exam for Electronic Engineering

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EE 1301: MODERN ELECTRONIC TECHNOLOGY

SESSION #25: GPS, PROPAGATION DELAY, WAVES

03/26/2018 Instructor: Joseph Cleveland, Ph.D.

Email: [email protected]

Thought for the Day

When you come to a fork in the road, take it

Yogi Berra

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Topics and Comments

• How GPS works – Why we need a very accurate clock

• Delays in digital computer circuits – Why there is an upper limit on your laptop clock

rate

• Lab #7 overview

• Waves and electronic technology

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The Global Positioning System

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GPS Network

• 24 GPS satellites orbit the earth at approximately 20,200 km (12,550 miles).

• These carry very stable atomic clocks that are synchronized with one another and to master atomic clocks at the National Institute of Standards and Technology (NIST) that operate the atomic clocks.

• Any drift from true time maintained at NIST in Bolder, CO, is corrected daily.

• GPS satellites continuously transmit their current time and position.

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GPS Fundamentals

• A GPS receiver monitors at least 4 satellites and solves equations to determine the position of the receiver.

• Question: How good is the position estimate?

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GPS Position Measurement

• First step: Measure the distance between the GPS receiver and a satellite

distance = speed of light x time

, , ⁄

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Requires the time it takes for the signal to travel from the satellite to the receiver.

GPS Position Measurement …

• In order to measure the travel time of the satellite signal, the receiver has to know when the signal left the satellite and when the signal reached the receiver.

• But how does it "know" when the signal left the satellite? – The GPS continuously computes the position of

every one of the 24 satellites

– It then determines the time delay for each signal it receives

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GPS Position Measurement …

• All GPS receivers are synchronized with the satellites so they generate the same digital code at the same time.

• A known random sequence of 1’s and 0’s

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GPS Position Measurement …

• When the GPS receiver receives a code from a satellite, it can look back in its memory bank and "remember" when the same code was sent.

, . ⁄

.

• Just how precise does this time need to be?

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tD

GPS Code in Memory

Received Code

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GPS Position Accuracy

• Studies by NIST yielded an 18 ns clock transfer accuracy more than 95% of the time:

∆ , ,

. • What technology provides this accuracy?

only the atomic clock

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Lab #7: Ring Oscillator

Logic Gate Time Delays

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Digital Logic Voltage Levels

• For a CMOS gate operating at a power supply voltage of 5 volts, the acceptable signal voltages range from 0 volts to 1.5 volts for a “0” logic state, and 3.5 volts to 5 volts for a “1” logic state.

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

R

D

G

S

5 V ID

Vin

If Vout is < 1.5 V, we call it is a “0”

If Vout is > 3.5 V, we call it is a “1”

Delays in Digital Logic Devices

• A CMOS gate structure looks like a capacitor

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Metallic Gate

Source Drain SiO2 Dielectric

P-Type Semiconductor

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5V

MOSFET Gate as a Capacitor

• CMOS gate structure as a capacitor

• Gate voltage charge and discharge

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Time to rise to a “1”

Time to fall to a “0”

Gate Propagation Delay

• When we switch a gate on or off it takes time to transfer electrons: to charge or discharge:

• The computer clock period cannot be shorter than this delay: clock rate < 1/tD

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

R

D

G

S

5 V ID

Vin

“1” level

“0” level

“1” level

“0” level

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Gate Propagation Delay …

• MC74VHCT series gate propagation delays

• Time delay for a full adder

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NOT: 4.7‐8.5 ns OR:  3.8‐6.5 ns

AND:  3.5‐7 ns NOR:  5.1‐8.5 ns

NAND: 3.8‐8.5 ns XOR: 4.8‐8 ns

Cin

S

Cout

8 ns 8 ns

7 ns 7 ns

6 ns 20 ns

16 ns

20 ns per bit pair for a full adder

Gate Delay & Ring Oscillator

• We can experience this delay with a ring oscillator made from NOT gates

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D

LED

LED ON

D

LED

LED OFF

Return pulse delayed 3

Return pulse delayed 3 again

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Gate Delay & Ring Oscillator

• The rate at which the LED turns on and off is determined by the total time delay around the ring

• Total time delay  6 to reset.

• We have a clock with a “tick” based on a physical parameter of a circuit.

• Use an oscilloscope to measure the wave period

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D

LED

LED ON

Waves

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Waves are Everywhere

• A Cesium atomic clock uses a microwave cavity that resonates at a wave frequency of 9,192,631,770 cycles per second

• GPS conveys time information by radio waves

• Barcode readers detect energy carried by light waves

• Electronic amplifiers increase the amplitude of a sound wave from a violin string

WAVES, WHAT ARE THEY?

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Waves are Everywhere …

• How do we use electronic technology to detect waves for use in: – Ribbon microphones

– Digital recording of music and video (CDs, DVDs)

– Radio and television (cable)

– ??

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Basic Variables of Wave Motion

• Wavelength  • Period T • Frequency f • Amplitude A • Speed v

• The wavelength is the minimum distance between any two identical points (such as the crests).

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Longitudinal Waves

• A wave that causes the particles of the medium to move parallel to the direction of wave motion is called a longitudinal wave.

• Sound is a longitudinal wave

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Direction of travel at speed v

Motion of air molecules



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Longitudinal Waves …

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High pressure

Low pressure

 peak

trough

distance

pressure amplitude

Transverse Waves

• A wave that causes the particles of the disturbed medium to move perpendicular to the wave motion is called a transverse wave.

• A vibrating violin string is a transverse wave

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Transverse Waves …

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 particle motion

displacement amplitude

direction of travel

Wave Generation

• Rate of wave generation is the frequency

• Wave speed: v – Sound: v = 1100 ft/sec or 343 m/sec at 20C – Violin “A” string: v = 281.6 m/sec

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

wave moves with speed v

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End of Session

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