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

Section 5.1 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

Section 5.2 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

Section 5.3: Transmitter Structure

transmitters and receivers are assembled from the following building blocks:

am modes

• CW, SSB, and AM are all types of amplitude-modulated signals
• generated by same transmitter structure
• analog transmitters: generate/modify signals with discrete electronic components
• DSP/SDR transmitters perform these operations in software, on a microprocessor

CW transmitters

• oscillator + amplifier
• crystal oscillator restricts operating frequency, but is stable
• variable frequency oscillator VFO controlled transmitter
• oscillator may contain buffer amplifier to protect from chirps - rapid change in frequency (key-down periods due to power supply, or load changes)
• output amplifier: two stages
  • driver stage
  • power amplifier stage
• VFO transmitter, mixers used to change TX output frequency without changing VFO frequency range
• schematic: VFO has a fixed range, fed into mixer. other feed into mixer is local oscillator - switch between three different crystals. the mixer output is sent to the filter, and combined with the keyer to send out a signal.

SSB transmitter:

• same arrangement, but the VFO is now replaced with a microphone circuit
• input to balanced modulator is carrier signal, plus microphone input
• output is DSB signal, so filter is required to remove the undesired sideband
• USB signals on 20 m, LSB on 80 and 40 m
• mixer then converts signal to correct frequency
• SSB transmitter can unbalance the modulator to generate AM signals
• SSB signals: carrier must be reduced or suppressed to at least 40 dB below signal's peak power output, to prevent interference
• output amplifier must be a linear amplifier, due to rapidly changing speech waveforms
• nonlinear amplifier will distort speech
• CW transmitter turns sine wave on/off, no need to be linear
• SSB/AM stages must all reproduce input signal accurately, via mixing, filtering, amplifying
• distortion in the transmit chain will create suprious signals, harmonics, mixing products, splatter
• SSB bandwidth should be 3 kHz or smaller
• AM bandwidth should be 6 kHz or smaller
• on 60 m, USB signals hsould be 2.8 kHz or narrower

FM transmitters:

• modulation and frequency changes work differently in FM TX than SSB/AM TX
• less expensive implementation of FM:
  • generate FM signal at low frequency
  • multiply it to reach new band
• for 2 m FM transmitter, modulated oscillator frequency is about 12 MHz, and multiplier selects 12th harmonic for transmission on 2 m (12 x 12 = 144 MHz)
• To transmit a signal at 146.52 MHz, you must use the 12th harmonic, so the oscillator frequency must be adjusted to 146.52/12 = 12.21 MHz
• Deviation from crystal is also multiplied
• to control deviation of 146.52 MHz to within 5 kHz, crystal oscillator deviation should be 5/12 = .416 kHz = 416.7 Hz
• good FCC practice requires bandwidth to be limited
• Carson's Rule: BW = 2 x ( Peak Deviation + Highest Modulating Frequency)
• If FM phone signal peak deviation is 5 kHz, and highest modulating frequency is 3 kHz, bandwidth = 2(5+3) = 16 kHz
• Repeater coordinators use 20 kHz channel, fits 16 kHz wide signal comfortably
• FM and PM phase modulation have constant power, so amplifier does not need to be linear

Signal quality:

• keep signals intelligible and prevent excessive bandwidth
• potential issues:
  • overmodulation - AM modes
  • Speech processing
  • Overdeviation - FM/AM
  • key clicks

Overmodulation - AM modes:

• if amplitude of AM or SSB signal is varied excessively in response to modulating signal, result is overmodulation
• overmodulation caused by speaking too loudly or by setting mic or audio gain too high
• overmodulation causes spurious signals in nearby frequencies
• modulation envelope - waveform created by connecting peaks of modulated signal
• cutoff - transmitter is turned off instead of following the modulated signal (floor is too high)
• flag-topping - transmitter reaches max output and cannot increase with signal (ceiling too low)
• transmitted audio is distorted
• spurious signals are created
  • distortion products
  • splatter
  • buckshot
• properly adjust mic gain
• use oscilloscope to check for clean signal at TX output
• note transmitter settings and meter behavior, and oscilloscope images
• flat topping happens when drive level to transmitter output or external amplifiers is beyond the max power level
• carrier cutoff occurs when output signal is totally cut off between peaks
• use normal speech/audio levels
• use monitor function to check own signal
• ALC - automatic level control
• two tone test for transmitter linearity:
  • modulate signal with pair of non-harmonic tones
  • adjust transmitter and amplifier for output without any distortion

speech processing:

• average power of SSB signal is low compared to CW
• energy spread over wider spectrum
• makes signals harder to understand through noise
• speech processing increases average power of speech signal without distorting signal
• some processors go between mic and radio, some go between radio and amplifier
• compression - increases gain at low input levels, holds gain constant for hi input levels
• amount of compression mesasured in dB
  • if low-level input signal is amplified to 10 dB more gain than high-level signal, you have 10 dB of compression
• overprocesssing can also be an issue
• processed signals require adjustment of transmitter modulation to avoid splatter
• speech processing can increase background noise too
• speech processing requires balance between increase in average power versus reduction in intelligibility

overdeviation

• distortion in FM signals happening due to overmodulation
• instead of envelope distortion, you get frequency deviation
• overdeviation of FM signals will cause interference nearby, just like AM

Key clicks:

• sharp transient clicking sounds heard on frequencies adjacent to CW signals
• caused by transmitter turning on and off
• reduce key clicks by adjusting TX configuration or by modifying keyer's control circuit
• use oscilloscope to monitor CW waveform
• leading adn trailing edges of CW output shoudl be 4-8 ms long, otherwise clicks will be generated from too-rapid on/off
• waveform: key clicks will show up as signals in sidebands

amplifiers:

