Radio Frequency Transmitters and Receivers - Assignment Example

Task 1 Question 1: Radio Frequency System of an Amplitude Modulated Transmitter There are a number of stages involved in the transmission of Amplitude modulated (AM) signals or radio frequencies. At the first stage (RF Oscillator stage), the carrier frequency that is intended to be used is normally generated using crystal oscillator circuitry/ capacitance inductance based Variable Frequency Oscillator (VFO). Next, the low power RF carrier output of the RF oscillator is then taken to the buffer amplifier stage where the output impedance of the carrier oscillator is marched with the input impedance of the frequency multiplier before they are isolated (Carr 44). At the Frequency Multiplier stage, higher harmonics of the carrier oscillator frequency are generated and the signals then move to the Power amplifier stage for amplification. On the other hand, the chain of the audio signal that is intended to be transmitted begins from the microphone and is then moved to the audio driver amplifier where the voltage of the signal is amplified in order to drive the audio power amplifier. As indicated in the block diagram, the Modulator stage is where all the audio information is normally impressed upon the carrier frequency. A class A or B power amplifier can then be used to amplify the audio signal power. Question 2: Illustration of an AM modulator Block diagram of an AM modulator Antenna The sound waves are first converted into electrical signals by the microphone. Next, the signals are then moved to the audio amplifier for signal amplification. At the amplitude modulator stage, the signals are modulated and the frequency for the carrier is then set by the frequency oscillator (Newkirk and Karlquist 15). Lastly, the modulated signal moves to the antenna where it is amplified and transmitted. Compared to the frequency of the massage, the measured frequency of the detected signals were relatively higher. Question 3: Single Sideband Amplitude Modulation In order to determine the lower side band at the output, the frequencies of the double sideband full carrier (DSBFC) spectrum presented above can be calculated by using a mathematical formulation: Lower sideband = Carrier frequency - modulating frequency  = fc-fm = 10 kHz 150 kHz = -140 kHz The lower side band at the output= 140 kHz Task 2 Question 1: Frequency-Modulated Transmitter In the frequency modulated transmitters, the modulating signals normally combine with the carrier to cause the variation of the frequency of the resultant wave with instantaneous amplitude of the modulating signal. In the first stage, the audio signals are amplified before they are moved to the modulator. The amplifier particularly converts the low power input audio signals into higher power signals while at the same time preserving the frequency and amplitude of the input signals. At the modulator, modulating audio signals are applied to a varicap diode thereby making the reactance to vary around the center frequency. This consequently changes the frequency of the oscillator in line with the modulation thereby providing Frequency modulation. After modulation, the signals move to the Oscillator stage where a stable sine wave signal is generated at the rest frequency but no modulation is applied. In the next stage, the oscillator output is fed to the frequency multiplier. This stage features a number of frequency multipliers which performs frequency multiplidation in order to increase the frequency of the modulated signals. Finally, after the multiplication of the frequency, the signals move to the power amplifier stage where the amplitude is increased to the required level for transmission. Just like in the AM transmitters, the impedance matching network then matches the antenna impedance to the correct load over the amplifier. Question 2: Angle modulation When the modulation index was changed to ᵝ=2 and ᵝ=8, a number of changes were observed in the FM output. For example, there was brighter sound when the modulation was changed to ᵝ=2 as the higher ß resulted in a broader series of sidebands and a significantly more complex spectrum in the FM output thereby leading to a brighter sound. On the other hand, changing the modulation index to ᵝ=8 resulted in more noise at the FM output since the modulation index was exceedingly high. The resulting changes observed in the FM output when the modulation index was changed to ᵝ=2 and ᵝ=8 are attributed to the fact that in any given audio-frequency (FM), modulation index( ß) is defined as the ration of carrier that deviates from the unmodulated frequency divided by the modulator frequency. ß = (Dfc-pk) / fm. It can therefore be argued that the Modulation Index determines the amplitude of the spectrum components of the FM output signal. Task 3 AM tuned-radio frequency (TRF) receiver Antenna Output A tuned radio frequency receiver refers to a radio receiver which is often composed of a number of multiple tuned