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1 |(a) |A music signal with frequency components from 50 Hz to 21000 Hz is Frequency modulated. If the maximum allowed frequency deviation is 50 kHz i) What is the modulation index? ii) What is the signal bandwidth using Carson’s rule? | |
|[10 Marks] |

|2 |(a) |Draw the block diagram for a Superheterodyne Receiver |
|[10 Marks] |

|3 |(a) |Explain the operation of the Superheterodyne Receiver |
|[10 Marks] |

|4 |(a) |The five basic characteristics that determine the quality of a receiver are Selectivity, Sensitivity, Signal to noise ratio, |
| | |Stability and Fidelity. Explain what is meant by any three of these. |
|[10 Marks] |

|5 |(a) |A 200 MHz carrier is frequency modulated by a 10 V peak-to-peak signal of 10 kHz. The instantaneous carrier frequency varies |
| | |between 199.90 and 200.10 MHz. |
| | | |
| | |Calculate (i) The modulator sensitivity. |
| | |(ii) The modulation index. |
| | |(iii) The signal bandwidth using Carson’s rule. |
|[10 Marks] |

|6 |(a) |A speech signal with frequency components from 300 Hz to 4 kHz is Frequency modulated. If the maximum allowed frequency |
| | |deviation is 30 kHz |
| | |What is the modulation index? |
| | |What is the signal bandwidth using Carson’s rule? |
|[10 Marks] |

|7 |(a) |Using a block diagram for a Superheterodyne Receiver explain how it operates |
|[10 Marks] |

|8 |(a) |Describe how frequency modulations operates using simple diagrams. |
|[10 Marks] |

|9 |(a) |Explain what frequency deviation is supporting your answer with a relevant equation. |
|[10 Marks] |

|10 |(a) |In frequency modulation what is the modulation sensitivity? |
|[10 Marks] |

|11 |(a) |Using a block diagram explain how a stereo transmitter operates. |
|[10 Marks] |

|12 |(a) |Using a block diagram explain how a stereo transmitter operates. |
|[10 Marks] |

|13 |(a) |In regards to a Superheterodyne receiver what is Image Channel Interference? |
|[10 Marks] |

|14 |(a) | |
|[10 Marks] |

|1 |(a) |(i) |
| | |Modulation index = frequency deviation/modulating frequency = 50 kHz/21 kHz = 2.38 |
| | |[3 marks] |
| | | |
| | |(ii) |
| | |Bandwidth is 2 (modulating frequency + frequency deviation) = 2 (21 kHz + 50 kHz) = 142 kHz |
|[10 Marks] |

|2 |(a) |[pic] |
|[10 Marks] |

|3 |(a) |Transmitted radio frequency signal is picked up by the receiving aerial and is selected by an RF stage. The RF stage, mixer and |
| | |local oscillator have manually ganged tuned circuits for selecting a desired station. One of the functions of the RF stage is to|
| | |couple the input signal to the mixer. The output of a self excited oscillator is also applied to the mixer. When selecting a |
| | |station the frequency of the local oscillator tracks the frequency of the input signal in such a manner that there exists a |
| | |constant frequency difference between the two signals. This frequency difference is known as the intermediate frequency (IF). |
| | | |
| | |The frequencies which appear in the o/p of the mixer include the sum and difference frequencies. The sum frequencies are |
| | |rejected and the difference or intermediate frequency is selected and amplified by a fixed tuned IF amplifier. The IF amplifier |
| | |may consist of one or more amplifier stages. The process of mixing moves the signal from the RF carrier to a lower frequency IF |
| | |carrier. The IF carrier still retains the modulation which was initially imposed on the RF carrier at the transmitter. |
| | |The output of the IF transmitter is fed to a detector. The AF signal which results from detection is amplified by audio |
| | |frequency amplifiers whose output activates the speaker. |
|[10 Marks] |

|4 |(a) |Any three of the below |
| | | |
| | |SELECTIVITY The ability to reject adjacent unwanted signals. The spacing between the carrier frequencies allocated to |
| | |different transmitters is limited by the available frequency spectrum. e.g. 9 kHz for broadcast in the medium waveband. The |
| | |selectivity of a receiver is its ability to reject signals at carrier frequencies adjacent to the wanted carrier frequency. In |
| | |the superheterodyne receiver see later) the selectivity is mainly determined by the gain versus frequency characteristics of the|
| | |IF amplifier. |
| | |The adjacent channel ratio is the ratio, in decibels of the input voltages at the wanted and adjacent channel signal frequencies|
| | |necessary to produce the same output power. |
| | |SENSITIVITY The ability to receive very small signals and produce an output of satisfactory signal to noise ratio. Usually |
| | |expressed as the minimum input signal (generally in micro volts), modulated at 400 Hz required to produce 50 mW output power |
| | |with a signal to noise ratio of 15 dB It is necessary to include a signal to noise in the measurement of sensitivity because it |
| | |would otherwise be possible for the output power to consist mainly of noise. |
| | |SIGNAL TO NOISE RATIO The ratio of signal power to noise power: S/N = 10 log (Ps/ Pn). The signal is the desired signal and |
| | |the noise is all other electrical signals, manmade or natural (noise is covered later). It is most desirable to have a high |
| | |signal to noise ratio. |
| | |STABILITY The ability to remain tuned to a station once it has been set |
| | |FIDELITY The ability to preserve the exact shape of the information envelope of the carrier while the signal progresses through|
| | |the receiver circuits |
|[10 Marks] |

|5 |(a) |(i) |
| | | |
| | |2 * Frequency deviation = 200.10 – 199.9 MHz = 200 kHz. |
| | |Frequency deviation = 100kHz |
| | |This is caused by a 10 V signal. |
| | |Therefore sensitivity is 100/10 kHz/V = 10 kHz/V |
| | | |
| | |(ii) |
| | | |
| | |The modulation index is 200 kHz / 10 kHz = 20. |
| | | |
| | |(iii) |
| | |BW = 2 (frequency of modulating signal + frequency deviation) 1 |
| | |= 2 ( fm + k * fd) 2 |
| | |= 2 ( 1 + ( ) * fm 3 |
| | | |
| | |Using any of the above |
| | | |
| | |BW = 420kHz |
|[10 Marks] |

|6 |(a) |(i) |
| | |Modulation index = frequency deviation/modulating frequency = 30 kHz/4 kHz = 7.5 |
| | | |
| | |(ii) |
| | |Bandwidth is 2 (modulating frequency + frequency deviation) = 2 (30 kHz + 4 kHz) = 68 kHz |
|[10 Marks] |

|1 |(a) | |
|[10 Marks] |

|1 |(a) | |
|[10 Marks] |

|1 |(a) | |
|[10 Marks] |…...

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