1-26
The principles of this circuit are similar to those of the block diagram circuit of figure 1-16. Notice in
figure 1-19 that the two generators are connected in series. Therefore, the resultant waveform of their
combined frequencies will determine when the cathode of D1 will be negative with respect to the anode,
thereby controlling the conduction of the diode. The new frequencies that are generated by applying these
signals to nonlinear impedance D1 are the sum and difference of the two original frequencies. The
frequency-selective device T1 may be tuned to whichever frequency is desired for use in later circuit
stages. Heterodyning action takes place, intentionally or not, whenever these conditions exist.
Heterodyning (MIXING) circuits are found in most electronic transmitters and receivers. These
transmitter and receiver circuits will be explained in detail later in this module.
Q-14. Define the heterodyne principle.
Q-15. What is a nonlinear impedance?
Q-16. What is spectrum analysis?
Q-17. What two conditions are necessary for heterodyning to take place?
AMPLITUDE-MODULATED SYSTEMS
Amplitude modulation refers to any method of varying the amplitude of an electromagnetic carrier
frequency in accordance with the intelligence to be transmitted by the carrier. The CARRIER frequency is
a radio-frequency wave suitable for modulation by the intelligence to be transmitted. One form of this
method of modulation is simply to interrupt the carrier in accordance with a prearranged code.
CONTINUOUS WAVE (CW)
The "on-off" KEYING of a continuous wave (cw) carrier frequency was the principal method of
modulating a carrier in the early days of electrical communications. The intervals of time when a carrier
either was present or absent conveyed the desired intelligence. This is still used in modern
communications. When applied to a continuously oscillating radio-frequency source, on-off keying is
referred to as cw signaling. This type of communication is sometimes referred to as an interrupted
continuous wave (icw).
Development
The use of a cw transmitter can be very simple. All that is required for the transmitter to work
properly is a device to generate the oscillations, a method of keying the oscillations on and off, and an
antenna to radiate the energy. Continuous wave was the first type of modulation used. It is still
extensively used for long-range communications. When Marconi and others were attempting the transfer
of intelligence between two points, without reliance on a conducting path, they employed the use of a
practical coding system known as Morse code. You probably know that Morse code is a system of on-off
keying developed for telegraph that is capable of passing intelligence over wire at an acceptable rate.
Morse code consists only of periods of signal and no-signal.
Figure 1-20 is the International Morse code used with telegraphy and cw modulation. Each character
in the code is made up of a series of elements referred to as DOTS or DASHES. These are short (dot) and
long (dash) bursts of signal separated by intervals of no signal. The dot is the basic time element of the
code. The dash has three times the duration of a dot interval. The waveforms for both are shown in figure
1-21. The elements within each character are separated by intervals of no signal with a time duration of
one dot. The characters are separated by a no-signal interval equal in duration to one dash. Each interval
