This is the class outline for week 3 of the SCARS Technician class 2013.
Week 3 Outline
Electricity
Quantity | Description | Symbol(s) | Unit(s) | Measured by: |
Current | Flow of electrons | I, i | Amps | Ammeter |
Voltage | Electromotive force (EMF) that causes current flow | V, v, E | Volts | Voltmeter |
- Electrons: negative electrical charges
- Current generally flows through a conductor (e.g., wire) or component (e.g., light bulb, transistor)
- Conductors: metals (copper, aluminum, gold, silver), graphite, water (pure: poor; salty: fair)
- Voltage difference
- Earth is reference (“zero”): earth ground, ground potential,ground
- Chassis (car body, metal box) also referred to as ground
- Voltage can be negative (-) or positive (+); like charges repel, so:
- Negative voltage repels electrons
- Positive voltage attracts electrons
Circuit
- Various components, usually connected by wire, copper trace on circuit board, chassis
- Parallel, serial circuits
Water flow analogy
- pump analogous to voltage source
- pipe analogous to conductor
- gallons/minute, liters/second, etc. analogous to current
- water wheel, turbine, pipe constriction analogous to load (resistance, or impedance, etc.)
Note: voltage measured across battery (parallel); current measured inline (series)
Multimeter Demo
Don’t measure current or resistance of battery
Parallel and Series Circuits
- (A) same current through all components
- (B) same voltage across all components
- Open circuit: infinite resistance (“OFF”)
- Closed (short) circuit: zero resistance (Bad mojo!)
Resistance
- resists, or impedes, current
- ohms, Ω (Greek upper case omega)
- measured with (gasp!) ohmmeter
- German physicist Georg Simon Ohm (mhos!)
Ohm’s Law
- expresses relationship between voltage, current and resistance
- R = E/I
- I = E/R
- E = I x R
Power
- rate energy is used (e.g., jogging requires 500 calories per hour)
- measured in watts (W), with a wattmeter
- your clothes dryer draws more power than your Ipod
- Power = voltage times current (curent can be determined by knowing voltage and power; appliance labels)
- P = E x I
- E = P/I
- I = P/E
Example: Toaster draws 1200 watts; how many amps will it draw off of your 20 amp circuit?
Combining Ohm’s law and Laws’s law
P = E x I, but E = I x R, so
P = I2R
I = E/R, so
P = E2/R
Example: My HF rig transmits 100 W. It is about 33% efficient, so it requires 300 W from the 12 V power supply. How many amps does it require from the power supply?
AC/DC
AC:
- alternating current
- alternates polarity
- current flows one way, then the other
- household current in US is 60 Hz, 120 V AC
- radio waves are (generated by) AC (starting about 30 kHz)
DC:
- same polarity
- constant voltage
- car battery: 12 V, DC
Components and Units
Basic Components
- resistors
- capacitors
- inductors (coils)
Resistors
- ohms, kilohms, megohms
- restricts current flow
- generates heat, or light (e.g., light bulb)
- variable resistors: potentiometers, or “pot”
- often marked with color code (red-violet-brown-gold: 270 ohms, tolerance 5%)
Capacitors
- farads, microfarads, nanofarads, picofarads
- stores electric energy in electric field
- smooths out voltage changes
- schematically, two metal plates separated by insulator
- blocks DC
- AC: higher the frequency, lower the reactance (a kind of impedance)
- variable capacitors
Inductors
- henrys, millihenrys, micro-henrys, nano-henrys
- stores electric energy in magnetic field
- smooths out current changes
- schematically, coil of wire
- passes DC
- AC: higher the frequency, higher the reactance
- coil may have iron or ferrite core to increase inductance
Transformer
- two inductors in proximity, used to change voltage (and current)
- voltage change proportional to turns ratio
Schematic of a transformer.
Typical transformer.
Power-pole transformer.
