Smoking and any food or drinks are not permitted in the Applications Lab!

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1 Pre-Lab Activities: None 220 Lab A Electrical Properties of Transmission Systems and the Local Loop Purpose of the experiment: Experiment with a telephone and view its properties under various different conditions: in standby, during a conversation, while producing DTMF tones, while ringing. Examine the transmission characteristics of the telephone line using a signal generator and the oscilloscope. Smoking and any food or drinks are not permitted in the Applications Lab! Equipment: Analog telephone set [This is a 2500 set.] Breakout box to gain access to the telephone line Multimeter for voltage measurements Oscilloscope Signal Generator Various cables and connectors Taking Data At the end of these instructions, you will find a sheet with several labeled sections. Use this sheet to record data as you take measurements. The lab instructor must initial the data sheet before you leave the lab. You must include the initialed data sheet with you lab report to receive full credit. Steps to be Performed: Preparation Before you start your work, the lab instructor will familiarize you with the use of the meter, the oscilloscope, and the function generator. The instructions below assume that you know how to take measurements using these pieces of equipment. Ask questions, and find the instructor if you are ever unsure about taking a measurement during the lab. On your table, you will find a telephone (labeled 5599 ) connected via the break-out box to a telephone jack. Our home-made break-out box is mounted on a length of 2x4. You may also move the utility phone labeled 2531 so it is within easy reach for use during the lab. Pick up the handset of 5599 to insure that you have dial-tone, and place a call to 2531 to insure that the telephone is correctly connected. Hang up the phones. Standby First, let s look at the voltage on the local loop when the phone is idle: Set the multimeter to DC-V. Turn the multimeter on and using the alligator leads - connect the two leads from the meter to red and green leads on the breakout box, labeled Line 1. Record the voltage displayed by the meter, being sure to indicate the correct units. Lift the handset. This will close the switch hook and allow some current to flow. This, in turn, will change the voltage. Record the meter reading. Ringing Telephone The multimeter is good for measuring constant or near constant voltages, but not for measuring alternating voltages, especially if the frequency is high. For this, we use a more complex tool, the oscilloscope. In this application, the oscilloscope screen shows the voltage ITS Printed 4/6/08

2 between the leads as the vertical axis of the display and time as the horizontal axis of the display. In general, be careful to get your units and/or metric prefixes right: m is for milli-, k is for kilo-. You should know which is which and what they mean. Turn the multimeter off and disconnect its leads from the breakout box. Connect the gray lead from the oscilloscope to the green lead on the breakout box; you won t need the alligator clips since the oscilloscope lead can clip directly to the wire. Leave the black lead from the oscilloscope disconnected (this is ground, which is not used in the local loop). On the oscilloscope, press the SETUP key. You will see a number at the bottom left of the display screen. Press the key below the number until it reads 4, then press RECALL. The oscilloscope has a lot of settings, we ve preset some for you and this chooses the 4 th setup. From 2531, call That will cause your phone to ring; don t pick up the phone, you want it to continue to ring. Watch the display on the oscilloscope. After you get a feel for the display, wait until a ring cycle starts, and then press STOP on the oscilloscope. Hang up the extension you called from to stop the ringing. Repeat the experiment above with the oscilloscope lead attached to the red lead on the breakout box. You will find that one wire carries a much larger voltage than the other one. Connect the oscilloscope lead to the wire with the higher voltage, call the phone, and freeze a clear trace. Sketch this latest trace you see on the oscilloscope on your data sheet. This shows how the voltage varies with time. Be sure to indicate the scale of the curve somehow. Use the measurement cursors on the oscilloscope to measure the ring voltage in the following way: Press the CURSORS key, select source 1, use the V1 cursor to locate the bottom (valley) of the display, then use the V2 cursor to mark the peaks. Record the difference between the two (displayed on the screen) as the peak-to-peak voltage. One-half of the peak-to-peak measure is what we normally think of as to ring voltage report that number as well. DTMF Tones On the oscilloscope, press the SETUP key. You will see a number at the bottom left of the display screen. Press the key below the number until it reads 2, then press RECALL. Pick up your phone, call 2531, and answer the call (you can set the hand-set down, but make sure to leave 2531 off-hook). Press any key on the phone and note the strength of the signal. On the breakout box, switch your probe to the other wire and repeat the test. Connect the probe to the wire that showed the stronger signal and leave it there for the rest of the steps. Talk into the phone and observe the oscilloscope display. Note that the trace varies wildly. Try whistling into it. Describe what you see. Are you whistling a sine-like sound wave? Press one of the number keys on the telephone, and observe the display. While you are holding down the key, press STOP on the oscilloscope to freeze the display. Sketch what you see. (Leave the phones off-hook). Use the cursors to determine what voltage is represented by this signal (use the same procedure you followed for the ring voltage). Now start the display on the oscilloscope, press and hold a key on 2531, and repeat the voltage measurement (no need to make another sketch). While you can guess from this display that there are a number of tones (or frequencies) involved, it is difficult to see which ones. In fact you know that the DTMF signal consists of two tones mixed together. Fortunately, we can use the oscilloscope s computing capability to help us identify these tones. On the oscilloscope, press the SETUP key. Press the bottom left key until the number 3 is displayed, then press RECALL. Your display will now show two traces. The bottom one is the same one you have seen earlier. ITS Printed 4/6/08

