Channels of Communication

HL – Option F.4

Mr. Jean

The plan:

• Video clip of the day

• Decibel Ratios – Further explanation of “Power”

• Methods of communication

• Practice Questions for Topic F– Solutions– Markers notes

Decibel:

• The decibel (dB) is a logarithmic unit used to express the ratio between two values of a physical quantity, often power or intensity.

• The decibel is commonly used as a measure of gain or attenuation, the ratio of input and output powers of a system.

• The number of decibels is ten times the logarithm to base 10 of the ratio of the two power quantities

The ratio of a power value P1 to another power value Po is represented by Ldb, that ratio expressed in decibels, which is calculated using the formula

Voltage and power increases:

• In electrical circuits, dissipated power is typically proportional to the square of voltage or current when the impedance is held constant.

• Taking voltage as an example, this leads to the equation:

Ways of Communicating

• Radio Waves– See F.1– With radio signals, keep in mind that the

signal is always analogue but it can transmit digital information

– Advantage is that no wires are necessary to transmit data

Geostationary Satellites

• Orbit the Earth at the equator and has an orbital period of one sidereal day– Time it takes the Earth to make one complete

rotation around its axis– This means that the satellite is always above

the same geographic position on Earth– Altitude is about 3.6 x 107 m up– Currently there are 5 or 6 at different positions

Geostationary Satellites

• Used for more direct transmission of signals– One satellite can get a signal to where it

would take many grounded antennas to get to– The frequencies range from

.1 – 10 GHz• Uplink and down link

frequencies must be different to avoid interference and unwanted noise

Polar-Orbiting Satellites

• A satellite that is in orbit around the Earth in a N/S or S/N path.– The satellite passes over both poles in its

orbit– Path is nearly normal to

the equator– Not geosynchronous and

so has a much lower altitude

– Passes along a different longitude with each trip

Comparing Satellites• Benefits of GS

– always above the same position on Earth so there is no need for tracking

– That location will always have access to the satellite

• Benefits of PS– Lower altitude so the signal doesn’t need as

much power to transmit• Also cheaper to put into orbit due to altitude

– GS can’t transmit more than about 60-70 deg. N or S of the equator

Issues with Communication

• Moral, ethical, economic, environmental

Operational Amplifier

• An amplifier is a device that takes a signal and makes its bigger.– An mp3 player gives out a very small pd that

you can hear with your headphones– If you connect it to a pair of speakers the

sound can be amplified with a power source

• An operation amplifier (op-amp) can be made to do more than just amplify signals, based on how it is wired

Op-Amp

• An actual op amp is a small black rectangle called a chip– The chip has 8 ‘legs’ or connections but only

5 are used for our purposes– Inside the chip is a complicated circuit composed of

transistors and resistors that we don’t need to be concerned with

Op Amp• In the top diagram (how an

op amp is represented in a circuit diagram);– The inputs are where the signal

is fed into the amp– The output is where the

amplified signal comes out– The +/- V is where the supply

(power is connected)• This is the energy that is used to

amplify the signal

– In a circuit diagram the V+ and V- are often omitted

The Ideal Op Amp

• An ideal operational amplifier has three primary characteristics– Infinite input impedance– Zero output impedance– Infinite gain

Input impedance

• This is essentially the resistance that would be measured across the input terminals– This means that the input current to the

amplifier is negligible– The amplifier measures the difference

between the two inputs

Output Impedance

• The output of an op amp is like a power source– It can be used to power loudspeakers,

lightbulbs, etc.– The output impedance is equal to the internal

resistance of the power source• In an ideal op amp this is zero

– To get a current to flow through a load resistor there must be a pd across it

• This is done by connecting the resistor to a wire at 0V. This is called an earth or ground

Open Loop Gain

• If a very small pd is applied between the inputs, a large potential is created on the output– The ratio: is called the open loop gain

of the amplifier• This value is typically in the range of 106

• This means that if the pd between the input terminals were 0.01V, then the output potential would be 10000 V

• In an ideal op amp this is infinite but it is controlled by the supply voltage

0

( )

V

V V

Gain

• A lone op amp is not particularly useful for amplifying music– If the open loop gain is 106, once the signal

goes above 0.000009 V, the output will be a constant 9V with a 9V supply

– The amplifier must be able to produce a signal that is the same as the input but with a larger amplitude

Inverting Op Amp• An op amp is said to be inverting if the +

(non-inverting) input is grounded and the – (inverting) input is connected to a resistor and an input signal– Rin is the input resistor

– Rf is the feedback resistor since it is feeding the output voltage back to the input

Inverting Op Amp

• When a voltage is applied to the inverting input (-) a voltage of the opposite polarity develops on the output– if Vin is +, then Vout is -; if Vin is -, then Vout is +

Inverting Op Amp

• Finding the Gain– The difference between V+ and V- must be

small so we can say they are equal• Since + is connected to Earth, • The potential at a point (P) between R1 and R2 is

therefore also 0V• Applying Ohm’s Law to R1 and R2

0V V

input

in

VI

Routput

f

VI

R

output f

input in

V R

V R

Non-Inverting Op Amp

• An op amp is said to be non- inverting if the – (inverting) input is grounded and the + (non-inverting) input is connected to an input signal– The polarity of the voltage at the output is the same

as that of the input (if Vin is +, then Vout is +; if Vin is -, then Vout is - )

Non-Inverting Op Amp

• The input signal is applied to the non-inverting (+) lead

• The amount the signal is amplified is determined by the value of resistors Rout and Rf

– If the difference between + and – is more than 9μV, the output will be a constant 9V so we can say the two inputs are about the same

– This means that the potential at point P is also Vinput

inputV V V

Non-Inverting Op Amp

• If the current through the resistors is I then we get

• To Rout alone;

• The 2 currents are equal so

output

out f

VI

R R

input

out

VI

R

output input

out f out

V V

R R R

1output f fout

input out out out

V R RR

V R R R

Comparator

• A comparator compares two potentials– If the potential on the + input is higher, then

the output is V+ and if the – input is higher, then the output is V-

Comparator• Fire Alarm

– At normal room temp all resistances are equal so both inputs equal V and the output is 0

– If the temp increases then the resistance of the thermistor drops and the pd across it drops. This results in an increase in the potential at the – input which results in an output of 9 V and the bell will ring

Schmitt Trigger

• A type of comparator that switches to a high output when the input is above a given value (upper threshold)– Only switches to a low output when the input

gets lower than another low value (lower threshold)

– Schmitt triggers are often used for reshaping digital signals to remove noise

Schmitt Trigger