Radio Communication Engineering
Radio Communication Engineering
Q1: Illustrate the radio frequency system of an amplitude modulated transmitter using a block diagram. Explain the function of each stage of the transmitter and how is this related to stages connected at the input and output of each stage.
The figure above is a representation of a simple am transmitter. An Amplitude modulated transmitter can be divided into two major sections. The two categories are dependent on the frequencies at which they operate. On the one side, there are the radio frequency units whereas on the other side there are audio frequency units. The radio frequency unit is that side that generates the RF carrier wave. As illustrated in the figure above, the carrier originates from the master oscillator where it is generated as a constant frequency and constant amplitude sine wave. The reason behind the constant amplification of the carrier wave is that the carrier wave is does not have sufficient amplitude and thus it must be passed through a number of stages before it attains the power required by the antennae. Save for the last stage of amplification, the amplifiers that are between the oscillator and the antennae are referred to as the INTERMEDIATE POWER AMPLIFIERS. As for the final stage, the amplifier used is called the FINAL POWER AMPLIFIER. During transmission, the AM signal is amplified by the power amplifier and then fed into the antennae with a characteristic antenna that is of ideally pure resistance.
Q2: Discuss the differences between Low level AM modulation and High level AM modulation.
To appreciate the difference between High level AM modulation and Low level AM modulation, it is imperative that we understand the difference in the definition of the two terms. On the one hand, Low-level modulation is simply modulating an RF source at a power level of mWs. This power is then increased to the transmission power level of the amplifiers. This process may be linear depending on the modulation method used. On the other hand, High-level modulation refers to when AM is applied to a RF power amplifier. In this case, the modulator is required to supply half of the carrier power.
Additionally, in low-level modulation, linear amplifiers are used after modulation whereas in high-level modulation, high efficiency class amplifiers are used. Also worth noting is the fact that in low level modulation the design of AF amplifiers is a bit cheap and easy given that it is done at low power. As for the high-level amplifiers, the process is complex and expensive given that it requires very high power. The other difference is in the application of the two modulators. While low-level modulators are sometimes used in TV transmitters, high-level modulators are used in high power broadcast transmitters.
Derive a system diagram for a frequency-modulated transmitter and explain the function of each stage
Block diagram of a frequency modulated transmitter
During frequency modulation, the modulated signal normally combines with the carrier to provide frequency that makes the resultant wave to vary with the instantaneous amplitude of the modulating signal. The figure above shows how the frequency modulator transmitter works. Firstly, the modulating signal that is applied to the varicap may result in the variation of the reactance. As shown, the varicap is directly connected across the tank circuit of the oscillator. When there is no modulation, the oscillator is supposed to provide a steady center frequency. However, when modulation is applied, the varicap is supposed to cause the oscillator frequency to vary around the center frequency according to the modulation signal. Eventually, the modulator output is then directed towards the frequency multiplier so as to increase the frequency and then to the power amplifier to increase the amplitude of the signal. Most FM transmitters have a range about nine meters. The best FM transmitter is that which can increase the range of transmission from the 9 meters to 23 meters. However, since the FM transmitters may provide a low frequency output, they are normally not suitable for use in urban center where they can be interrupted with other radio signals.
Draw a block diagram of an AM tuned-radio frequency (TRF) receiver and explain its operation
A domestic radio receiver separates the low frequency audio signal from the high frequency carrier and amplifies the audio signal into a form that is meaningful to our normal human ears. As shown in the diagram, the RF amplifier in the TRF amplifies all the incoming signals while the BFO frequency oscillator receives CW signals. The RF amplifier amplifies all the incoming signals from the antenna before it is passed to the detector. The detector or the demodulator recovers the AM signal and amplifies the signal provided by the RF amplifiers. Once the signal is recovered, it is passed through the audio amplifier, which makes the signal louder so that it can be heard in the loudspeaker. Additionally, the BFO recovers the CW signal. This is normally referred to as detector mixing. Tuning in the TRF is meant to select which signal is to be used in the set. The tuning system has a capacitor and an inductor in parallel. The inductor is normally fixed while the capacitor is varied. The RF amplifiers used in the tuned circuit uses a tuned circuit. The capacitor in the tuner is ganged with the capacitor in the circuit so that they both vary their values at the same time. Ganged tuning ensures that selective amplification is achieved. The detector used in this system strips the audio signal of the amplified RF signal.
