Download ECE 471 Assignment 1:

ECE 471 Assignment 1
ASK and PSK Modulation and Demodulation
Fall 2004
ECE 471 Assignment 1:ASK and PSK Modulation and Demodulation
Using two supplied mixers and an op amp and the equipment in the electronics lab configure the
following circuit to perform modulation and demodulation using PSK and ASK.
Task 1. ASK (20 points)
a. ASK modulation: set a signal generator to 2 Volts peak-peak, return to zero with a frequency
≤ 100Hz square wave. (Obtain the non return to zero waveform using the offset control).
Verify using oscilliscope and specturm analyzer.
b. Modulate square wave using ≈ 1-2KHz sin wave with 4Vpeak-peak. Verify and demonstrate
using oscilliscope and specturm analyzer.
c. Demodulate by splitting sin wave and using 2nd mixer. Verify and demonstrate using
oscilliscope and specturm analyzer.
d. Design and implement a low pass filer to recover the data signal. Verify and demonstrate
using oscilliscope and specturm analyzer.
Task 2. PSK (20 points)
* PSK the offest is set to zero (or turned off)
a. PSK modulation: set a signal generator to 2 Volts peak-peak, non-return to zero with a
frequency ≤ 100Hz square wave. Verify using oscilliscope and specturm analyzer.
b. Modulate square wave using ≈ 1-2KHz sin wave with 4Vpeak-peak. Verify and demonstrate
using oscilliscope and specturm analyzer.
c. Demodulate by splitting sin wave and using 2nd mixer. Verify and demonstrate using
oscilliscope and specturm analyzer.
d. Design and implement a low pass filer to recover the data signal. Verify and demonstrate
using oscilliscope and specturm analyzer.
Task 3: (10 points)
Write a short report describing your experiences and observations. Include graphs, sketches,
diagrams, and tables as appropriate. Present your results during the demonstration.
Basic Schematic of Modem Experiment
- Watch your power levels! Turn power to minimum and verify with oscilliscope or meter.
- see specific IC’s for detailed schematic and pin-outs
- Set RZ for ASK and NRZ for PSK using the DC offset on the function generator. Verify
with O-scope.
Sig
Gnd
Sig
+V
out
Gnd
Gnd
-V
Operational Amplifier: differential amplifier w/ large voltage gain
inputs:
very high input impedance
"inverting" or (-)
"noninverting" (+)
single low output impedance
1. simple dual polarity power supply using two 9 volt batteries.
+9V
-9V
2. Inverting Amplifier w/ resistors RA & RB
input applied in series with RA

output is connected back to the inverting input through RB


noninverting input connected to ground reference or center tap of dual polarity power
supply

input signal moves pos/neg  output will move neg/pos
out/in voltage ratio depends RA, RB

voltage at the inverting input remains constant or zero: Vout = -(RB/RA) Vin
RA
1K
RB
10K
Vin
1V
Ic
1mA
Vout
-10V
RB
RA
+
3. Noninverting Amplifier:

input applied directly to noninverting input (+), RA is grounded

input impedance (seen by signal) >> input will following applied signal (not held constant by
feedback current)

signal moves in either direction  output will follow in phase to maintain the inverting input
at same voltage as the input (+)

voltage gain is always more than 1 and can be worked out from Vgain = (1+ RB/RA)
RA
1K
RB
10K
Vin
1V
Vout
11V
+
RB
RA
4. Voltage Follower (buffer)
high input impedance, low output impedance, unity gain
Vin  Vout by equal amount.
+
5. Opamp Filters
1st Order LPF or HPF: fc (-3dB point) =
1
2R1C1
R1
R2
C1
R1
R2
C1
+
+
2nd Order LPF or HPF: fc (-3dB point) =
1
2 R1 R2C1C 2
attenuation: 12dB/octave (25%) outside of passband
R1
C1
R1
C1
C2
R2
+
R2
C2
+-
Example for 2Khz cutoff frequency -R1= R2 =7.95K, C1 = C2 = 0.1uF
R
C
C
C
R
+
R
R
C
+
741 Op-Amp Pin Out
offset-null 1
8 not used
invert-input
2
-
7 +V
non-invert-input
3
+
6
-V
4
output
5 offset-null