2-2
TRANSMITTER FUNDAMENTALS
Basic communication transmitters include continuous wave (cw), amplitude modulated (AM),
frequency modulated (fm), and single sideband (ssb) types. A basic description of each of these
transmitters is given in this chapter.
CONTINUOUS WAVE TRANSMITTER
The continuous wave is used principally for radiotelegraphy; that is, for the transmission of short or
long pulses of rf energy to form the dots and dashes of the Morse code characters. This type of
transmission is sometimes referred to as interrupted continuous wave. Cw transmission was the first type
of radio communication used, and it is still used extensively for long-range communications. Two of the
advantages of cw transmission are a narrow bandwidth, which requires less output power, and a degree of
intelligibility that is high even under severe noise conditions. (For example, when the receiver is in the
vicinity of rotating machinery or thunderstorms.)
A cw transmitter requires four essential components. These are a generator, amplifier, keyer, and
antenna. We have to generate rf oscillations and have a means of amplifying these oscillations. We also
need a method of turning the rf output on and off (keying) in accordance with the intelligence to be
transmitted and an antenna to radiate the keyed output of the transmitter.
Let’s take a look at the block diagram of a cw transmitter and its power supply in figure 2-1. The
oscillator generates the rf carrier at a preset frequency and maintains it within close tolerances. The
oscillator may be a self-excited type, such as an electron-coupled oscillator, or a quartz crystal type,
which uses a crystal cut to vibrate at a certain frequency when electrically excited. In both types, voltage
and current delivered by the oscillator are weak. The oscillator outputs must be amplified many times to
be radiated any distance.
Figure 2-1.—Cw transmitter block diagram.
The buffer stage or first intermediate power amplifier stage (referred to as the ipa) is a voltage
amplifier that increases the amplitude of the oscillator signal to a level that drives the power amplifier
(pa). You will find the signal delivered by the buffer varies with the type of transmitter and may be
hundreds or thousands of volts.
The buffer serves two other purposes. One is to isolate the oscillator from the amplifier stages.
Without a buffer, changes in the amplifier caused by keying or variations in source voltage would vary
the load of the oscillator and cause it to change frequency. It may also be used as a frequency multiplier,
which is explained later in this text.
