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TELEMETRY & DATA ACQUISITION SYSTEM (MULTIPLEXING SYSTEM (Concept…
TELEMETRY & DATA ACQUISITION SYSTEM
DATA ACQUISITION SYSTEM
Definition
A system used to measure and record signals obtained
signal originating from direct measurement or electrical quantities
DC & AC voltages
Frequency or resistance
Signals originating from transducers
strain gauge
thermocouple
Application
biomedical
aerospace
telemetry industries
Types
Analog
for wide bandwidth requirement
used when low accuracy can be tolerated
simple system
system block diagram
Digital
for narrow bandwidth requirement
for high accuracy & low cost requirement
complex system
system block diagram
MULTIPLEXING SYSTEM
Definition
the process of simultaneously transmitting two or more individual signals over a single communications channel
Application
Telemetry
telephone
satellite communications
Advantages
to making the system practical and for justifying the expense.
increasing the number of communication channels so that more information can be transmitted.
Concept
Multiple input signals are combined by the multiplexer into a single composite signal that is transmitted over the communications medium
the multiplexed signals may modulate a carrier before transmission
a demultiplexer is used to sort out the signal into their original form at the other end .
Types
TDM-TIME DIVISION MULTIPLEXING
for digital & analog signal
each signal can occupy the entire bandwidth of the channel
each signal is transmitted for only a brief period of time
the multiple signals take turns transmitting over the single channel
time scaled is shared
need sync pulse
low interference
simple circuit
efficient utilization
types
syncronous
asyncronous
method
1.each time slot might contain 1 byte from each channel. One channel transmits 8 bits
2.The third channel then transmits its data word and so on
3.One transmission of each channel completes one cycle of operation called a frame.
4.The cycle repeats itself at high rate of speed
5.the data bytes of the individual channel are simply interleaved
The resulting single –channel signal is a digital bit stream that must somehow be deciphered and reassembled at the receiving end.
FDM-FREQUENCY DIVISION MULTIPLEXING
for analog signal
a number of signal can share the bandwidth of a common communications channel
Signal spectrum
method
The multiple signal to be transmitted over this channel are each used to modulate a separate carrier
Each carrier is on a different frequency
The modulated carriers are then added together to form a signal complex signal that is transmitted over the single channel
The modulator outputs containing the sideband information are added together in a linear mixer
In a linear mixer, all the signals are simply added together algebraically
The resulting output signal is a composite of all carriers containing their modulation
This signal is then used to modulate a radio transmitter.
the composite signal itself may be transmitted over the single communication channel
system block diagram
allocating each signal a portion of the spectrum within that bandwidth
frequency is shared
need guard band
high interference
complex circuit
inefffective
TELEMETRY
Definition
the transmission of data for monitoring and control over long distance
A set of telecommunication system
collect data at remote and inconvenience place & relay data to the evaluation station
block diagram
Application
testing of moving cars, aircraft, missiles
Distance
long distance
DcV converted into audio tone
use modem
on modulation & demodulation process
MODULATION
DEMODULATION
short distance
data in DC voltage or current format sent in few meters
Types
Landline
Power lines, Telephone lines, Electrical wires
Distance: 50m to 1km
for lab, industries
types: current, voltage, position
radio-frequency
radio links from 1 km to 5km at 4MHz
distance >50km, microwave links range 890Mhz to 30GHz
repeaters used for every 30km to 60km distance transmission
FINAL
QUESTIONS
