Mr. Rogers' IB Physics Topics Syllabus 1st Quarter 2nd Quarter 3rd Quarter 4th Quarter IB Objectives Core Thermo HL Thermo Core Energy Core Waves HL Waves HL Digital Tech Opt SL/HL EM Waves Opt SL/HL Com Core Nuclear HL Nuclear Opt HL Relativity Opt HL Medical
Topic 4

IB Physics Standards: Items directly related to the standards are shown in blue

Option F: Communications

 Objectives Essential Question: What's the difference between an AM and FM radio signal?

1. Describe what is meant by the modulation of a wave. The modification of a carrier wave by a second time dependent signal.

2. Describe the nature of amplitude modulation (AM) and frequency modulation (FM).

3. Solve problems based on the modulation of the carrier wave in order to determine the frequency and amplitude of the information signal.

4. Sketch and analyze graphs of the power spectrum of a carrier wave that is amplitude-modulated by a single‑frequency signal. (see figure 1)

5. Define (see ref):

sideband frequencies

bandwidth--The width of the electromagnetic spectrum that a signal occupies

modulation--The process of adding the information contained, for example, in the human voice to a suitable electromagnetic carrier.

demodulation--The process of recovering the information contained, for example, in the human voice, which had been previously added to a suitable electromagnetic carrier.

Figure 1: Power Spectrum of a Carrier Wave  Amplitude-Modulated by a Single‑Frequency
 fc = carrier wave or broadcast frequency fm = modulating frequency or frequency of the information being broadcast.

1. Solve problems involving sideband frequencies and bandwidth.

 AM FM quality Poor: bandwidth limited but this is a property of current broadcast systems not of AM. However, amplitude is susceptible to noise and attenuation Good to excellent: generous band width available for stereo-audio but this is a property of current broadcast systems not of FM. However, frequency generally is not susceptible to noise or attenuation. bandwidth Low (10 kHz): typically broadcast at lower frequencies, 540 to 1600 kHz for 160 stations. (ref) but this is a property of current broadcast systems not of AM. Higher (200 kHz): typically broadcast at higher frequencies, 88.1 to 108.1 MHz for 100 stations. (ref) but this is a property of current broadcast systems not of FM. range Better with all things equal, but this is primarily caused by the lower frequencies used, not by AM. Worse with all things equal but this is primarily caused by the higher frequencies used, not by FM. cost Lower: broadcast equipment is less complex. Higher: broadcast equipment is more complex
1. Describe, by means of a block diagram, an AM radio receiver.

• aerial--creates an electromagnetic field by moving electrons back and forth in the antennae. Best performance generally occurs when the antennae system is designed to resonate.

• tuning circuit--filters out all but the desired signals

• RF amplifier--radio frequency amplifier boosts signal

• demodulator--filters out the carrier signal

• AF amplifier--audio frequency amplifier boosts loudness

• loudspeaker

 Essential Question: How has digital data transmission revolutionized communication?

Digital signals

1. Solve problems involving the conversion between binary numbers and decimal numbers.

• bit--a binary digit

• least-significant bit (LSB)  10111001

• most-significant bit (MSB) 10111001

• the highest number expressed by n bits = 2n -1

• the largest number of objects that can be represented by n bits = 2n

1. Distinguish between analogue and digital signals.

• analog--infinite number of possible values between the lowest and highest output

• digital--finite number of possible values between the lowest and highest output determined by the number of bits

1. State the advantages of the digital transmission, as compared to the analogue transmission, of information. (see ref)

 Analog Digital Accuracy of pure signal precise depends on number of bits Error checking no yes, example: parity check Error Correction no yes Susceptibility to noise high low Flexibility low high. Can send many types of information over the same connection Multiplexing difficult easy Bandwidth Required for Transmission lower higher

1. Describe, using block diagrams, the principles of the transmission and reception of digital signals.

• sample‑and‑hold,

• clock

• parallel‑to-serial converter

• serial-to-parallel converter

• digital-to-analogue converter (DAC)

1. Explain the significance of the number of bits and the bit-rate on the reproduction of a transmitted signal.

• number of bits--resolution

• bit-rate--audio frequency range (sampling rate must be 2x the highest frequency) or amount of information. Generally, for broadcasts, the higher the carrier frequency the higher the information transfer rate.

1. Describe what is meant by time‑division multiplexing. Several channels of information are sent over a single carrier by allocating time slots to each channel.

