MTI 7605 ASK Modulation and Demodulation

Transcription

1 Page 1 of 1 MTI 7605 ASK Modulation and Demodulation Contents Aims of the Exercise Learning about the functioning principle of amplitude shift keying (ASK) and its demodulation Measurement and evaluation of the frequency spectrum of ASK Overview of Exercises Amplitude Shift Keying, ASK Presentation of the signal course at the output of the ASK modulator Demodulation of the ASK signal Frequency spectrum of the ASK signal

2 Page 1 of 2 MTI 7605 ASK Modulation and Demodulation Introduction Amplitude Shift Keying (ASK) Modulation With amplitude shift keying, ASK, the amplitude of a high frequency, sinusoidal carrier signal is varied by the lower frequency, squarewave modulation signal, according to the block diagram shown below. During this modulation, the frequency of the carrier signal remains constant. Fig. 1:Block schematic diagram, ASK modulator In the so-called two-stage ASK(2-ASK) used here, a "High" level on the modulating signal switches the carrier signal through to the output of the ASK modulator; no switching occurs when a "Low" level is applied.

3 Page 1 of 2 MTI 7605 ASK Modulation and Demodulation Exercise Assembly With all voltages removed, the ASK modulator/demodulator is inserted into the Experimenter. The supply voltages, 0V, +15V and -15V are connected via the bus system of the Uni-Tr@in basic unit. Guidelines will be found in the relevant operating instructions. It is recommended to have these worksheets readily to hand. They provide extra details for the circuit diagrams. Sub-assemblies and Components Required Qty. Description Order No. 1 UniTr@in-I Interface with virtual instruments SO4203-2A 1 Experimenter SO4203-2B 1 ASK modulator / demodulator SO4201-9H 1 Measuring line set 2mm UniTr@in I SO5146-1L

4 Page 2 of 2 Operator Elements and Sockets A B C Modulator input, TTLin Equipment earth, GND Trimmer, for adjusting the sinusoidal shape of the carrier D Signal input, ASKin E Signal output, ASK F Test signal, Carrier G Potentiometer, Carrie H Test signal, Det. in I Signal output, TTLout Fig. 2: Front panel of the ASK modulator/demodulator

5 Page 1 of 5 MTI 7605 ASK Modulation and Demodulation ASK Modulation Exercise 1 Displaying the signal at the output of the ASK modulator Connect the "Carrier" socket to channel A on the oscilloscope. Select a timebase of 100µs/div. Set the input selector to 1V/div and trigger on A. In the lower part of the operating bar of the oscilloscope, the button will be seen for the cursor function. Set this for channel A. Also, two amplitude markers are available for measuring voltages and two time markers for measuring time or frequency. Set time marker 1 to the first zero pass of the sinewave carrier. Set time marker 2 so that at the top right, a frequency of 2.1kHz is shown (approx divisions). Now, with the "Carrier" potentiometer, set the carrier frequency to 2100 Hz. Apply a modulation signal from the function generator with the following data: f M = 150 Hz (frequency range 10) Signal shape TTL Level: 1:1, 50% (5V level) Connect the output of the function generator to the TTLin input of the ASK modulator. On channel A of the oscilloscope, measure the ASK signal at output ASK out and on channel B, the modulation signal at the TTL input. Trigger on B. Classify the results of the measurement according to the known types of modulation. Result: X = 1 ms/div X/T (B) Chan. A= 1 Chan. B= 2 V/DIV DC V/DIV DC

6 Page 2 of 5 Fig. 1: Channel A, ASKout; Channel B, Modulation signal TTLin Exercise 2 Demodulation of the ASK signal Connect the output of the ASK modulator, ASK out to the input ASK in of the ASK demodulator. On channel B of the oscilloscope, measure the input voltage to the ASK modulator at the test point TTL in and on channel A, the demodulated signal at output TTL out.trigger on B. Compare the modulation voltage with the output voltage of the demodulator. Result: X = 1 ms/div X/T (B) Chan. A= 2 Chan. B= 2 V/DIV DC V/DIV DC

7 Page 3 of 5 Fig. 2: Channel A, TTLin, Channel B, output signal TTLout Exercise 3 Frequency spectrum of the ASK signal On the ASK modulator (SO H), connect the input TTL in to +5V (High signal) and using the "Carrier" potentiometer, set the carrier frequency to f T = 2100 Hz. Connect the analog test input A of the UniTr@in-Interface to the ASK out output. Select the spectrum analyser and load the default settings for the "ASK". Now, start a continuous measurement. Result:

8 Page 4 of 5 Fig. 3: Frequency spectrum, ASK modulator, f = 2100 Hz On the function generator, set the modulation signal as follows: f M = 150 Hz (frequency range 10) Signal shape: TTL Level: 1:1, 50% (5V level) Measure the frequency spectrum of the signal with the same settings on the spectrum analyser as previously. Then, vary the frequency of the modulation signal at the function generator between 50 Hz (= 100 Baud) and 300 Hz (= 600 Baud). Describe what is observed! Result:

9 Page 5 of 5 Fig. 4: Frequency spectrum, ASK modulator, f M =150 Hz, f T = 2100 Hz