Download Balanced modulator

Palestine Polytechnic University
College Of Engineering & Technology
Analog Communication Lab
Unit 1 .
Exp. Name:
Introduction To Analog Communications
Supervisor: Eng. Omar Abu-saif
Group Name:
Rawan Hassan Abu-Khreabeh
Yusra Na’el Arafeh
3/3/2014
Objectives:
Describe the basic principles of analog radio comm.
To be Familiar with ANALOG COMMUNICATIONS circuit bored.
To know principal forms of modulation .
Recognize a message signal, carrier signal , and modulated signal
Equipment required:
F.A.C.E.T base unit.
Analog Communications circuit bored .
Oscilloscope “dual trace “
Generator ,sine wave .
Theory:
Communication :is the transfer of information from place to
another, have two kind :
1. Unidirectional :the system that have just one direction
Transmitter –› Receiver
2. Bidirectional : the system operates in opposite directional .
Modulation :the process of changing a char. Of the carrier signal with
the message signal
In analog we can change 1 char. Of carrier according message signal
,but in digital we can change 2 char. Of carrier according message.
Principle forms of modulation :
1. Amplitude mod. (AM)
2. Frequency mod. (FM)
3. Phase mod. (PM)
Analog Modulation:
AM: changing of amp. Of carrier according change of amp. Of
message .
AM message –› freq. carrier
AM message –› phase carrier
Freq. bands :
LF, HF , UHF , EHF , ELF
{300 - 3000 }MHz , most usage freq.
We can be obtained message signal from F.G and carrier signal from
VCO-LO .
T o see modulated signal in the present form we must do :
Put message on ch1.
Put AM modulated signal on ch2.
Press auto .
Press acquire ,to choose mode to be peak detected .
Demodulation :the process of recovering or detecting the message
signal from the modulated carrier freq.
Balanced modulator :
Vout = K*Vm*Vc{cos[(Wm+Wc)t+cos[(Wm-Wc)t+p]}
Such that:
Vout: instantaneous DC output voltage.
K: constant depend on the load.
P: phase diff. between inputs.
BFO: Beat Freq. Oscilator
Notes:
1. Between mod. And demod. We have an operator just for
amplify the signal ( RF power amp.)
2. By potentiometer we controlled amp. Of carrier ,and by
negative supply we controlled freq. of carrier .
3. Negative supply : cores to control high freq. and fine to control
low freq.
4. In VCO-HI we use positive supply to control freq.
5. Switches:
 S1:to adjust modulator to be balanced (DSB)
 S2: to adjust the mixer to be balanced
 S3: adjust antenna matching to be 330 ohm
6. Radio signal transmitter:
 Radiated through space
 Conducted through a wire
7. Bandwidth : the difference between highest and lowest freq.
8. Freq contains message –› sideband freq.
Results:
Ex #1:
Scope_00
 By using F.G we generate a message signal with 200 mVp-p ,2KHz
Scope_01
 By using VCO-LO and negative supply knob on the base unit we generate a
carrier signal with 200 mVp-p ,1MHz
 We put channel 1 of oscliioscope on the message signal .while channel 2
on the modulated signal (output) .
Scope_02
 We just change the freq. of message signal to 2KHz
Scope_03
 We just change the freq. of message signal to 5KHz
Scope_04
 message signal with 200 mVp-p ,2KHz
Scope_05
Scope_06
 Modulated signal (output after modulation )
Scope_07
 Demodulation ( get message signal from carrier freq.)
Scope_08
 We can see the AM signal amplitude change to correspond to the
change in the amplitude of message signal .
Ex #2 :
Scope_00
 Carrier signal from VCO-LO with freq 743.5 KHz
Scope_01
 Just we change amp. Of carrier signal by potentiometer knob
Scope_02
 We change freq. of carrier signal by change positive supply
Scope_03
 We change freq. of carrier signal by change positive supply
Scope_04
Scope_05
Scope_06
Scope_07
Division
=
=
1
𝑓𝑟𝑒𝑞.∗𝑠𝑤𝑒𝑒𝑝 𝑠𝑖𝑡𝑡𝑖𝑛𝑔
1
452,000∗.5∗10^−6
= 4.42
Division
=
1
𝑓𝑟𝑒𝑞.∗𝑠𝑤𝑒𝑒𝑝 𝑠𝑖𝑡𝑡𝑖𝑛𝑔
1
=
1455,000∗.2∗10^−6
= 4.43
Conclusions:
AM mod. Means we change the amp. Of the carrier correspond to
the variations to the amp. Of message signal .
If we change the amp. And/or the freq. of the message signal then
this make a change in amp. And/or the freq of AM signal .
1
F= ,if T decrease –› f will increase .
𝑇
Oscillator given high freq.
Function of modulator is combine between message signal and
carrier .
Filter is the component that is when we tune into our favourit
radio staion.
VCO-LO freq. increase when we change +ve supply and time
decrease
We need 3 post connector to connect SSB in basic unit
We can adjust the mod. Of the carrier by message by tuning
potentiometer on the modulator .
Radio signal is type of angle mod.
To connect trans. With receiver we need two post connector
between FROM TRANSMITTER and R8 .
Balanced modulator have 2 input (message & carrier) ,two output
( +output & -output).
Balanced modulator can be used as mixer with max. freq. and
min. freq.