General/Chapter 5 Study Guide

Chapter 5: Radio Signals and Equipment

Section 5.1: Signal Review

• a radio signal at one frequency, at constant power, is CW - continuous wave
• adding information to a signal (via changes in frequency, phase angle, or amplitude) is modulation
• the method of modulation is asignal's mode
• an unmodulated signal carries no information
• to recover information from a signal is demodulation
• voice mode/phone mode - voice/speech is information
• data mode/digital mode - data is information
• analog - information can be understood by human
• digital - information can be understood by computer

AM

• amplitude modulation
• information contained int he signal's envelope, or maximum instantaneous power for each cycle
• AM signals have a carrier and 2 sidebands
• AM signals with the carrier removed are double sideband DSB
• AM signals with one sideband and the carrier removed are single sideband SSB

angle modulation

• varying freq to add info is frequency modulation FM
• amt that signal frequency varies is called deviation
• phase angle can be varied with respect to reference phase, this is phase modulation
• FM/PM can be decoded with same circuits
• FM: changes amount of time signal takes to make a 360 degree cycle

PM - changes relative phase difference between signal and reference phase

• FM and PM signals have one carrier, multiple sidebands
• FM/PM are constant power (modulated or not)

bandwidth

• definition of bandwidth - width outside of which the average power of the signal is attenuated by 26 dB below the mean power
• typical bandwidth values:
  • TV - 6 MHz
  • AM - 6 kHz
  • FM - 5-16 kHz
  • SSB - 2-3 kHz
  • Digital - 50-300 Hz
  • CW - 100-300 Hz

Summary:

• The process that changes the phase angle of an RF wave to convey information is phase modulation
• the process that changes instantaneous frequency of RF wave to convey information is frequency modulation
• Instantaneous power level of RF signal used to convey information in amplitude modulation AM
• The phone emission with the narrowest bandwidth is SSB single sideband

Section 5.2: Radio's Building Bloccks

Oscillators, mixers, multipliers, modulators

Oscillators:

• produce a pure sine wave, as close to 1 frequency as possible
• oscillator has two parts:
  • amplifier to increase gain
  • feedback circuit to route some output back into input
• if product of amplifier gain and amt of feedback are > 1, circuit's output will be self-sustaining - this is called oscillation
• oscillator output frequency can be fixed or varied

fixed frequency oscillators FFOs: 3 types

• RC (resistors/capacitors)
• LC (inductors/capacitors)
• crystal (acts like LC, orders of magnitude more precise)

variable frequency oscillators VFOs: 3 types

• LC circuit with variable capacitor
• PLL phase locked loop
• DDS direct digital synthesizer (software-controlled, has stability of crystal oscillator)

Mixers:

• changing frequency of signal is key function in RX/TX
• this is what mixer does
  • in: f1, f2
  • out: f1 +/- f2
• mixers combine 2 frequencies f1 and f2
• combine to form f1 + f2 and f1 - f2
• heterodyning - the mixing of 2 frequencies (f1 +/- f2)
• input f1 is called RF input (transmitted signal)
• input f2 is called local oscillator LO (locally-generated reference signal)
• outputs from mixer are called mixing products

Multipliers:

• this unit creates a harmonic of the input frequency (multiplies by an integer)
• low-frequency oscillators are easier/smaller to build, so run low-frequency oscillator signal through a multiplier to make a VHF/UHF signal

Modulators:

• modulators add information to a carrier signal
• information added to a signal as amplitude, frequency, or phase variations
• input 1 is carrier input f1
• input 2 is modulating input f2
• output is f1 modulated by f2

Amplitude Modulators:

• first created by varying the power supply voltage of a CW signal
• as voltage changes, amplituded of output signal's envelope follows along
• also called plate modulation, or drain modulation, or collector modulation
  • plate modulation: voltage being varied connects to a vacuum tube
  • drain modulation - voltage being varied connect to transistor's drain
  • collector modulation - voltage being varied connects to a transistor's collector
• modulation transformer then adds/subtracts amplified voice signal to power supply voltage
• AM circuits cannot generate SSB signals
• AM, double sideband can be generated by balanced modulator
• balanced modulator:
  • input 1: carrier signal f1
  • input 2: modulating signal f2
  • output signal: double sideband f1 +/- f2
  • the pairs of sidebands, above and below, are due to heterodyning, f1 +/- f2
• DSB - double sideband requires a balanced modulator, so the carrier frequency will cancel out
• AM - requires an unbalanced modulator, so the carrier frequency will survive heterodyning

Frequency and Phase Modulators:

• FM - frequency of modulated signal changes with modulating signal's amplitude
• PM - frequency of modulated signal (deviation of signal) is proportional to modulating signal's amplitude and frequency
• FM/PM sound the same, demodulated with same circuit
• angle modulation performed with a reactance modulator
• two types of reactance modulators:
  • FM reactance modulators
  • PM reactance modulators
• FM reactance modulator: amp feeds into reactance modulator, output is frequency modulated output
• PM reactance modulator: amp and fixed-frequency oscillator feed into reactance modulator, output is phase modulated output

Summary:

• if a 3 kHz LSB signal has a carrier frequency of 7.178 MHz, it occupies 7.175-7.178 MHz
• if using 3 kHz USB signal on 20 m, how close to edge of band should carrier signal be? 3 kHz below edge of band
• basic components of a sine wave oscillator are an amplifier and a feedback circuit with a filter
• frequency of LC oscillator can vary depending on component ratings - individual inductances and capacitances
• a transceiver controlled by a direct digital synthesizer is that you have stability of a crystal, with variable frequency
• a reactance modulator connected to RF amplifier stage generates phase modulation or frequency modulation
• carrier suppression in SSB phone - the advantage over AM is that transmitter power can be used more efficiently (narrower bandwidth)
• the receiver stage that combines a 14.250 MHz signal with a 13.795 MHz oscillator to produce a 455 kHz intermediate frequency is a mixer (heterodyning)
• the mixing of two signals is called heterodyning
• in a VHF/UHF transmitter (FM), the stage that generates a harmonic of a low-frequency signal is the multiplier