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CHAPTER 3 DEMODULATION | Up
Neets Module 12-Modulation Principles | Next
HETERODYNE DETECTION |
3-2
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.
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