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Chapter 1: Introductory Topics. EET 2323C: Electronic Communications I - Analog Miami Dade College. Chapter 1: Introductory Topics. Communication Systems Decibels Electrical Noise Sources Measurements Information and Bandwidth Fourier Analysis LC Circuit Review Oscillator Review
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Chapter 1: Introductory Topics EET 2323C: Electronic Communications I - Analog Miami Dade College
Chapter 1: Introductory Topics • Communication Systems • Decibels • Electrical Noise • Sources • Measurements • Information and Bandwidth • Fourier Analysis • LC Circuit Review • Oscillator Review • Troubleshooting Electronic Systems
1.1 Communication Systems The function of a communication system is to transfer information from one point to another via some communication link.
1.1 Communication Systems Basic to this transfer in electrical communications is the process of modulation: • Modulation – process of putting information onto a high-frequency carrier for transmission Thus, the transmission happens at the high frequency, which “carries” the lower frequency information.
1.1 Communication Systems The low-frequency information that is transferred is often called the intelligence signal, or simply, intelligence. The process of extracting the intelligencefrom the high-frequency carrier is called demodulation.
1.1 Communication Systems Well, let’s try an experiment! On the count of three, yell out your birth date. Modulation / Demodulation sounds complicated, why would we want to go through the trouble? Why not just transmit the intelligence? Human voice ranges from about 20 to 3000 Hz, if we all transmitted at those frequencies the intelligence would be lost! …. And it would be very noisy! (We would also need HUGE antennas)
1.1 Communication Systems Modulation helps solve this issue. If we all transmit at different frequencies, I can “tune” into only frequency that is carrying the intelligence that I want.
1.1 Communication Systems There are three basic methods of putting low-frequency intelligence onto a higher frequency: Where v = instantaneous voltage = peak value or amplitude = angular velocity = 2f =phase angle Amplitude Modulation (AM) FrequencyModulation (FM) Phase Modulation (PM)
1.2 Decibels Decibels (dBs) - a logarithmic unit of measurement that expresses the magnitude of a physical quantity (usually power or intensity) relative to a specified or implied reference level.
1.2 Decibels Other useful forms: and if R1 = R2, as is often the case, then:
1.2 Decibels Often, decibels are used to provide “standard” reference levels. Commonly used standards: dBm = dB level using a 1 mW reference dBm(600) = dB level using a 1 mW reference with respect to a 600- load. dBm(n) = dB level using a 1 mW reference with respect to a n- load.
1.2 Decibels Show that when making a dBm measurement, a measured value of 1mW will result on a 0 dBm power level.
1.2 Decibels A microwave system requires a +8-dBm audio level. Determine the voltage level required to produce a +8-dBm level assuming a 600- audio system.
1.2 Decibels A laser diode outputs +10 dBm. How many watts is this?
1.3 Electrical Noise One of the primary concerns in the design of communication systems is electrical noise: Electrical Noise= any undesired voltages or currents that ultimately end up appearing in the receiver output. To the end user, this often manifests as Static.
1.3.1 Source of Electrical Noise Noise can arise from two sources: External Noise– noise in a received radio signal that has been introduced by the transmitted medium Internal Noise – noise in a radio signal that has been introduced by the transmitter/receiver
1.3.1 Sources of External Noise Human-Made Noise – Usually most troublesome form of external noise. Often produced by spark-producing mechanisms such as: • engine ignition systems • fluorescent lights • commutators in electrical motors • other large sources include power lines Occurs randomly at frequencies up to about 500 MHz.
1.3.1 Sources of External Noise Atmospheric Noise – caused by naturally occurring disturbances in the earth’s atmosphere: • Lightning discharge most common contributor • Frequency spread over the entire radio spectrum • Intensity is inversely related to frequency Not a significant noise factor for frequencies exceeding about 20 MHz.
1.3.1 Sources of External Noise Space Noise – noise arriving from outer space. • Solar noise originates from our sun, and usually peaks every eleven years • Cosmic noise originates from all other stars, and while their individual effects are small, but the additive effects of billions of them are problematic Space noise occurs at frequencies from about 8 MHz up to 1.5 GHz (it actually occurs at < 8 MHz, but is absorbed by the ionosphere).