• hf operators use amps to strength signals
• VHF/UHF amps are solid state bricks
• HF amps use vacuum tube circuits
• SSB modes require linear amplifier, so wave form input not distorted
• amplifier circuit = stage
• four classes of amplifier stages:
  • Class A - most linear, least efficient, on all the time, gain is limited
  • Class B - push-pull, pair of amplifiers switching on and off with cycle, good efficiency, linearity is possible
  • Class AB - midway between A and B, amplifier device is active for more than half of the cycle. Linearity not as good as Class A.
  • Class C - active for less than half of cycle, highest efficiecy, only work for CW and FM due to non-linearity

Tuning Linear Amplifier:

• 3 primary operator adjustments
  • band
  • tune
  • load/coupling
• band - configures input and output impedance matching or filter circuits
• tune/load adjust components in output matching circuit (pi network)
• start by setting the band switch properly
• apply drive power to amplifier, wtch current meter
• adjust tune control for a minimum setting, which indicates output matching circuit is resonant with input signal
• adjust load to maximize output power, then adjust tune to plate-current minimum
• this will enable max output power without max plate current being exceeded
• drive power important, esp. for grid-driven amplifier circuits
• too much drive causes excessive grid current
• modern amplifiers have circuitry to protect circuits against excessive grid drive
• operate amp according to mfg specs!
• excessive drive power can also destroy solid state amplifiers with power transistors

neutralization:

• for oscillations, positive feedback must lead to a gain > 1
• HF amplifiers using triode tubes can become self-oscillating at VHF frequencies
• main path for positive feedback in a grid-driven amplifier is inter-electrode capacitance (voltage difference between plate and control grid)
• self-oscillation geneates spurious signals, can damage circuit components
• neutalization: creates negative feedback at VHF - connect an out-of-phase signal with input signal to cancel unwanted positive feedback
• neutralization is performed by connecting small variable apacitor between amplifier's output and input

Section 5.3 Summary

• Transceiver operated in "split mode" means it transmits/receives on different frequencies
• on a vacuum tube RF power amp, the plate current meter indicates correct plate tuning control when there is a dip or minimum
• use ALC auto level control with RF power amp to reduce distortion due to excessive drive
• in a solid state RF amplifier, excessive drive power can cause damage
• the correct adjustment for the load/coupling control of a vacuum tube RF power amplifier is maximum power output - while not exceeding maximum plate current
• transmitter keying circuits include time delay so that transmit-receive changeover operations can complete properly before RF output is allowed
• common use for a dual VFO feature on transceiver is to allow monitoring of 2 different frequencies
• to conduct a two-tone test, use two non-harmonically related signals
• the two-tone test analyzes transmitter linearity
• on modern transciever, speech processor is used to increase intelligibility of voice signals
• for ssb phone signal, speech processor will increase average power
• incorrectly adjusted speech processor will result in all of these:
  • distorted speech
  • splatter
  • excessive bkgd noise
• efficiency of RF power amplifier is obtained by: Efficiency = (RF output power)/(DC input power)
• Class A linear amplifier has the following characteristic: low distortion (but inefficient)
• Class C power stage is used to amplify CW signals (not SSB, not AM, because nonlinear and distorts speech waveform)
• Amplifier with highest efficiency is Class C, the most non-linear. The least efficient is Class A, the most linear.
• The final amplifier stage of a transmitter is neutralized to eliminate self-oscillations
• a linear amplifier is an amplifier that preserves the output wave form
• in some ssb transmitters, a filter sits between the balanced modulator and the mixer
• in some ssb transmitters, a balanced modulator combines the carrier oscillator and speech amp signals and sends the results to a filter
• an effect of overmodulation is excessive bandwidth
• to adjust proper ALC settings, on ssb, adjust transmit audio or mic gain
• flat topping means signal distorition caused by excessive drive
• an AM signal's modulation envelope is the waveform created by connecting peak values of modulated signal

Section 5.4: Receiver Structure

• Receivers have many more adjustments than transmitters

Superheterodyne receivers:

• mixing of signals is called heterodyning, leads to f1 +/- f2
• superheterodyne receivers are sensitive to very weak signals
• basic circuit:
  • RF signals are amplified and sent to mixer.
  • LO also sends signal to mixer.
  • signal at intermediate frequency IF is produced, filtered and demodulated.
• iF filter removes nearby signals selectively
• IF used because easier to tune filters/hi gain amps for single frequency
• to convert signal at 14.250 to an IF of 455 kHz, local oscillator should be at 14.250 +/- .455 (13.795 or 14.705 MHz)
• to cover entire 20 m band, 14.0 - 14.35, need local oscillator to tune from 13.545 MHz to 13.895 MHz
• Demodulation of amplified IF signal is done by the product detector, a type of mixer
• If AM being used, envelope detector is used for demodulation
• Output of product or envelope detector is used to recover modulating signal
• Product/envelope detector outputs audio signal
• Front end of superheterodyne: antenna feeds into Rf amp. Mixer mixes signal from RF amp and from local oscillator. That is front end.
• Front end processes weak signals at original frequency, so must owrk for wide frequency range and weak/strong signals
• preselector sometimes used between antenna and RF amp to reject strong, out of band signals
• these can fry a receiver if too strong, overwhelming circuitry
• if additional sensitivity needed, preamplifier can be used
• FM receivers: similar to superheterodyne, but key differences
• FM only frequency matters
• special non-linear amplifier called limiter replaces the linear IF amplifier
• limiter: amplifies received signal until all amplitude modulation information (e.g., noise) is removed and only square wave remains, square wave of varying frequency
• audio information: recovered by discriminator or quadrature detector (in place of the product detector in the AM/SSB/CW circuit)