Procedure: Introduction to SIMULINK; by z0Ze2Va

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									EE423 Lab#4.



Lab 4: Amplitude Modulation/Mixer

Where Used: In communication circuits requiring a DSB-AM or DSB-SC modulator, a
frequency translator (mixer), or a synchronous detector.

CAUTIONS:
  1. The circuit in Figure 1 requires three different power supply voltages. Their
     values are somewhat critical so please adjust the power supply values using a
     digital multimeter.

   2. Double-check your circuit connections before powering up the circuit. The
      communications IC can be easily damaged if incorrectly connected.

   3. The 1496 IC is very sensitive to the carrier input level. Be sure to follow
      recommendations given in the text. The carrier input should NEVER exceed 2
      volts peak-to-peak.

PURPOSE:
To demonstrate the use of the MC1496P communications IC as a DSB-AM modulator, a
DSB-SC modulator, and a mixer.

TEST EQUIPMENT:
  1. Spectrum Analyzer
  2. Oscilloscope
  3. Krohn-Hite Filter
  4. Digital Multimeter
  5. Function Generator
  6. Power Supplies (+12vdc, +5 vdc, -8 vdc)

REFERENCE:
Read data sheet of the MC1496 for information on how to use this IC. (See link from the
lab web page: http://www.eng.iastate.edu/ee423/EE423/labs/mc1496rev4f.pdf )

BACKGROUND:
The MC1496P communications IC may be used for a variety of mixer applications. It is
designed so that the local oscillator input port (carrier input) is balanced. This means that
it can be adjusted to null the carrier frequency component at the output. This feature
permits the generation of double-sideband modulation with full-carrier, vestigial –carrier,
and suppressed-carrier waveforms. The circuit has been designed to be very linear if the
carrier input signal level at pin 10 is 25 millivolts peak, or less, and the modulation input
signal at pin 1 is 1 volt peak, or less. For the circuit of Figure 1, this limit is set by the
product I 5 Re = 1 volt.




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EE423 Lab#4.


A carrier input of 0.3 volts peak will cause the unmodulated carrier waveform to be early
a square-wave. This will produce output harmonics that also have double-sideband
modulation.

PROCEDURE:

HINT: Make sure that all of your waveforms are correct and make sense. Ask the lab
instructor to explain them if necessary.

   1. Construct the circuit of Figure 1, apply power and connect a 40 millivolt peak-to-
      peak 15 kHz sinewave signal to the carrier input. Do not apply modulation yet.
      Observe the output on the oscilloscope while adjusting the carrier balance control.
      The output carrier should be a maximum at each end of the potentiometer and
      reach a null near the pot midpoint. Does it operate as expected? If not, check the
      circuit to make sure that it is properly wired. Now set the balance pot to one end
      so the carrier output is a maximum. Now slowly increase the carrier input until
      the output appears to be distorted. At what input level (so not exceed 2 volts peak-
      to–peak) does the carrier sinewave output just begin to appear distorted? At what
      input level, if any, does the carrier appear to be peak limited (where obvious
      clipping has occurred)?

   2. With a peak-to-peak carrier input of 50 to 100 millivolts, adjust the balance
      control for a minimum carrier output. Try to measure this minimum level or
      report an upper bound for it. Now apply a 1-volt peak-to-peak 500 Hz sinewave to
      the modulation input (pin 1). Observe the output waveform and determine if it is
      DSB_SC (If the 500 Hz modulation signal appears in the output and obscures
      your measurements, remove it by using the Krohn-Hite filter as a bandpass filter
      ahead of the oscilloscope. Its bandwidth should be set wide enough to pass both
      the sidebands in the output. This step is not necessary.). Synchronize the
      oscilloscope horizontal sweep to the modulating signal.

   3. While observing the output waveform, adjust the carrier balance control (and also
      the modulation level, if necessary) to get 100% modulation. Now adjust the
      balance control to obtain overmodulation.

   4. SKETCH the waveforms observed for DSB-SC, 100% modulation, and
      overmodulation.

   5. Connect the spectrum analyzer or the oscilloscope’s FFT module to the output.
      Measure and record the spectral components for 100% modulation and for DSB-
      SC. Are distortion sidetones present? For DSB-SC, what is the carrier suppression
      in dB relative to the input carrier? If you used the FFT module, what window and
      sampling rate did you use?

   6. Check the spectrum for a large carrier input (limiting occurs).




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EE423 Lab#4.


   7. Check the operation of the circuit as a mixer by applying two signals at the two
      input ports where each signal has the same amplitude and the frequency
      separation is sufficient to allow you to measure the sum and difference frequency
      components at the output.

LABORATORY REPORT:
Include all measurements, waveform sketches, and calculations for the steps given above.




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EE423 Lab#4.



                      Appendix
1. Circuit Diagram:




                        Fig. 1


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EE423 Lab#4.


2. Resistance Color Code:

http://www.electrician.com/resist_calc/resist_calc.htm


3. 100% Modulation and Overmodulation:




                          E m ax  E m in
Modulation percentage 
                          E m ax  E m in




            100% Modulation                              Overmodulation




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EE423 Lab#4.


4. Preparation for Lab#4 and Lab#5:
In the following two weeks, we will do FM modulation and demodulation. Again, check
in advance you have all the circuit components except the chips.




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