PRINCIPLE OF TELEMETRY

by: anjani Kumar B.K.N. Govt. Polytechnic Narnaul

Telemetry

(Measuring at a distance)

A technology which allows user to collect

information from inaccessible and inconvenient

location & to transmit it to the accessible places to

process.

input stage intermediate stage

O/p stage

-Recording -Indicating - Display -Control

Block diagram of general telemetring system

Measurand Primary

detector

Telemeter

Receiver

Telemeter

Transmitter

Telemeter

channel

End

Device

Types of telemetry systems:-

1. According to the link between the telemeter

transmitter and the telemeter receiver:

A. Land line telemetry system

I. Voltage telemetry system

II. Current telemetry system

III. Ratio telemetry system

B. Radio frequency telemetry system

I. frequency telemetry system

II. Pulse telemetry system

2. According to the form in which the

information is transmitted

I. Analog telemetry systems

II. Digital telemetry systems

The “land line telemetry system” comes

under a category of a system with direct

physical link. This physical link between the

transmitter and receiver is nothing but a

“telemeter channel”. This link can be a cable

used for telephony or telegraphy or a power

line carrier.

This system is most suitable for short distance

telemetry.

Radio frequency telemetry:- There is no physical link between transmitting end and the receiving end is established through the “radio links” . The link between the transmitting end receiving end is established through the radio links. Example: The controlling of aircraft on test flight, rockets and spacecraft. There is no physical link. The RF telemetry systems are more suitable for transmission of data over distances more than 1 km.

Transmission path: 1. Transmission path termed as line communications. 2. Radio waves termed as radio communications. Line communications: The most common form of the line

communication is telephone. These are carried out using overhead lines

on poles and also by use of buried cables. In UHF range and above line

communications are carried out by the use of wave guide.

Radio communication: Radio waves are commonly termed as unbounded waves. The propagation of signals through atmosphere is used. Non-directional antennas are employed for broadcast transmission.

Voltage telemetry system

Advantage:

Such systems use primary sensing

elements such as:

Microphones

Tachometers etc.

Disadvantages:

The voltage telemetry system is limited for

transmission up to 300 meters distances.

Current telemetry system

Advantage of current telemetry system

Simple DC milliammeters can be used.

The received signal can be added or subtracted directed.

Disadvantage:

This system is also not suitable for long distance

since the current output is varied by means of an

adjustable resistance in the line.

Ratio or Position telemetry system

Advantages:

Requires no intermediate amplifiers or conversions

Minimum moving parts, so the maintenance is low.

Fast response.

Disadvantage:

These systems affected by line resistance.

Major radio frequency bands

Name frequencies uses

VLF below 30KHz……………radio location equipment

LF 30KHz to 300KHz………wartime radio navigation

MF 300kHZ to 3MHz……………include AM radio BB

HF 3MHz to 30MHz………………Radio

VHF 30MHz to 300MHz……………includes AM radio &

Television VHF

channels

UHF 300MHz to 3GHz…......includes TV UHF channels

SHF 3GHz to 30GHz…...........Satellite communications

EHF 30GHz to 300GHz……….Satellite communications

In a radio receiver by providing an RF amplifier the following advantage: Signal to noise ratio is improved . Image frequency rejection is improved. Spurious frequencies are prevented. Better selectivity. Improved coupling of the receiver to the antenna.

Modulation is the process of varying one or more

properties of a high frequency periodic wave form ,

called the carrier signal, with a modulating signal which

contains information to be transmitted.

What is modulation?

Need of modulation

To strengthen the weak signals.

To reduce the antenna height.

To reduce the noise and distortions.

To change the information from one form to another form

(encryption of data).

methods of modulation

Modulation

Analog modulation

Frequency Modulation

Phase Modulation

Amplitude Modulation

Pulse modulation

Pulse Amplitude modulation

Pulse width modulation

Pulse position modulation

The Complex Envelope of an AM signal is given by

Ac indicates the power level of AM and m(t) is the Modulating Signal

If m(t) has a peak positive values of +1 and a peak negative value of -1

AM signal 100% modulated

Representation of an AM signal is given

by

( ) [1 ( )]cosc cs t A m t t

Envelope detection can be used if % modulation is less than

100%.