Variable components:
Reactance and Impedance
- resistor does not affect phase of current and voltage
- capacitor: current changes lead voltage changes
- inductor: voltage changes lead current changes
- inductive reactance: X, increases with frequency (low-pass filter)
- capacitive reactance: X, decreases with frequency (high-pass filter)
combination of resistance and reactance: impedance, Z
Resonance
- at some frequency (resonant frequency), capacitive reactance and inductive reactance cancel
- impedance is minimum
- filter: passes some frequencies, blocks others
- tuned circuit is (narrow) band-pass filter
Diodes, Transistors, and Integrated Circuits
- response depends on voltage, current and/or polarity
- made from semiconductor
- silicon, germanium…
- “doped” with impurities to allow positive or negative charges to pass
- P-type, N-type
- PN junction allows more current flow in one direction than the other
Diode
- PN
- allows current flow in one direction
- heavy-duty diode: rectifier, converts AC into pulsating DC
- has anode and cathode
- diode across tuned circuit, hooked up to an antenna: “crystal” radio
- LED: light-emitting diode
Transistors
- can control large voltage with small voltage (gain)
- amplifiers, switches
- two common types:
- BJT: bipolar junction transistors
- PNP, NPN
- base, emitter, collector
- FET: field-effect transistors
- FET: field-effect transistors
- conducting path or channel of N- or P-type material
- ends of channel form the source and drain electrodes
- gate electrode controls current flow through channel
- BJT: bipolar junction transistors
Integrated Circuits
- IC, or “chip”
- many components in a single solid-state device
Protective Components
- fuses, circuit breakers, surge protectors, UPSs, lightning arrestors
- protect equipment
Circuit Gatekeepers
- switches, relays
- open and close circuits
- number of poles and throws
- e.g., SPST is simplest switch
Schematics and Component Symbols
Types of Radios and Radio Circuits
Transmit-Receive switches
Oscillators and amplifiers
- oscillator produces steady signal at one frequency
- driver is a stable, low-level amplifier
- power amplifier further amplifies signal
Filters
- pass certain frequencies
- attenuate other frequencies
- passive filter: tuned circuit, capacitors and inductors
- active filter: electronic curcuts that selectively amplify certain frequencies
- high-pass, low-pass, band-pass, notch
Modulators
- discussed earlier, puts information (voice, data, morse code, etc.) onto carrier
- on-off, AM, FM, SSB, phase-modulation…
Mixers
- combine 2 radio frequencies
- result is sum and difference of the frequencies
- Example:
- radio receives bands at 3.5 MHz, 7.0 MHz, 14.0 MHz, 21.0 MHz, 28.0 MHz
- mix 3.5 MHz signal with 7.0 MHz => difference of 3.5 MHz
- mix 10.5 MHz signal with 14.0 MHz => difference of 3.5 MHz
- mix 17.5 MHz signal with 21.0 MHz => difference of 3.5 MHz
- mix 24.5 MHz signal with 28.0 MHz => difference of 3.5 MHz
- mixing makes multi-band receiver easy to implement
Demodulators
- opposite of a modulator
- extracts information (voice, data, morse code, etc.) from RF signal
- detectors (for AM: envelope detector)
- product detectors (for SSB, CW)
- frequency discriminators (for FM)
Receivers
simple direct-conversion receiver
simple superheterodyne receiver
FM receiver
Transverters
- uses superheterodyne principle of mixing frequencies
- converts entire transceiver to operate on a different band
- Example:
- There are few 220 MHz-band SSB and CW radios
- transverter can allow transceiver for 10-meter band (28 MHz – part of every HF rig) to operate on 220 MHz band
T7A – Station radios; receivers, transmitters, transceivers
T7A01 (C)
What is the function of a product detector?
A. Detect phase modulated signals
B. Demodulate FM signals
C. Detect CW and SSB signals
D. Combine speech and RF signals
T7A02 (C)
What type of receiver is shown in Figure T6?
A. Direct conversion
B. Super-regenerative
C. Single-conversion superheterodyne
D. Dual-conversion superheterodyne
T7A03 (C)
What is the function of a mixer in a superheterodyne receiver?
A. To reject signals outside of the desired passband
B. To combine signals from several stations together
C. To shift the incoming signal to an intermediate frequency
D. To connect the receiver with an auxiliary device, such as a TNC
T7A04 (D)
What circuit is pictured in Figure T7, if block 1 is a frequency discriminator?
A. A double-conversion receiver
B. A regenerative receiver
C. A superheterodyne receiver
D. An FM receiver
T7A05 (D)
What is the function of block 1 if figure T4 is a simple CW transmitter?
A. Reactance modulator
B. Product detector
C. Low-pass filter
D. Oscillator
T7A06 (C)
What device takes the output of a low-powered 28 MHz SSB exciter and
produces a 222 MHz output signal?
A. High-pass filter
B. Low-pass filter
C. Transverter
D. Phase converter
T7A07 (B)
If figure T5 represents a transceiver in which block 1 is the transmitter
portion and block 3 is the receiver portion, what is the function of block 2?
A. A balanced modulator
B. A transmit-receive switch
C. A power amplifier
D. A high-pass filter
T7A08 (C)
Which of the following circuits combines a speech signal and an RF carrier?
A. Beat frequency oscillator
B. Discriminator
C. Modulator
D. Noise blanker
T7A09 (B)
Which of the following devices is most useful for VHF weak-signal communication?
A. A quarter-wave vertical antenna
B. A multi-mode VHF transceiver
C. An omni-directional antenna
D. A mobile VHF FM transceiver