Lab 8 Amplitude Modulation

http://physics.usask.ca/~angie/ep311/lab8.htm

Lab 8 Amplitude Modulation 

Introduction
AM radio stations use a transmission system called Amplitude Modulation (AM).  This consists of a high frequency "carrier" wave that is "modulated" by a low frequency (audio range) wave that contains the actual signal to be transmitted. The carrier frequency is used because it makes it possible to transmit over much larger distances. Modulation is achieved by multiplying the carrier and the modulator. This gives you a frequency spectrum composed of the carrier frequency (fc) and two sidebands: the carrier minus the modulator (fm), and the carrier plus the modulator. Each sideband contains the information for both frequencies,  so either one can be selected using a bandpass filter, and then "demodulated" - the high frequency carrier is filtered out, leaving the modulator.
Output of an AM Circuit:
When the carrier and the modulator are multiplied together, the output is is a waveform where the carrier wave is multiplied by the modulation index,m. The relative amplitudes of the modulating signal and the carrier signal determine what the modulation index will be.

You can view the output of an AM circuit on the oscilloscope (in "Stop" mode). It will look like the figure on the left. The modulation index is then defined as:
m = (message signal peak amplitude) / (unmodulated carrier peak amplitude)
m = (A - B) / (A + B)

The frequency components can also be displayed by using the FFT function on the oscilloscope. A typical plot is shown below.

This shows the frequencies of the signal components and their relative amplitudes in dB. The modulation index can be calculated from this plot, using the following equation:

m = 2 / 10 ^ (delta/20)

where delta is the difference between the carrier and the sideband power (dB).

      An AM Circuit:

A simple AM circuit is shown below. The AM signal is transmitted at the "antenna". The circuit uses a non-inverting amplifer in which the carrier wave is applied to the non-inverting input. The modulating signal is applied to the gate of a JFET (Junction Field Effect Transistor). The JFET acts like a voltage controlled resistor, so it can be used to vary the gain of the amplifier based on the applied modulating signal. This varying gain acts as the "multiplier" that creates the AM modulated output.

Breadboard and test the AM circuit shown above, using a carrier signal in the 70 - 90 kHz range (use the function generator on the IDL-800 breadboard).  Use the benchtop EZ function generator to supply a modulating signal of around 10 kHz. It will require an approximately -0.6V DC offset to provide a quiescent operating point in the linear range of the JFET. To set this offset, pull the DC Offset button the function generator and adjust it for the desired offset. The relative amplitude settings for the two waveforms are important. Set the modulating signal very close to the minimum amplitude possible on the function generator (make sure the "Attn+ button is pressed in for extra attenuation) and the carrier frequency just above the minimum value as well. Too high of an amplitude for either signal will cause distortion of the output.
Set the oscilloscope by using "Autoset", and adjust the trigger level, if necessary, to obtain a clear output. Then press "STOP" to see the modulated waveform. Sketch the waveforms, measure the amplitude of the signal and determine the modulation index, m.
Set the oscilloscope back on"RUN" and go to the MATH menu. Select the "Operation" (top right hand side of the display screen) to be "FFT". Now the frequency will be displayed on the x-axis and the amplitude(dB) of the signal on the y-axis. Use the "Zoom" option (bottom right hand side of the display screen) to zoom in on the area of interest, and the cursors to determine the difference in the frequencies and the amplitudes. Sketch the output, label the carrier frequency and the sidebands, and calculate the modulation index from this information.
You can also "capture" the screen output of the oscilloscope on your computer in Excel, then include it in your report. To do this, follow these instructions:

Launch Excel
On the Menu bar, go to: Tools, Add-Ins
Browse...
C:\VXIpnp\WINNT\TekVISA\ExcelToolbar\TekXLToolbar.xla
TekXL Toolbar now appears in Excel
Click the left button in Toolbar ("TekXL Connection")
Click ASRL2::INSTR, click OK
Click middle button in Toolbar (looks like a dual waveform)
Click Capture.

      An AM Receiver Circuit:

The AM signal can be received using the circuit shown below.

The tuned LC Circuit would filter out the specific sideband for the signal you want to receive (like selecting the station on an AM radio dial). The rest of the circuit retrieves the original modulating signal, which contains the information to be transmitted, by rectifying and filtering out the carrier frequency. A speaker can be used to listen to the final output.