)](1[)( tmAtg c

AMPLITUDE modulation

]0 [i.e., modulation of absence in the envelope AM of Level -

)](1[ of valueMinimum -

)](1[ of valueMaximum -

min

max

m(t)A

tmAA

tmAA

c

c

c

Definition: The percentage of positive modulation on an AM signal

is max% Positive Modulation 100 max ( ) 100c

c

A Am t

A

min 100 min ( ) 100c

c

A Am t

A

The percentage of negative modulation on an AM signal is

max minmax ( ) min ( )

% Modulation 100 1002 2c

m t m tA A

A

The percentage of overall modulation is

AM – Percentage Modulation

Amax = 1.5Ac

Amin = 0.5 Ac

% Positive modulation= 50%

% Negative modulation =50%

Overall Modulation = 50%

If m(t) has a peak positive values of +1 and

a peak negative value of -1

AM signal 100% modulated

AM-PERCENTAGE MODULATION

Under modulated (<100%) 100% modulated

Envelope Detector

Can be used

Envelope Detector

Gives Distorted signal

Over Modulated

(>100%)

AM-NORMALIZED AVERAGE POWER

The normalized average power of the AM signal is

tmAtmAA

tmtmA

tmAtgts

ccc

c

c

2222

22

2222

2

1

2

1

212

1

12

1

2

1

If the modulation contains no dc level, then

The normalized power of the AM signal is

2

1

2

1 2222 tmAAts cc

Discrete Carrier

Power

Sideband power

0tm

FREQUENCY modulation

Frequency modulation is the process by which frequency of the carrier signal c(t) changes with respect to the modulating signal m(t).

FM modulated signal s(t) is a nonlinear function of the modulating signal m(t), thus it is known as nonlinear modulation process.

If the modulating signal is m(t) then

and the modulated signal s(t) is

Where, β is the modulation index.

Modulation index:- Modulation Index is the ratio of the

maximum deviation frequency to the frequency of modulation. In other

words it is the ratio of the spread in frequency spectrum to the

frequency that was used to modulate the carrier.

For FM,

modulation index is given by the formula

β = ∆f/fm

where,

β = modulation index for FM

∆ f = difference in carrier frequency

fm = frequency of the modulating signal

BANDWIDTH :- The bandwidth in frequency modulation is given as

BW = 2 (∆ f + fh )

where:

∆ f = change in frequency

fh = highest frequency of modulating signal

Carrier

Signal

Modulating

Signal

Modulated

Signal

Depending to this modulation index FM can ne classified into two types –

1. Narrowband FM : modulation index is smaller than one radian.

2. Wideband FM : modulation index is larger than 1 radian.

Comparison between am & fm

AMPLITUDE MODULATION FREQUENCY MODULATION

In AM, carrier wave is modulated in Amplitude.

AM has smaller bandwidth.

AM has poor sound quality due to smaller bandwidth.

AM radio ranges from 535 to 1705 kHz.

In FM carrier wave is modulated in Frequency.

FM has larger bandwidth.

FM has great sound quality due to larger bandwidth.

FM radio ranges from 88 to 108 MHz

phase modulation

In Phase Modulation the angle of the carrier wave is varied by the

modulating signal.

The characteristics of Phase modulation and Frequency

Modulation are similar.

If the modulating signal is sinusoidal, the spectrum and waveforms

of Phase modulated and frequency modulated signals coincides.

]cos[)( ttAts cc tmDt p

)](cos[)( tmDtAts pcc Resulting PM wave:

and

Where Dp is the phase sensitivity of phase

modulator.

The Phase Modulation Index is given by: p

Where ∆θ is the peak phase deviation

The total bandwidth required for PM can be determined from the

bandwidth and maximum amplitude of the modulating signal:

BPM = 2(1 + β)B

Where = 2 most often.

Pulse modulation Pulse modulation is in reality, mot a different type of modulation. It refers, rather, to the type of modulating signal involves communication using a train of recurring pulses.

We may then have pulse amplitude modulation or pulse frequency modulation.

? How the information is represented by pulse or pulses.

Let us consider the amplitude of the pulse as the important parameter, or its width, or its location in a series of pulses, and so on. We may consequently have a great variety of pulse modulation techniques. All will use AM or FM as the mode of transmission. Pulse modulation may be used for the transmission of both digital and analog data.

Pulse amplitude modulation

Message information encoded in the form of the amplitude of

pulses.

Pulse transmitted every T seconds, amplitude of the pulse is

quantized to Q values, for PAM-Q.