2. Solve problems involving analogue‑to-digital conversion.

example: how many bits would be needed to represent a 0 to 1000 degree C temperature scale?

2 ^ (bits) -1 = 1000

ln[2 ^ (bits)] = ln 1001

0.69315 (bits) = 6.9088

(bits) = 6.91 / 0.693

= 9.97 since we always round up = 10

1. Describe the consequences of digital communication and multiplexing on worldwide communications.

• cost and availability to the general public

• quality of transmission

• communication and data sharing such as the Internet.

1. Discuss the moral, ethical, economic and environmental issues arising from access to the Internet.

 Essential Question: How can glass transmit data?

Optic fiber transmission

1. Explain:

• critical angle--the angle needed for total internal reflectance

• .
• total internal reflection--when a ray of light strikes the boundary between two transparent materials and is reflected back into the first material without passing into the second.

• Importance of total internal reflectance to Fiber optics: If a fiber optic cable is designed for total internal reflectance, then light shined in one end will stay inside the cable until it exits out the other end even if the fiber is bent. Turning the light off represents a zero. Turning it on represents a 1, hence, the fiber can be used for high speed transmission of digital data. This data would look like pulses of light separated by dark regions traveling through the fiber.

1. Solve problems involving refractive index and critical angle.

θcr = sin-1 (n1 / n2)

where:
n1 < n typically material 2 is more dense than material 1

Derived from Snell's Law:

n1 sin( θ1) = n2 sin(θ2)

Where:

n = index of refraction

= (speed of light in a vacuum) / (speed of light in the media)

θ = incident angle

1. Apply the concept of total internal reflection to the transmission of light through a step-index optic fiber. (ref)

step-index fiber--the fiber core is clad with a material having a slightly lower index of refraction (n). Done properly, this enhances total internal reflection.

Δ = (ncore - ncladding) / ncore << 1

ncore = typically between 1.44 and 1.46

Δ = typically between 0.001 and 0.02

1. Describe the effects of material dispersion and modal dispersion. (attenuation per unit length measured in dB km–1)

material dispersion--the light used to transmit the signal always has some spectrum width (combination of different frequencies of light). Since materials transmit different wavelengths of light at different speeds (the reason prisms create rainbows), the different wavelengths of light in the signal spread out from each other as they travel through the optical fiber. Using monochromatic light eliminates this problem

modal dispersion--the light used to transmit the signal enters the fiber over a range of angles with respect to the axis of the fiber (we're modeling light using rays). Generally, the higher the angle the greater the number of times the ray of light will be reflected and the longer the distance the ray will travel through a given length of cable. This causes the pulse of light to be spread out. Using laser light greatly reduces this problem

1. Explain what is meant by attenuation and solve problems involving attenuation measured in decibels (dB). (attenuation per unit length measured in dB km–1) (ref)

for power dB = 10 log (P1/P2)

for amplitude dB = 20 log (A1/A2)

 Attenuation (dB) Remaining Power 1 0.79 3 0.50 10 0.1 20 0.01 30 0.001 40 0.0001
1. State what is meant by noise in an optical fiber.

2.   Noise is any random degradation of information carried by the system. Noise makes the edges of a pulse of light traveling through the fiber look fuzzy.
• External Sources: Unlike wires which are susceptible to random E and B-fields, Noise in optical systems typically does not come from external sources.

• Internal Sources: Noise in optical systems can come from optical amplifiers.

1. in optic fiber transmission, describe the role of

• amplifiers--boosts the power of the light signal

• reshapers--reduces the effects of noise. Makes the edges of the pulse of light sharply defined.

1. Solve problems involving optic fibres.

 Essential Question: How is data communicated?

Channels of communication

1. Outline different channels of communication:

• wire pairs--two wires with currents flowing in opposite directions are twisted together so that the EM field produced by each wire cancels each other. Likewise any current induced in the pair of wires from an outside EM field will also be canceled out. For example, CAT 6 cable is for gigabit transmission rates with performance of up to 250 MHz. maximum allowed length of a Cat-6 horizontal cable = 100 meters (330 ft). Wires are relatively inexpensive and easy to install.

• coaxial cables--a conductor is placed inside a hollow conductive outer sheath that acts as the return wire. The sheath is connected to ground and shields the inner conductor from noise as well as prevents it from becoming a broadcast antennae. These cables tend to be thick, stiff, and expensive. They require special fittings on the end that are difficult to install.

