E11 Lab #2
Diodes and Demodulation

Before you start this lab, be sure to read the lab rules.

Before you start this lab you should read the introduction to diodes and modulation.

Lab breakdown

1.      Diode rectifier

2.      AC to DC

3.      Modulation

4.      Demodulation

5.      Report


Lab Procedure

Half-Wave Rectifier

Connect the circuit shown below with a 10 kW resistor (use a breadboard).  Vin should be, approximately, a 4 volt peak to peak sine wave at 1 kHz.  Use the Wavetek function generators (the same ones you used last week).  This circuit is called a half-wave rectifier.

         Circuit 1

Set the oscilloscope so that Channel 1 is observing Vin and Channel 2 is observing Vout.  Make sure the two channels are on the same voltage scale.  Predict and then observe the output.  Make sure you can explain the output in terms of the input and the diode characteristics.  Measure the difference between Vin and Vout at the peak (Do this using the voltage cursors -- in the "Measurement" section on the oscilloscope front panel).  Record (print) the oscilloscope output with the voltage cursors displayed.  

AC to DC

Add a 0.1 uF capacitor in parallel with the 10 k resistor from circuit 1.  Explain the output. Record (print) the oscilloscope output Predict qualitatively what will happen if you replace the 10 k resistor with a 1 k resistor (using only 0.1 uF in your circuit), then actually do it to check your reasoning.  Replace the 1 k resistor with a 10 k resistor and predict what will happen if you double the capacitance by adding another 0.1 uF capacitor in parallel, then do it.

In the last step you built a simple circuit to convert AC energy to DC energy.  A circuit like this is in most electronic appliances because power is transmitted as AC but most electronics use DC energy.

Build a modulator

Set the Wavetek function generator to approximately 1 kHz and 4 volts peak to peak.  Make sure the DC offset is off.  Connect the signal  to the breadboard and listen to the tone through a set of headphones.  Look at the signal on the oscilloscope before and after the headphones are attached and explain what happens.  Hint: What is the Thevenin equivalent of the function generator?  The headphones are a 64 Ohm load.

Now take the output of the Wavetek function generator and put it into the input labeled AM IN on the back of one of the blue Exact function generators like the one shown below (Exact is the brand name).  Set the output of the Exact function generator to about 20 kHz.  Set the AM button to "ON", turn off the DC offset and set the VAR RAMP/PULSE to off.

Adjust the Modulation and Carrier knobs (highlighted in blue and orange) which control the modulation process until you get an image on the scope like the one below; the function of these controls is discussed below.  You may also need to adjust the Amplitude knob.  Channel 1 is the signal from the Wavetek generator, Channel 2 is the signal from the Hi output of the Exact generator.  Note the different voltage scales.

 Recall that a modulated signal can be represented by the equation.

Each of these quantities is shown below.

         "A" is the midpoint of the variations in the sinusoidal envelope.  You can vary this with the Carrier knob.

         "B" is the amplitude of the envelope.  You can vary this with the Modulation or Amplitude knob on the Exact generator.  You can vary this with the Amplitude knob on the Wavetek generator.

         , is the frequency of the carrier signal, which you can change with the Frequency control on the Exact generator.  Tc is the period of the carrier signal.

         , is the frequency of the modulated signal, which you can change with the Frequency control on the Wavetek  generator.  Ts is the period of the modulated signal.

The quantities A and B should both be about 2 volts, but make sure that B is a little smaller than A (there is no particular need for precision for either quantity - just try to get an image similar to the one shown).  When your image looks like the one above, connect the headphones to the Hi output of the Exact generator.  What do you hear?  (Note: the output of the Exact generator drops much less than the output of the Wavetek when the headphones are attached because it has a 50 Ohm impedance, compared to 600 Ohms for the Wavetek generator).

Build a demodulator

Before continuing, make sure you have read the background on demodulation.  Take the AM modulated wave from the Exact generator and put it through a half-wave rectifier like the one at the beginning of this lab.  Vin should be from the Hi output of the Exact generator.

Build the circuit with a capacitor of your own choosing, and verify that it works as you expect (what do you expect?).   Guidelines for calculating the time constant are given in the background page.  

Display the modulated signal (Vin) and the demodulated signal (Vout) on the oscilloscope to verify that your circuit works.   Record (print) the oscilloscope output.  I want you to listen to the sound before continuing, but there are several problems we will attack one at a time.

 

The rest of the lab is time consuming, and optional. It also uses some concepts we haven’t covered in class. Finish these parts only if you want to. (You must still clean up the lab before leaving – the last part of every lab).

1.      The first problem is that Vout is not centered around zero volts, so if we attached the headphones there would be a lot of DC current flowing through them.  We will add a capacitor and resistor to Vout.Since no DC current can flow through the capacitor or the 1 MOhm resistor, the DC voltage at Vout will be zero.  The time varying voltage will pass across the capacitor with and appear as a voltage across the resistor (we will prove this later in the semester).  Because the 1 MOhm resistor is so large, it will have little effect on your circuit.  Build this circuit and verify that it works.  Vout should be the demodulated signal without a DC component - i.e., centered around ground.
                         

2.      The second problem is that we can't simply put attach the headphones to Vout because they are a 64 Ohm load, and this would effect the operation of the rest of the circuit.  To avoid this we use a simple op amp circuit with a gain of one, which won't change the output but will allow us to connect the headphones to Vout without changing the signal.  The pin numbers for the opamp (741) are shown.  Don't forget to connect +Vcc and -Vcc.  Build this circuit and verify that it works.  Keep in mind that the last three components added don't change the nature of the signal, they just make it easier to listen to.  
              

While listening to the headphones vary the signal frequency (the Wavetek generator) .  Over what frequency range, approximately, does this circuit work?  What are the limiting factors.   I have a portable CD player that you can use to hear what a more interesting signal sounds like.

Improved Circuit.

Increase the modulation frequency (the Exact generator) by a factor of 10 up to 200 kHz and decrease the value of capacitance you chose for the demodulator by a factor of 10.  Now what is the frequency range, approximately, over which the circuit works?   Try listening again to the output from my CD player, it should sound much better.


Before You Leave

Make sure the lab is cleaned up, and that you have all the information you will need for your report (see below).  Make sure the resistors and capacitors are put back in the proper drawers -- either measure them with a meter, or read the code (how to read the resistor and capacitor codes).

Your Report

Make sure you have all the information you will need for your report.