Example shown above is the PAM encoded (blue) signal

corresponding to a sinusoidal (red) input.

Pulse amplitude modulation

Examples:

- Telephone modems faster than 300bits/sec use PAM.

- Ethernet uses PAM.

100 BASE-T2 as well as 1000BASE-T use PAM-5.

To achieve full-duplex operation, we can do one or two things-

- Use some kind of carrier sensing (as in Ethernet, which uses

carrier sense multiple access)

- Or use some flavor of time division multiple access.

Pulse width modulation

Here we modulate the width of pulses (or their duty cycle) to

convey information.

Example above shows the PWM signal (bottom picture)

corresponding to a sinusoidal signal (top picture).

The PWM signal is typically generated using a saw-tooth

waveform and a comparator.

Pulse width modulation

Popular in digital circuits

- generation of PWM signal easy, demodulation typically uses

counters and digital to analog convertors.

Three flavors of PWM-

- pulse center is in the center of the time window.

- pulse leading edge coincides with leading edge of time window

and the trailing edge is modulated.

- pulse trailing edge coincides with trailing edge of the time window

and the leading edge is modulated.

Applications

- voltage regulators

- class D audio amplifiers (feed PWM signal to speaker after

filtering to block carrier which are highly efficient).

Pulse code modulation

Means to represent an analog signal in a digital manner.

Sample the analog signal in every T seconds, into P values.

- P is usually a power of 2.

Transmit log2P bits every T seconds.

Typically sampling is done via an ADC (analog to digital convertor).

Many such PCM data streams can be multiplexed on to a high

bandwidth medium in a time division multiplexing fashion.

- Example:- Voice signals sent over a phone network, or data

sent over an optic fiber.

Pulse code modulation

Demodulation is done by collecting log2P entries, and feeding them to

a digital to analog convertor (DAC).

- Possibly need to do decompression before this.

Applications-

- Digital audio in computers and CDs.

- Straight PCM is not used in video standards (DVD, DVR) since

it needs a high bit rate.

Some PCM techniques transmit the difference between two adjacent

samples, rather than the raw sample values. This effectively compresses

the transmitted data.

Pulse Position modulation

Suppose I want to send one of M message bits every T seconds.

PPM modulates the message by transmitting a single pulse in one

of 2M time slots

- Each time slot is T/2M seconds long.

Problematic for communication media where multi path

interference dominates

- Urban environments

- Media which exhibit frequency dependent fading.

Pulse Position modulation

Commonly used in communication over optic fibers

- Multi path fading is nominal

- No need for phase locked loop at the receiver (i.e. can use

non-coherent receiver). Coherent receivers are prohibitively

expensive for optical communication systems.

Also used in communication for RC aircraft/cars etc

- The demodulation is very simple and easy, allowing for a low

cast receiver.

- Fancier RC systems use PCM (more expensive)

Pulse frequency modulation

Conceptually, we could do PFM as well.

Pulses of constant amplitude are generated, at a rate which is

modulated by the signal frequency.

Problem: arrival rate of pulses is random, and hence demodulation is

hard.

Therefore PFM is mostly a curiosity.

MULTIPLEXING

Whenever the bandwidth of a medium linking two devices is

greater than the bandwidth needs of the devices, the link can be

shared.

Multiplexing is the set of techniques that allows the (simultaneous)

transmission of multiple signals across a single data link. As data and

telecommunications use increases, so does traffic.

Dividing a link into channels

Categories of multiplexing

Frequency-division multiplexing (FDM)

FDM is an analog multiplexing technique that combines analog signals.

It uses the concept of modulation.

FDM process

Time Division Multiplexing (TDM)

In TDM, each information signal is allowed to use all available bandwidth

In theory, it is possible to divide the bandwidth or the time among the users of a channel.

Continuously variable signals, such as analog, are not well adapted to TDM because the signal is present all the time

Time Division Multiplexing Process

TIME DIVISION MULTIPLEXING

TIME DIVISION MULTIPLEXING

TIME DIVISION MULTIPLEXING

TIME DIVISION MULTIPLEXING

FRAMES

synchronous Time division multiplexing

In synchronous TDM, the data rate of the link is n times faster, and the

unit duration is n times shorter.

TIME DIVISION MULTIPLEXING

SAMPLING AN ANALOG SIGNAL

EFFECT OF SAMPLING