In this laboratory exercise, you will examine a simple circuit that is frequency dependent.  There are numerous circuits that can best be thought of as having frequency dependent behavior.  For example, when you tune a radio, you are selecting a range of frequencies from which you want to extract audio signals.  In this lab exercise, you will encounter a frequency dependent circuit and explore the capabilities of that circuit - and its limitations.

        The circuit is one of the simplest possible frequency-dependent circuits, and it is really a voltage divider composed of a resistor and a capacitor.  The circuit is shown below.

        You need to do the following.

• Compute the critical frequency for the circuit (otherwise known as the corner frequency or bandwidth).
• The ratio of output signal amplitude to input signal amplitude (for a sinusoidal input signal) is
• 
• The critical frequency occurs when the squared term is 1.  (Much below that frequency, the squared term is negligible.  Much above that frequency, the squared term dominates).
• The critical frequency is then:  w = 1/RC, or f = 1/(2pRC)
• Connect the circuit above.  Use these values:
• R = 10 KW
• C = .022 mf
• Compute the critical frequency.
• Check out the filter.  There are several important points to check.  (To check the frequency response, input a sinusoidal signal from the function generator.  Put the function generator output/filter input on Channel 1 of the oscilloscope, and put the filter output on Channel 2.)
• The output of the filter at low frequencies (much below the critical frequency) should be the same as the input.  Is it?_____________
• The output of the filter at high frequencies (well above the critical frequency) should drop off as 1/f.  Check the using a frequency at 10x the critical frequency and 50x the critical frequency.
• The output of the filter at the critical frequency should have a phase shift of 45^o, and should be 0.707 times the input.  Determine if that is true________________.  If it is not true, determine where the critical frequency really is for your filter by determining where the output is 0.707 times the input.
• You can check all of the above using a sinusoidal signal from the function generator as the input and observing the input (the function generator signal) and the output signal on the oscilloscope using two channels to observe both signals simulatneously.
• Put a music CD into the CD player (sound card), and play some music.  We have provided some CDs with music sample.
• Use the special cables that let you connect to the output of the CD player.
• Use only one channel from the CD player.  (Unless you are are willing to do the extra work to get two channels.  It's called stereo because there are two music channels that don't have the exact same signal, and which gives you the stereo effect when you play those two channels into different ears.)
• First, observe the signal from the CD player on the oscilloscope.  Note what you see.
• Keeping the connection to the oscilloscope - so that you can observe the signal - connect the headphone to play the signal at the output of the filter..
• Listen to the music and write a description of what you hear in your lab notebook.
• Actually, there is a problem here.  A headset loads the circuit because it draws too much current and the circuit no longer works as a filter.  You should observe that the output isn't what you expect.  There probably isn't much of a signal at all.
• The circuit below will eliminate the loading.  This shows all of the connections except the power supply connections for the op-amp isolation amplifier.  Click here for a pinout for the operational amplifier.  (A pinout is a graphical description of terminals and pins for the integrated circuit chip.  The pinout diagram will open in a separate window so that you can keep this window open and still have the pinout window open.) Click here for a short note on the isolation amplifier.  This note includes a sketch of what the isolation amplifier looks like when it is connected on a circuit board.
• Put the integrated circuit chip (LM741 operational amplifier) into your circuit board.  Be sure to straddle the center "groove" so that you do not short pins together.
• Set the power supply to +12 and -12 on the positive and negative supplies.  Turn the output OFF.
• Connect the positive supply to pin #7, and the negative supply to pin #4.
• Connect the circuit shown below.
• The output is pin #6.
• The inverting input (with the minus sign) is pin #2.  Connect the operational amplifier output from pin #6 to here.
• The non-inverting input (with the plus sign) is pin #3.  Connect the output of the filter (the voltage from the capacitor in the filter circuit) to this pin.
• Connect the output of the inverting amplifer circuit to the osciloscope.
• Be sure that the ground from the filter circuit is connected to the ground on the oscilloscope and the common on the power supply.  (Actually, you should usually connect the common to the ground on the power supply, but you are doing that, in effect, when you connect the common to the grounds on the other instrument - because those grounds and the power supply ground are all connected together.)
• When you have all of the wiring above completed, then you can turn on the signal source and turn on the power supplies (+12 and -12v as indicated above) by turning the outputs ON.
• Now, listen to what you get when you have the headphones directly connected to the DC player output, and when you have them connected to the output of the isolation amplifer.
• Record your observations, using terms you are familiar with (things like treble, bass, etc.) and try also to put that into electrical engineering terms (like high frequency, low frequency, etc.)

• Ideally, this circuit should not draw much current from the R-C circuit, and it should also be able to drive the headset.  In your notes in your lab book explain why that is so.  If you have any confusion about why that happens, you need to check the picture link below.

• Listen to the same music using the circuit above.
• Here is another circuit.  Connect this circuit using the isolation amplifier as above.  (Use this circuit to replace the original circuit.)  Listen to the same music and record your observations.

        In class, we will work toward understanding what happens in both of these circuits.