1. DATA TRANSMISSION AND RETRIEVAL
This Chapter includes
Demodulation
Data Transmission and Retrieval
FAX
Modem
Created by C. Mani, Principal, K V No.1, AFS, Jalahalli West, Bangalore

2. Demodulator D C C’ RL Output AF signal
Amp. Modulated Signal
Demodulator
L1 L2 D C C’ RL
Output AF signal
C’ offers low impedance to RF signal and high impedance to AF signal since Xc = 1/2C’

3. Data Transmission AF Processor and Filter AF Power Amplifier
Transmitting Antenna
AF Processor and Filter
AF Power Amplifier
AF Amplifier
AF Data
Line
Fi l ter
Modulator
Power Amplifier
HF Amplifier
HF oscillator

4. Data Retrieval HF Amplifier-I HF Amplifier-II Demodulator
Amp. Modulated Signal
Data Retrieval
Receiving Antenna
Line
Fi l ter
HF Amplifier-I
HF Amplifier-II
Demodulator
AF Data Processor
AF Power Amplifier
AF Pre-Amplifier
AF Data

5. FAX (Facsimile Telegraphy)
* The electronic reproduction of a document at a distant place via Telephone line
Original Document
Optical Scanner
Analog Signal
Telephone Line
Sensor
Modem
Light Signal
Digital Signal
Printer
FAX Copy
Functions Performed by FAX:
i) Optical Scanning ii) Conversion of data for transmission
and reception iii) Printing a copy at the receiver’s end

6. FAX Modem i) Optical Scanning ii) Conversion of data for transmission
Written document
Rollers
Metallic Plate
Mirror
Lens
Light Source
Charge coupled device (Semiconductor Chips)
Digital Signal
Analog Signal
Telephone Line
Modem

7. MODEM (MOdulator and DEModulator) DATA SET
* Converts digital signal to analog signal at the transmitting end and vice versa at the receiving end.
AM Signal
Digital Signal
Modulator Portion
Demodulator Portion
Digital Signal
Communication Channel
Demodulator Portion
Modulator Portion
Digital Data Machine eg. Computer
Digital Data Machine eg. Computer
AM Signal
Modem I
Modem II
Terminal-1
Terminal-2
* Modem is also called as data set because it inter connects data source with a transmitter or receiver.

8. Modems differ in: a) Speed of data transmission:
i) low speed : upto 600 bits per sec (bps)
ii) medium speed: 600 – 2400 bps
iii) high speed : above 2400 bps
b) Modulation methods:
i) frequency shift keying (FSK) which shifts a carrier
frequency to indicate a space or mark
ii) phase shift keying (PSK) which shifts the phase of a
carrier wave
c) Bandwidth:
i) low frequency range (below 4 kHz)
ii) wide band (in multiples of 4 kHz)

9. Modes of Modems: 1) Simplex mode:
It operates as either send-only or receive-only device; i.e. data signals can be transmitted between Transmitter and Receiver only in one direction.
2) Half-duplex mode:
In this, data signals can be transmitted between Transmitter and Receiver in both the directions but only in one direction at a time.
3) Full-duplex mode:
In this, data signals can be transmitted between Transmitter and Receiver in both the directions simultaneously.
END