Lab 5 Writeup

# Lab 5 Writeup - 16 The sound from the AM receiver changes...

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16) The sound from the AM receiver changes as follows: as we increase the audio frequency of the signal generator, it sounds like the pitch is rising in the AM receiver, and vice versa. When we vary the amplitude of the signal generator, we end up with a louder sound in the AM receiver with a larger amplitude, and vice versa. This is just as we expected. 17) When we increase the pitch of whistle or sing at a different note, there is a noticeable increase in the number of waves in the waveform for a given time period. 18) If R5 is changed to 5100 Ohm, we then have an op-amp DC offset of 7*(5100 / (5100 + 4700)) = 3.64 V. We then have a maximum peak of 7 – 3.64 = 3.36 V for our AC signal before the waveform starts to clip. This translates to a maximum modulation percentage of (3.36 / 3.64) * 100 = 92.3 % . 19) Going back to the resistor value of R5 = 3300 Ohm, we have an op-amp DC offset of 7*(3300 / (3300 + 4700)) = 2.89 V. At 50% modulation, we need to solve for V ac in (V ac / 2.89) = 0.5 V ac = 1.45 V. We then solve for the AC gain of the op-amp from Point D to Point B: (V D – V - ) / 50k = (V - - V ac ) / 100 With V - at AC ground, we solve to get V ac = -2V D , which implies an AC gain of -2 for the op-amp. Therefore, the amplitude of the sine wave at V D corresponding to a V ac of 1.45 V is then 1.45 / -2 = -0.725 V . 20) The wavelength we are dealing with for the operating frequency of 600 kHz is based on the equation (wave velocity) = (wavelength)*(frequency). Wavelength is then (3e8 / 600e3) = 500 m. To build an efficient wire for our AM transmitter, we then need it to be from a quarter to a half-wavelength long. This translates to a wire antenna between 125 to 250 meters long, which is not practical to build in a lab.

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