Experiment 2 Design of Envelope Detector

Transcription

1 Group No Grade Student number and name Student number and name Student number and name Experiment 2 Design of Envelope Detector Preliminary Work 1. Read and understand the working principle of the envelope detector. 2. Design an envelope detector circuit and simulate it in PSpice. Apply a tone modulated AM signal to the input terminals of your circuit. You may use the part MULT in PSpice in order to generate an AM wave. 3. You are free to choose the frequencies of the message and carrier signals. But keep in mind that the signal generators and oscilloscopes in the laboratory works in the frequency range 0 to 20 MHz. 4. You are free to choose the amplitudes of the message and carrier signals. Remember that the signal generators in the laboratory can provide AC amplitudes up to 10 V and DC offsets up to 5 V. 5. Be careful while choosing the capacity and resistance values. You should be able to get them in the market. You must use a variable capacitor and/or a variable resistor in order to observe their effects in the circuit. 6. Observe the result for µ=0.5, µ=1, and µ= Your preliminary report should include Your design decisions Your circuit ATTENTION Waveforms at the input and output of the circuit for µ=0.5, µ=1, and µ=1.2 generated by PSpice. Be careful about the values of modulation index. Wrong modulated waves don t get partial credit! You are not allowed to join the laboratory if you don t submit a preliminary report. Each group will submit only one preliminary report. 1

2 1 THEORY The process of demodulation is used to recover the original modulating wave from the incoming modulated wave; in effect, demodulation is the reverse of the modulation process. As with modulation, the demodulation of an AM wave can be accomplished using various devices; here, we describe a simple and yet highly effective device known as the envelope detector. Some version of this demodulator is used in almost all commercial AM radio receivers. For it to function properly, however, the AM wave has to be narrow-band, which requires that the carrier frequency be large compared to the message bandwidth. Moreover, the percentage modulation must be less than 100 percent. An envelope detector of the series type is shown in Fig.1, which R s AM wave, s(t) C R l output Figure 1: Envelope detector consists of a diode and a resistor-capacitor (RC) filter. The operation of this envelope detector is as follows. On a positive half-cycle of the input signal, the diode is forward-biased and the capacitor C charges up rapidly to the peak value of the input signal. When the input signal falls below this value, the diode becomes reverse-biased and the capacitor C discharges slowly through the load resistor R l The discharging process continues until the next positive half-cycle. When the input signal becomes greater than the voltage across the capacitor, the diode conducts again and the process is repeated. We assume that the diode is ideal, presenting resistance r f to current flow in the forward-biased region and infinite resistance in the reverse-biased region. We furtherassumethat theam wave appliedtodieenvelopedetector issuppliedby avoltage source of internal impedance R S. The charging time constant (r f + R S )C must be short compared with the carrier period 1/f c, that is, c(t) = A c cos(2πf c t) (1) where A c is the carrier amplitude and f c is the carrier frequency. Let m(t) denote the baseband message signal. Amplitude modulation is defined as a process in which the amplitude of the carrier wave c(t) is varied about a mean value, linearly with the baseband message signal m(t) An amplitude modulated (AM) wave may thus be described as a function of time as follows: (r f +R s )C << 1 f c (2) so that the capacitor C charges rapidly and thereby follows the applied voltage up to the positive peak when the diode is conducting. On the other hand, the discharging time constant R l C mustbelongenoughtoensurethatthecapacitor discharges slowlythroughtheloadresistor R l between positive peaks of the carrier wave, but not so long that the capacitor voltage will 2

3 not discharge at the maximum rate of change of the modulating wave, that is 1 f c << R l C << 1 W (3) where W is the message bandwidth. The result is that the capacitor voltage or detector output is nearly the same as the envelope of the AM wave. 2 PROCEDURE a. Construct the circuit you designed in your preliminary work. Note the frequency of the message signal f m and carrier signal f c below. f m = f c = b. Write the necessary calculation steps to determine the value of R l and C c. Check your result and determine if your demodulator should work. R l (calculated)= C(calculated)= d. Connect the signal generator outputs to the oscilloscope. Measure the exact frequency of message and carrier signals. f m (measured)= f c (measured)= e. Adjust the amplitudes of message and carrier signal in order to obtain the modulation index µ = 0.5. f. Measure the output voltage of the circuit. Draw the waveform with AM signal in dual mode below. Determine the peak amplitude and frequency of the output signal. f demod (measured)= 3

4 Figure 2: AM and demodulated signal for µ = 0.5 Volt/div= time/div= g. Adjust the amplitudes of message and carrier signal in order to obtain the modulation index µ = 1. h. Measure the output voltage of the circuit. Draw the waveform below with AM signal in dual mode. Determine the peak amplitude and frequency of the output signal. f demod (measured)= 4

5 Figure 3: AM and demodulated signal for µ = 1 Volt/div= time/div= i. Adjust the amplitudes of message and carrier signal in order to obtain the modulation index µ = 1.2. j. Measure the output voltage of the circuit. Draw the waveform with AM signal in dual mode below. Determine the peak amplitude and frequency of the output signal. f demod (measured)= 5

6 Figure 4: AM and demodulated signal for µ = 1.2 Volt/div= time/div= 6

7 3 Conclusion 3.1 Student Name and ID: 7

8 3.2 Student Name and ID: 8

9 3.3 Student Name and ID: 9