3 The top trace is new. It displays the frequencies the oscilloscope has found in the signal. [Remember that any signal can be expressed as the sum of sine waves. The horizontal axis is frequency and the vertical axis indicates the strength of the contribution that frequency makes to the overall signal.] Press the 1 key on 5599 and hold it for about a second. While still holding the phone key, press STOP on the oscilloscope. Observe the top trace. You should see two peaks in the trace [Ignore the left edge of the screen, that peak corresponds to the DC voltage you measured earlier.] The oscilloscope has determined that the leads are seeing a combination of DC and two AC frequencies. We want to measure which frequencies. Again use the CURSORS key on the oscilloscope. Select F2 as the Source; at the bottom of the screen you will now see the notation f1 and f2 above one of the keys. You can press the key repeatedly to highlight f1, f2, or both. Move the two cursors to point to the two peaks and record the frequencies displayed at the bottom of the screen. You ll get the best results if you try to have equal areas of the peak on either side of the cursor instead of putting the cursor precisely at the highest point. Press the RUN button and repeat the steps above for the remaining keys on the key pad. Frequency response of the telephone circuit. We will now determine if this telephone connection transmits different frequencies the way we expect based on our readings and in-class discussion. There s a connector on the table with a red jack and speaker wire. One of the two wires has a thin white stripe in the insulation. Plug the jack into the frequency generator, connect the oscilloscope probe to the wire without the white stripe, and connect the black clip to the wire with the stripe. Turn the signal generator on and select Shift-Recall-1-Enter sequentially, not all at once. Adjust the (left) Volts/div knob until you see a square wave display on the oscilloscope. Press Stop, and examine the frequency display. As you can see, a square wave is actually made up of a number of frequencies. Select the Cursor display on the oscilloscope and make sure that F2 is shown under Source. Position the f1 cursor at the 350Hz peak. Position the V2 cursor so it measures the height of the peak. For the remainder of these measurements, do not move V2. Now move f1 to the next peak and record the frequency. Position the V1 cursor at the top of this new peak (make sure V2 is still at the top of the 350Hz peak). The V(F2) readout tells you the height of the second peak relative to the first one (in a unit called db, or decibels). The bigger the number, the smaller the second peak is relative to the first one. Repeat your measurements for the remaining peaks. Position the frequency cursors at the two peaks immediately higher than the 350 Hz peak. Press Run and Stop a few times and note the effects of noise, particularly variations in the shapes of the peaks. Disconnect the probe clips from the audio wires, and reconnect the grey clip to the red wire on the breakout box. Connect the speaker in place of the audio test wire. Place a call from 2531 to 5599 and answer the call. Place the speaker next to the mouthpiece of the 2531 phone. Start the oscilloscope display and adjust the Volts/division knob to get a stable display at the bottom of the oscilloscope. Start and stop the display a few times, again noting the effects of noise. Note that the time display is not a square wave anymore, and that the collection of peaks looks qualitatively different. Reposition the V2 cursor to the top of the 350Hz peak. For the remainder of these measurements, do not move V2. Measure V for the peaks you had located above. Remember that a larger V means that the new peak is weaker (relative to the lowest one). From your results, estimate at what frequency the telephone line filter starts cutting the signal off. Note: To make that determination easier, arrange your two sets of measurements in a table with three columns: frequency, V measured directly, V measured through the telephone circuit. ITS Printed 4/6/08

4 General Rules Requirements for your lab report Your report must be typed, except that drawings may be made by hand. While your raw data sheet must be attached, all relevant data must be copied into your typed report. Do not put things like see data sheet into your typed report. Things to put into your lab report A header section with your name, your teammates names, group number, and date/time of the lab. The initialed raw data sheet (always attach this at the end of the report) Each of the subsections below requires a brief description of what you measured, your results, and when requested a reference to and a quote from a reference source that you can compare your measurement to (for example, one of our readings or an internet source). On-hook and off-hook voltage. Compare on-hook voltage to a reference. Ring voltage (sketch and number). Compare the ring voltage to an outside reference. Voltage measured for the DTMF signal. Frequency pairs for all DTMF Tones. Find a reference that defines the DTMF frequency assignments NOT your class notes and compare your answers to that reference. Square wave analysis and analysis of the way the telephone circuit transmits frequencies. o o o Show diagrams of the wave forms. Show your measurements and your conclusion regarding where the circuit starts to cut frequencies off. Find an outside reference to compare your conclusion to. ITS Printed 4/6/08

5 ITS 220 Lab A Data Sheet Name: Group: Group Number: Date/Time: Voltmeter measurements (on-hook and off-hook voltages) Ring voltage display and ring voltage measurements (note which wire, red or green, you used) Oscilloscope display during conversation/whistling. Oscilloscope display during DTMF tone, and signal voltage (incoming and outgoing). ITS Printed 4/6/08

6 Frequency pairs for all keys on the telephone key pad * 0 # ITS Printed 4/6/08

7 Frequencies and V measurements for the peaks that make up the square wave signal. V values for the peaks measured over the telephone line. Peak # Frequency V from 1 st peak Frequency received at breakout box V from the 1 st peak at the breakout box ITS Printed 4/6/08