1.3.1 Sources of External Noise Summary of External Noise FrequenciesSources of External Noise 0 – 500 MHz Human-Made Noise 0 – 20 MHz Atmospheric Noise 8 MHz – 1.5 GHz Space Noise
1.3.1 Sources of Internal Noise Internal noise is introduced by the receiver itself, and is most problematic in the early stages of the amplification process.
1.3.1 Sources of Internal Noise Thermal Noise (aka Johnson Noise, White Noise) – noise caused by thermal interaction between free electrons and vibrating ions in a conductor: • Causes rate of arrival of electrons at either end of a resistor to vary randomly • Temperature dependant • Spread equally throughout the entire frequency spectrum
1.3.1 Sources of Internal Noise Thermal noise power is given by where k = Boltzmann’s constant (1.38 x 10-23 J/K) T = resistor temperature in Kelvin (K) f = frequency bandwidth of the system The noise in voltage across a resistor can be estimated as: where en is the rms noise voltage
1.3.1 Sources of Internal Noise Determine the noise voltage produced by a 1-M resistor at room temperature (17oC) over a 1-MHz bandwidth.
1.3.1 Sources of Internal Noise Shot Noise – noise that occurs in semiconductors (primarily transistors) caused by the discrete-particle nature of the current carriers. • Caused by random paths electrons travel within device • No good way to estimate, must refer to device sheet Excess Noise – little understood noise that occurs at low frequencies (< 1kHz). Thought to be the result of imperfections of the crystal surface.
1.3.1 Sources of Internal Noise Transit-time Noise – noise that increases rapidly in the vicinity of the device’s high-frequency cutoff. Occurs when transit time of carriers crossing a junction is comparable to the signals period.
1.3.2 Noise Designation and Measurement Signal-to-Noise Ratio– fundamental relationship between the desired signal power and noise power: often described in decibels:
1.3.2 Noise Designation and Measurement Noise Figure (NF)– used to describe how much additional noise a particular device has injected: where Si/Ni= signal-to-noise power ratio at input So/No= signal-to-noise power ratio at output often reported as a device’s Noise Ratio (NR)
1.3.2 Noise Designation and Measurement If the device were ideal, then If the device under consideration were ideal (injected no additional noise), what would be it’s noise factor?
1.3.2 Noise Designation and Measurement A transistor amplifier has a measured S/N power of 10 at its input and 5 at its output. Calculate the NR and the NF.
1.3.2 Noise Designation and Measurement Example Datasheet NF Curve (2N4957)
1.3.2 Noise Designation and Measurement As discussed previously, noise in early stages of amplification is most important. Friiss’s Formula quantifies this:
1.3.2 Noise Designation and Measurement Equivalent Noise Temperature (Teq) = another common way of manufacturers specifying noise. where T0= 290 K *Note that equivalent noise temperature is used for convenience, but is not an actual temperature.
End of Lecture Review 1. Shot noise is directly related to what? a. DC current b. AC voltage c. resistor value d. Boltzman constant
End of Lecture Review 2. What device superimposes information onto a high frequency signal for transmission? a. a demodulator b. the carrier c. modulator d. the intelligence
End of Lecture Review 3. The device which extracts the information signal from the high frequency carrier is called a a. transducer b. transmitter c. modulator d. demodulator
End of Lecture Review 4. Which is NOT an example of internal noise? a. noise produced by resistors b. noise produced by transistors c. excess noise d. space noise
End of Lecture Review 5. Which is NOT a term for noise produced by resistors? a. White noise b. Shot noise c. Johnson noise d. Thermal noise
End of Lecture Review 6. If the noise produced by a resistor in a radio receiver is 2.4 uV and the resistance is then doubled, the resulting noise produced by the new resistance is approximately what? 3.4 uV
End of Lecture Review 7. Two types of noise that occur at extremely low and high frequencies in amplifiers are known as a.excess noise and transit time noise b. flicker noise and shot noise c. 1/f noise and thermal noise d. Johnson noise and pink noise
End of Lecture Review 8. A figure of merit that best describes how much noise a device creates is known as: a. signal to noise ratio b. Noise figure c. Friis noise d. effective noise bandwidth