radio frequency amplifiers which are followed by circuits for purposes of detection and amplification of the audio signal (Carr 168).The above shown block diagram divides the T.R.F receivers into three major sections. The first section is the RF section which contains a single, double or three stages of R.F amplification. The tuning of the receiver will often take place at this stage. Coming after the R.F amplifiers is the detector. At this section, an A.F component is split from an R.F portion of a carrier wave. The A.F wave is then sent on to the third section of the audio frequency amplifier where additional amplification occurs. The last procedure gets accomplished if the audio signal eventually appears within loudspeakers or ears phones as sound. Each tuned RF stage constitutes an amplifying device alongside a tuned circuit which carries out filtering. In a classic TRF receiver of the 1920s, an amplifier would be a triode vacuum tube. Found in every pair of stage was some air-core RF coupling transformer which helped in coupling a signal from a plane circuit of a given tube to the input grid circuit of the tube that followed. Task 4 Principle of Operation of the Superheterodyne Receiver The superheterodyne receiver constitutes all the main components of the T.R.F alongside three additions. Signals will enter a front end circuitry from the antenna. The front end amplifier and tuning block is charged with two roles. First, it applies broadband tuning to the RF stage, a move that is aimed at accepting signals on the wanted frequency while rejecting those on an image frequency. Second, it sufficiently amplifies signals while aiming at achieving a proper signal to noise ratio. At the mixer, a signal that is tuned and amplified will enter a single port of the mixer while a local oscillator signal gets into the other port. At this stage, mixer performance remains crucial in view of the general performance of the receiver (Sharma 372). At the local oscillator, there might be a variable frequency oscillator which could be tuned through alteration of the setting on variable capacitors. After a signal leaves a mixer, it enters the IF stages. The stages have majority of amplification inside the receiver plus the filtering which makes it possible to differentiate signals based on frequencies. After passing through the IF stages, there arises a need to have signals demodulated. This achieved at the demodulator stage. The kind of demodulator used is dependent on the transmission type. Out of the demodulator comes a recovered audio as an output. This will be passed to the audio stages during which they will be amplified then presented to the loudspeakers or headphones. Task 5 The necessary Arrangements for Transceiver Operation In order to overcome some of the difficulties that when both a transmitter and a receiver are joined together in one circuit board to obtain a transceiver, a special kind of arrangement is normally needed to ensure that the transceiver is able to operate effectively. The necessary arrangement normally involves the use of single aerial antenna in order to protect the receiver section. This can be particularly achieved by placing a pair of opposed diodes from the receiver input to the ground to ensure that the transceiver is able to safely pass potential excess voltages to the ground. For example, the necessary arrangement can be achieved using the antenna relay by ensuring that any time the transmit button is pushed; the relay pulls in thereby removing the antenna from the receiver and connecting it to the transmitter. On the other hand, another pair of contacts should also be used to connect the receiver input to a grounded outlet. In addition, pin diodes are can provide an important alternative to relay since they operate faster and can be connected in series to provide extra attenuation of the signals moving to the receiver. Generally, this kind of arrangement will present an open circuit to the signal voltages while at the same time clamping excess voltages from their forward bias voltages. The additional front end protection offered by the arrangement not only protects the operations of the transceiver but also ensures operation within the required voltage ranges. This is critically important because transmitters normally generate excess voltage that is sufficient enough to interfere with the operations of the receiver by destroying the first amplifier of the receiver when it is joined together with the transmitter. Works Cited Carr, Joseph J. Practical Radio Frequency Test and Measurement: A Technician's Handbook. Boston: Newnes, 1999.Web. 12 Jun. 2014. Sharma, S.P. Basic Radio and Television. New Delhi: Tata McGraw-Hill, 2003. Print. A.P.Godse and Bakshi U.A. Communication Engineering. London: Technical Publications.2009. Print. p. 36 Newkirk, David and Karlquist, Rick. Mixers, modulators and demodulators: The ARRL Handbook for Radio Communications. Newington: ARRL. 2004. Print. , pp. 15.1–15.36. Read More