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

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Fig. 29: AM detector circuit with AGC. The main function of the AM detector is to recover the modulated signal. It has another function called automatic gain control (AGC). AGC is required in the superhetrodyne receiver to regulate the receiver gain as the input carrier amplitude varies due to a variety of reasons. Since these variations are very slow the low pass filter for the AGC has a very low cut off frequency (i.e. 1Hz). Choosing a resistor value of 100kOhms we get: 1 f AGC = = 1Hz 2πRC 1 C = = 1.59uF 2πf R AGC Ref. [2] Appendix B Choosing a resistor value of 400kOhms for the audio out low pass filter with a resonant frequency of 10kHz: 1 C = 2 π f audio R = 39.79 pF However this capacitor is very small and is the order of stray capacitance. Choosing R=100kOhms gives C= 0.159nF. This will be implemented in the hierarchy structure. Note: the reason the resistor values are chosen at 100k is they are much larger than the resistor value in the CR high pass filter at the amp output to avoid loading the circuit. Fig. 30 below shows the AGC and audio signals. 30
Fig. 30: AGC and audio signals. The AGC signal should be constant. From Fig. 30 it can be seen to sit at dc and it varies by a slight amount (in the order of milli-volts). PART 7: DESIGN OF PRE-AMPLIFIER AND POWER AMPLIFIER. Class A, B, and C amplifiers are used in transmitters. Class A amplifiers are used in low-power stages where device dissipation and efficiency are not critical. Class B and C amplifiers are used where high power and efficiency are required. In the superhetrodyne receiver the pre-amplifier stage and power amplifier stage are combined and implemented together as an audio amplifier. Therefore, class C amplifiers cannot be used in audio amplifiers because the output current flows for less than one-half of the input signal cycle. Class B operation is achieved when the active device (in this case the BJT) is biased at cut-off, so that the output current will flow for only one half of the input signal cycle. Efficiency can reach as high as 78.5% and for linear system operation must be used in a push-pull circuit configuration. Fig. 31 below shows the circuit of the audio amplifier using a class B output stage. Ref. [4], Ref. [5] & Ref. [6] Appendix B 31
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: Fig. 28: Enlarged section of diode
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 and 46: Fig. 50: Modulating frequency in AM
Page 47 and 48: * Working on the IF stages of the d
Page 49 and 50: are very important) combated this p
Page 51: Fig. 18: Local Oscillator (Amplifie

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