1. Channels of Communication HL – Option F.4 Mr. Jean

2. The plan: Video clip of the day Decibel Ratios –Further explanation of “Power” Methods of communication Practice Questions for Topic F –Solutions –Markers notes

3. 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

4. The ratio of a power value P 1 to another power value P o is represented by L db, that ratio expressed in decibels, which is calculated using the formula

5. 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:

7. 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

8. 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 10 7 m up –Currently there are 5 or 6 at different positions

9. 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

10. 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

11. 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

12. Issues with Communication Moral, ethical, economic, environmental

13. 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

14. 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

15. 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

16. The Ideal Op Amp An ideal operational amplifier has three primary characteristics –Infinite input impedance –Zero output impedance –Infinite gain

17. 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

18. 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

19. 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 10 6 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

20. Gain A lone op amp is not particularly useful for amplifying music –If the open loop gain is 10 6, 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

21. 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 –R in is the input resistor –R f is the feedback resistor since it is feeding the output voltage back to the input

22. Inverting Op Amp When a voltage is applied to the inverting input (-) a voltage of the opposite polarity develops on the output –if V in is +, then V out is -; if V in is -, then V out is +

23. 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

24. 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 V in is +, then V out is +; if V in is -, then V out is - )

25. 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 R out and R f –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 V input

26. Non-Inverting Op Amp If the current through the resistors is I then we get To R out alone; The 2 currents are equal so

27. 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-

28. 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

29. 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

30. Schmitt Trigger