operator in distinguishing between various signals. Since the carrier is unmodulated, the af voltage can be developed by using the heterodyne procedure discussed in chapter 1. The procedure is to mix the incoming cw signal with locally generated oscillations. This provides a convenient difference frequency in the af range, such as 1,000 hertz. The af difference frequency then is rectified and smoothed by a detector. The af voltage is reproduced by a telephone headset or a loudspeaker. Consider the heterodyne reception of the code letter <emphasis type="b">A</emphasis>, as shown in figure 3-3, view (A). The code consists of a short burst of cw energy (dot) followed by a longer burst (dash). Assume that the frequency of the received cw signal is 500 kilohertz. The locally generated oscillations are adjusted to a frequency which is higher or lower than the incoming rf signal (501 kilohertz in this case), as shown in view (B). The voltage resulting from the heterodyning action between the cw signal [view (A)] and the local oscillator signal [view (B)] is shown in view (C) as the mixed-frequency signal. ENVELOPE (intelligence) amplitude varies at the BEAT (difference) frequency of 1,000 hertz (501,000 - 500,000). The negative half cycles of the mixed frequency are rectified, as shown in view (D). The peaks of the positive half cycles follow the 1,000-hertz beat frequency. Figure 3-3A. - Heterodyne detection. RECEIVED CW SIGNAL
Figure 3-3B. - Heterodyne detection. LOCAL OSCILLATOR SIGNAL
Figure 3-3C. - Heterodyne detection. MIXED-FREQUENCY SIGNAL
Figure 3-3D. - Heterodyne detection. RECTIFIED MIXED-FREQUENCY SIGNAL
Figure 3-3E. - Heterodyne detection. AUDIO-BEAT NOTE FROM FILTER
The cw signal pulsations are removed by the rf filter in the detector output and only the envelope of the rectified pulses remains. The envelope, shown in view (E), is a 1,000-hertz audio-beat note. This 1,000 hertz, dot-dash tone may be heard in a speaker or headphone and identified as the letter A by the operator. The heterodyne method of reception is highly selective and allows little interference from adjacent cw stations. If a cw signal from a radiotelegraph station is operating at 10,000,000 hertz and at the same time an adjacent station is operating at 10,000,300 hertz, a simple detector cannot clearly discriminate between the two stations because the signals are just 300 hertz apart. This is because the bandpass of the tuning circuits is too wide and allows some of the other signal to interfere. The two carrier frequencies differ by only 0.003 percent and a tuned tank circuit cannot easily discriminate between them. However, if a heterodyne detector with a local-oscillator frequency of 10,001,000 hertz is used, then beat notes of 1,000 and 700 hertz are produced by the two signals. These are audio frequencies, which can be distinguished easily by a selective circuit because they differ by 30 percent (compared to the 0.003 percent above). Even if two stations produce identical beat frequencies, they can be separated by adjusting the local-oscillator or BEAT-FREQUENCY OSCILLATOR (bfo) frequency. For example, if the second station in the previous example had been operating at 10,002,000 hertz, then both stations would have produced a 1,000-hertz beat frequency and interference would have occurred. Adjusting the local-oscillator frequency to 9,999,000 hertz would have caused the desired station at 10,000,000 hertz to produce a 1,000-hertz beat frequency. The other station, at 10,002,000 hertz, would have produced a beat frequency of 3,000 hertz. Either selective circuits or the operator can easily distinguish between these widely differing tones. A trained operator can use the vaiable local oscillator to distinguish between stations that vary in frequency by only a few hundred hertz. Q.3 What is the simplest form of cw detector? |
 
|
|
|
Integrated Publishing, Inc. - A (SDVOSB) Service Disabled Veteran Owned Small Business
|