• optic fibers--can handle very high (gigabit) transmission rates over longer distances as compared to wires. Harder to install, but also tend to be more secure (harder to tap) and much less susceptible to noise.

• radio waves and satellite communication--with microwave frequencies, can handle high data transfer rates over long distances without wires or cable. Allows for mobility

1. Discuss the uses and the relative advantages and disadvantages of wire pairs, coaxial cables, optic fibres and radio waves.

• noise

• attenuation

• bandwidth

• cost and handling

1. State what is meant by a geostationary satellite. The satellite hovers about 25, 000 miles above a stationary point on the equator.

2. State the order of magnitude of the frequencies used for communication with geostationary satellites, and explain why the up-link frequency and the down-link frequency are different.

3. Discuss the relative advantages and disadvantages of geostationary and polar-orbiting satellites for communication.

4. Discuss the moral, ethical, economic and environmental issues arising from satellite communication.

moral: Satellite communication is highly useful for military purposes. Needless to say military might can do great harm if it is not tempered by moral considerations.

ethical: Broadcast band with is a limited resource. Without controls over when and how it can be used, broadcasts on the same frequencies can jam each other, making communication impossible. Fairness and the public's benefit has to be carefully weighed against profitability.

economic: Rapid global communication is a key element in international trade and the global economy. Satellite communication also allows under-developed nations, without communication infrastructure, such as a conventional telephone system, to develop economically by communicating with potential customers in other countries.

environmental: Space junk.

 Essential Question: What is an op-amp and why is it revolutionary?

Electronics

1. State the properties of an ideal operational amplifier (op-amp). A DC-coupled high-gain electronic voltage amplifier with differential inputs and, usually, a single output.

 infinite input impedance at the input terminals--draws zero input current zero output impedance--drawing current from the device does not drop its output voltage. infinite open-loop gain (i.e., when doing theoretical analysis, limit should be taken as open loop gain Gopenloop goes to infinity) infinite bandwidth infinite slew rate (i.e., the rate of change of the output voltage is unbounded) zero offset voltage (i.e., when the input terminals are shorted so that V + = V − , the output is a virtual ground).

 Draw circuit diagrams for both inverting and non-inverting amplifiers (with a single input) incorporating operational  amplifiers. (ref) Derive an expression for the gain of an inverting amplifier and for a noninverting amplifier. Students should be aware of the virtual earth approximation.

1. Describe the use of an operational amplifier circuit as a comparator.

• draw appropriate circuits.

• Output devices for comparator circuits may include light-emitting diodes (LEDs) and buzzers.

 Describe the use of a Schmitt trigger for the reshaping of digital pulses. (ref1, ref2) triggers at a specific level and remains triggered until input falls below the chosen level. can convert a given waveform to a square wave output. Solve problems involving circuits incorporating operational amplifiers.

 Essential Question: Are mobile phone communications private?

The mobile phone system

 State that any area is divided into a number of cells (each with its own base station) to which is allocated a range of frequencies. Students should know that frequencies are allocated so as to avoid overlap between cells. conventional broadcast between X and Y -- the signal has to be powerful in order to reach the desired distance. While X is broadcasting, no one in an area greater in radius than the distance from X to Y can use the frequency without causing interference. cell phone communication between A and B --"A" broadcasts a weak signal to a local tower that then relays the signal via telephone lines to a distant tower. This tower then broadcasts a weak signal to the receiver at B. Only a very small area is prevented from using the same frequency. Describe the role of the cellular exchange and the public switched telephone network (PSTN) in communications using mobile phones. The role of the cellular exchange in the selection and monitoring of base stations and the allocation of channels should be understood. Discuss the use of mobile phones in multimedia communication. high carrier frequency enables high band width and high data rate digital signal enables flexibility for text, video, or graphics spread spectrum provides some level of privacy Discuss the moral, ethical, economic, environmental and international issues arising from the use of mobile phones. moral: Driving while talking on a cell phone is similar to driving drunk. ethical: While not easy, cell phones can be tapped. Even without GPS a cell phone caller's approximate location can be determined. economic: 3rd world countries with little infrastructure can more easily implement phone service. The use of smart phones is opening up internet availability to a wider variety of socio-economic groups. environmental: Cell phone towers have to be built at regular intervals. These can be visually displeasing. Micreowave transmissions of cell phones may have unintended environmental impact. However, there is little data to indicate that it is a problem. international: Enabling rapid communication to remote or lower socio-economic areas enhances their ability to participate in the global economy.

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