Explain the principle of operation of the super heterodyne receiver
For a super-heterodyne receiver, receiving a radio signal requires an antenna. The antenna is normally built in the receiver just in the case of a radio that uses AM waves. The output of the antennae is normally very small. For a radio signal to be realized there must be a suitable antenna. In the case of, the antennae is normally inbuilt. This is often built into a receiver; especially in the case of. The output of the antennae is measured in microvolts. The signal received is tuned and amplified using an RF amplifier. The local oscillator also helps in the tuning process. As for a triode radio frequency amplifier, if the plate and the anode are connected, capacitive coupling between the plate and the grid will oscillate the amplifier if the gain is more than one. Traditionally, dozens of low gain triodes were connected in groups to make the whole equipment work. This process produced large amounts of power in its operations. It also required a dedicated team of engineers to service and maintain it.
Principle of operation
The operation of the super-heterodyne largely depends on frequency mixing. Once the signal from the antennae is filtered, it can reject the image frequency possibly amplify the signal received. A sine wave that is produced from the oscillator mixes with the signal eventually shifting it to a particular intermediate frequency. The intermediate frequency is normally a low frequency. Additionally, the modulator, instead of using the original frequency, it uses the IF signal to recreate the original information.
Explain the necessary arrangements for transceiver operation
Q1: Having a transmitter and receiver working separately such as radio station (only transmitter) and home radios (only receiver) is not difficult. Difficulties arise when both transmitter and receiver are joined in one circuit board to obtain a transceiver that transmit and receive signals.
Trans-receivers are known to have the following challenges:
- Due to high frequency, multi stage
amplifiers are susceptible to breaking into oscillation.
- As gain of RF amplifier is very high, a small feedback from output to input with correct phase can lead to oscillations.
- Correct phase means a positive feedback and it takes place due through stray capacitances
- As reactance of stray capacitances decreases at higher frequencies resulting in increased feedback.
- Forcing the device to work as an oscillator instead of an amplifier.
- The bandwidth is inconsistent and varies with the center frequency when tuned over a wide range of input frequencies.
- As frequency increases, the bandwidth ( f/Q) increases. Thus, the selectivity of the input filter changes over any appreciable range of input frequencies.
- The gains are not uniform over a very wide frequency range.
- Due to higher frequencies, ability to select desired signal is affected.
Q2: In addition to the block diagram, provide a basic block diagram of transceiver and explain the operation of each block
As the name suggests, a trans-receiver is a device that transmits signals and receives signals as well. The transmitter and receiver do not need separate circuits. Instead, they are contained on one particular circuit. Trans-receivers are normally of different types. For instance, the RF trans-receiver normally uses an RF module during high-speed data transmission. RF circuits are normally very tiny microelectronic circuits that have the capacity of working at high speeds of about 100 Ghz. The designers of this circuit wanted to bring digital domain close to the antennae. Software defined radio is used in ensuring that both the receiver and transmitter ends are close to the antennae.
In wired telephones, the handset has both the transmitter and receiver. For a person to hear someone on the phone and top communicate effectively, the phone must have both the transmitter and the receiver. On a portable mobile telephone, the entire phone system is called a trans-receiver for radio and audio. A modem can also be referred too as a trans-receiver given that it sends and receives signals. However, the modem uses modulation and demodulation. It modulates and demodulates the signal on transmitting and receiving the signal respectively. When someone is using a cordless telephone, audio and radio trans-receivers are used while a radio trans-receiver is provided for the base station. However, when a speakerphone is included in the communication, the base becomes the audio trans-receiver in addition to the cordless phone. .
Trans-receiver block diagram