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# AM transmitter circuit

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For the AM transmitter circuit shown below it is shown that the circuit has two parts, the radio frequency oscillator (carrier waveform oscillator) and the audio amplifier (generates the modulating signal). The message signal is a voice signal that is generated via the electrical condensed microphone and then amplified by the audio amplifier transistor Q1 to get the final modulating signal m(t). the frequency oscillator (Colpitts oscillator circuit) represented by Q2 and then connected to an LC tank circuit. The tank circuit is tuneable in which the capacitance value is varying to change the carrier frequency (remember that parallel LC circuit works like a Band Pass Filter BPF. Therefore the range of frequencies can change from 0.5 MHz to 1.6 MHz (within the AM frequencies Band).

After you speak into the microphone, a voice signal will be generated, in this project we can replace the microphone with a function generator (with less than 5KHz frequency) to generate a tone signal (to get a tone AM modulated signal). You will use the DC power supply to get the B1 9V source, you will need oscilloscope to display the transmitted modulated time domain signal just before the antenna, you will need insulated copper wire to be used as an antenna. You will need resistors, capacitors and an inductor as shown in the figure. We will use a variable decade capacitance box for C3, we will use low power NPN transistor for Q1 and Q2, and finally, we will use 5KOhm potentiometer instead of R5.

1) Simulate the circuit above using MULTISIM or PSPICE.

2) Use a function generator with 3KHz and 2 Vpp instead of the Microphone shown in the circuit. Explain what happens when the peak to peak voltage changes.

3) Change the R5 resistance value and explain what happens at the output (just before the antenna)

4) Change the C3 capacitance value and find the centre frequency value and explain what changes occur at the output.

5) What is the carrier frequency? how much is the modulation index?

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• 3 weeks later...

good

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