simulation of a superheterodyne receiver using pspice - School of ...

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CHAPTER 5: CONCLUSIONS. * One of the things that I have learned personally from this project is to adopt a methodical approach to problem solving. From the outset of the project the aim was to design and simulate a complete superhetrodyne receiver. Rather than tackle the problem in one large section, the superhetrodyne receiver was looked at in block diagram form. Each block was designed and tested separately to ensure each individual circuit worked correctly prior to simulating the complete circuit. This practical approach to problem solving enabled diagnosis of errors and faults to exact locations and solving of problems was therefore easier. * The aim of this project was to design and simulate a complete superhetrodyne receiver using Pspice. Knowledge of analogue design of circuits greatly helped in the design of the project. DC formulae and circuit configurations studied in the process of three years of Electronics gave good background knowledge of the type of circuits to be implemented in the superhetrodyne receiver. Another aspect that helped was the previous use of the Pspice simulation package. Now having spent the duration of the project working with Pspice I would have to say that my knowledge of the package has been greatly enhanced, as too is my understanding of amplifiers and other circuits in general. * One thing that was helpful in some of the solving of problems and errors, especially with the RF and IF stages, was viewing the dc conditions of the circuit with the aid of Pspice. Often component values had to be changed and viewing voltages and currents in circuits ensured that the component values were reasonable. Many simulation errors occurred due to unrealistic dc conditions in circuits. 46
* Working on the IF stages of the design provided a problem is so far as that the main objective of this stage to provide good selectivity. Selectivity is best achieved at low frequencies, especially when using tuned LC circuits. However, the problem arises when low frequencies lead to interference and distortion due to what is known as image frequencies. This is when two signals are received that differ by twice the intermediate frequency. If we have an image frequency: f = f + 2 f image RF IF Where f RF is the desired frequency. Mixing the image frequency with f LO , where f = f + f , will give: LO RF IF f = f ± IF image f LO Taking the difference frequency: f IF = f image − f LO Therefore the difference frequency is: = ( f + 2 f ) − ( f + f ) = f IF RF IF RF IF Therefore the image frequency generates a signal at f IF also. * The selection of the Q values in the RF and IF blocks was very important as an engineering compromise occurs. High selectivity is required which can be achieved with LC circuits with high Q values. However, circuits with high Q values have narrower bandwidths. It must be taken into consideration that the bandwidth must be large enough to pass all the information (i.e. the carrier and both sidebands). * Undoubtedly, the biggest conclusion to be drawn from the project is in the area of loading. In the hierarchy design the major problems in simulation of the circuit were due to the loading effects from one hierarchy block to another. Inspection of the overall design showed that some parts of the design were suppressing other parts and preventing them from functioning correctly. The 47
Page 1 and 2: SIMULATION OF A SUPERHETERODYNE REC
Page 3 and 4: CHAPTER 1: PROJECT ORGANISATION. Th
Page 5 and 6: modulating signal (Sensitivity). A
Page 7 and 8: amplitude of the carrier wave varie
Page 9 and 10: Fig. 3: Amplitude Modulator. Settin
Page 11 and 12: This is a quite high value, which m
Page 13 and 14: R Total IN IN 5 = R1 R2 R = 80k 50k
Page 15 and 16: was assumed that the drain-source r
Page 17 and 18: A V = g m R L = 5.56X10 −3 X1X10
Page 19 and 20: 1 Substituting for ω 2 = . LC T β
Page 21 and 22: −3 3 VD = VDD − I DRD = 20V −
Page 23 and 24: Ref. [6] Appendix B Fig. 21: Mixer
Page 25 and 26: Values for the second LC tuned circ
Page 27 and 28: Fig. 24: Double tuned IF amplifier.
Page 29 and 30: Fig. 28: Enlarged section of diode
Page 31 and 32: Fig. 30: AGC and audio signals. The
Page 33 and 34: I C 22.02mA BJT Q12 (pnp transistor
Page 35 and 36: The middle section of the hierarchy
Page 37 and 38: Fig. 40: Block 4 - Local oscillator
Page 39 and 40: Fig. 44: Block 8 - Audio amplifier.
Page 41 and 42: The first major test for the superh
Page 43 and 44: spectrum. However, as long as the I
Page 45: Fig. 50: Modulating frequency in AM
Page 49 and 50: are very important) combated this p
Page 51: Fig. 18: Local Oscillator (Amplifie

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