1. Learning the transmission of the message signal with ASK and FSK modulation. 2. Learning how to demodulate the received signal and reconstruct the message signal.

1. Digital modulation
MD. Toufiq Hasan Anik
Objective:
1. Learning the transmission of the message signal with ASK and FSK modulation.
2. Learning how to demodulate the received signal and reconstruct the message signal.
Theory:
ASK
In many situations, for example in radio frequency transmission data cannot be transmitted directly but
must be used to modulate a higher frequency sine wave carrier. The simplest way to modulating a carrier
with a data stream is to change the amplitude of the carrier every time the data changes this techniques
is known as amplitude shifting keying.
Generation:
The simplest form of amplitude shifting keying is on-off keying , where the transmission outputs the
sinewave carrier whenever the data bit is a ‘1’ , and totally suppresses the carrier when the data bit is a
‘0’. In other words the carrier is turned on for a ‘1’ and off for a ‘0’.
m(t)*c(t)=Sask
Here we multiplied the message signal with the carrier signal and got Sask .
So, the carrier signal wii only exist on 1 points.
Reconstruction:
In receiver we firstly pass it through the diode to cut only positive term. Then we pass it through LPF to
get smooth curve and after that we pass it through the Voltage level comparator and get the desire
message signal.

2. FSK:
Frequency is used to determine the 1 and zero state .For example 1 is represented by high frequency and
0 is represented by low frequency. Depending on this frequency level we can again regenerate the
message signal.
Generation:
First we multiply the message signal with the higher frequency. Then again multiply the inverse message
signal with another lower frequency message signal. Then sum the both signal and create Sfsk(t).
m (t)* c1 (t)+ m’(t)*c2(t)=Sfsk (t)
Reconstruction:
First we receive the message signal then pass it through PLL(phase locked loop) which detect the
frequency level and generate a voltage level graph . Which has higher ripple. The make it smooth we
pass it through LPF. Then pass it through Voltage level comparator. Which regenerate the message signal
again .
Experimental Setup/Block Diagram:
ASK:
Generation:

3. Reconstruction:
FSK:
Generation:

4. Reconstruction:

5. Results and Simulation Diagrams:
m(t)
c(t)
OOK

6. S-ASK
Demodulation:
Without noise

7. Reconstructed
FSK:
S-fsk (m(t)*c1(t))
S-fsk (m’(t)*c2(t))

8. S-FSK
PLL (1)
LPF (2)

9. Comparator (3) Shifted frequency
Reconstructed
Discussions:
From ASK and FSK we learned how the basic principal of transmission and reconstruction occurs in
communication system and here both of this modulation has higher application in most of the
communication sectors. We learned how to encode and transmit by carrier frequency and then again
reconstruct it from received signal.