AM Transmitter :
Transmitters
that transmit AM signals are known as AM transmitters. These transmitters are
used in medium wave (MW) and short wave (SW) frequency bands for AM broadcast.
The MW band has frequencies between 550 KHz and 1650 KHz, and the SW band has
frequencies ranging from 3 MHz to 30 MHz. The two types of AM transmitters that
are used based on their transmitting powers are:
- High Level
- Low Level
High
level transmitters use high level modulation, and low level transmitters use
low level modulation. The choice between the two modulation schemes depends on
the transmitting power of the AM transmitter. In broadcast transmitters, where
the transmitting power may be of the order of kilowatts, high level modulation
is employed. In low power transmitters, where only a few watts of transmitting
power are required , low level modulation is used.
High-Level and Low-Level Transmitters
Below
figure's show the block diagram of high-level and low-level transmitters. The
basic difference between the two transmitters is the power amplification of the
carrier and modulating signals.
Figure
(a) shows the block diagram of high-level AM transmitter.
In high-level transmission, the powers of the carrier and
modulating signals are amplified before applying them to the modulator
stage, as shown in figure (a). In low-level modulation, the powers of the two
input signals of the modulator stage are not amplified. The required
transmitting power is obtained from the last stage of the transmitter, the
class C power amplifier.
The various sections of the figure (a) are:
- Carrier oscillator
- Buffer amplifier
- Frequency multiplier
- Power amplifier
- Audio chain
- Modulated class C power amplifier
Carrier oscillator
The carrier oscillator generates the carrier signal, which lies
in the RF range. The frequency of the carrier is always very high. Because it
is very difficult to generate high frequencies with good frequency stability,
the carrier oscillator generates a sub multiple with the required carrier
frequency. This sub multiple frequency is multiplied by the frequency
multiplier stage to get the required carrier frequency. Further, a crystal
oscillator can be used in this stage to generate a low frequency carrier with
the best frequency stability. The frequency multiplier stage then increases the
frequency of the carrier to its required value.
Buffer
Amplifier
The purpose of the buffer amplifier is twofold. It first matches
the output impedance of the carrier oscillator with the input impedance of the
frequency multiplier, the next stage of the carrier oscillator. It then
isolates the carrier oscillator and frequency multiplier.
This is required so that the multiplier does not draw a large
current from the carrier oscillator. If this occurs, the frequency of the
carrier oscillator will not remain stable.
Frequency
Multiplier
The sub-multiple frequency of the carrier signal, generated by
the carrier oscillator , is now applied to the frequency multiplier through the
buffer amplifier. This stage is also known as harmonic generator. The frequency
multiplier generates higher harmonics of carrier oscillator frequency. The
frequency multiplier is a tuned circuit that can be tuned to the requisite
carrier frequency that is to be transmitted.
Power Amplifier
The power of the carrier signal is then amplified in the power
amplifier stage. This is the basic requirement of a high-level
transmitter. A class C power amplifier gives high power current pulses of the
carrier signal at its output.
Audio Chain
The audio signal to be transmitted is obtained from the
microphone, as shown in figure (a). The audio driver amplifier amplifies the
voltage of this signal. This amplification is necessary to drive the audio
power amplifier. Next, a class A or a class B power amplifier amplifies the
power of the audio signal.
Modulated Class C Amplifier
This is the output stage of the transmitter. The modulating
audio signal and the carrier signal, after power amplification, are applied to
this modulating stage. The modulation takes place at this stage. The class C
amplifier also amplifies the power of the AM signal to the reacquired
transmitting power. This signal is finally passed to the antenna., which
radiates the signal into space of transmission.
Figure shows the block diagram of a low-level AM
transmitter.
Modulation takes place at the stage, and the power of the modulated signal is amplified to the required transmitting power level. The transmitting antenna then transmits the signal.
