Chapter 5
Radio Signals and Equipment
Signal Review
• Modulation is the process by which
information (voice or data) is impressed
into the transmitted radio signal.
• Demodulation is the process by which
information is extracted from the received
signal.
• The more information a signal carries, the
wider is its bandwidth.
• Bandwidth is the width of the frequency
band outside of which the average
transmitted power is attenuated at least 26
dB.
• Amplitude Modulated Modes
– Amplitude modulation (AM) contains a carrier
with two identical sidebands above and below
the carrier.
– Single sideband (SSB) removes the carrier
and one of the sidebands - upper sideband
(USB) and lower sideband (LSB). More
transmitter power is contained in a smaller
bandwidth.
– The duty cycle (% of time the transmitter is at
full power) is 100% for AM, with or without
modulation. The duty cycle for SSB is about
20 to 25% and 0% without modulation.
• Angle Modulated Modes
– Frequency Modulation (FM) varies the
frequency in proportion to the signals
amplitude.
– Phase modulation (PM) varies the phase
(time shift) of the signal.
– FM and PM signals are generated differently,
but can be demodulated by the same circuits.
– FM and PM signals have a carrier and many
sidebands.
– The duty cycle of FM or PM is 100% with or
without modulation.
Amateur Signal Bandwidths
Signal
Bandwidth
AM Voice
6 kHz
SSB Voice
2 to 3 kHz
FM Voice
5 to 15 kHz
SSB Digital
0.05 to 3 kHz (50 to 3000 Hz)
CW
0.1 to 0.3 kHz (100 to 300 Hz)
Amateur Television 6000 kHz (6 MHz)
Digital Modes
• Digital information is sent as binary data or
bits (two states, 0 or 1).
• A symbol can be a signal bit, or a group of
bits. The symbol rate (Baud rate) is rate at
which symbols are transmitted. As the
symbol rate increases, the signal’s
bandwidth also increases.
• The duty cycle for a digital tranmission is
nearly 100%. Therefore, you should
reduce your transmit power to about 50%
of maximum power.
• Frequency shift keying (FSK) is a shift in
the transmitter’s carrier frequency to
represent the binary data.
• A more common approach is audio
frequency shift keying (AFSK). This
method uses two audio frequencies to
represent the binary data. The advantage
of AFSK is that the tones can be fed
directly into the microphone input of a
transmitter. LSB is used for AFSK.
• As the symbol rate increases, the two
tones must be spaced further apart.
• Radioteletype (RTTY) is an old form of digital
communications which uses a mark (1) and
space (0). The initial bit is a start bit and the
characters are separated by a stop bit. RTTY
uses the 5 bit Baudot code.
• On HF, most RTTY signals are 45 baud and use
a 170 Hz shift between mark and space.
• Multiple-frequency shift keying (MFSK16)
uses 16 separate tones separated by
15.625 Hz. This fits within a 500 Hz CW
bandpass filter.
– MFSK16 provides good weak signal
performance.
– MFSK16 does not provide error correction.
Phase Shift Keying
• Phase shift keying (PSK) is when the
phase of the tones or signal are varied to
create the binary signal.
• PSK31 is a popular mode that uses a 180̊
phase shift of the tone to create the binary
signals.
– The baud rate is 31.25
– Uses a varicode where the characters do not
have a fixed length (similar to Morse code).
Packet Modes
• A packet is a structured data group. An
entire packet must be received to be
processed correctly at the receiving end.
• Errors can be detected by calculating a
checksum. This is similar to an ARRL
radiogram with a letter count. Like a letter
count, a checksum is not robust and may
incorrectly say a message has no errors.
A more sophisticated count is the cyclic
redundancy check (CRC).
• If an error is detected, the receiving
system responds with NAK (not
acknowledged) message and the packet is
resent. This protocol is called Automatic
Repeat Request (ARQ).
• Forward error correction (FEC) includes some
redundant information in the packet to allow for
some error correction at the receiving end
without a resend.
• Packet radio is used in VHF and UHF bands at
baud rates of 1200 or 9600 baud.
• PACTOR I uses ARQ and FSK for reliable HF
communications. PACTOR II and PACTOR III
use PSK.
• WINMOR also uses ARQ and FSK or PSK for
HF communications.
• Winlink 2000 uses either PACTOR or WINMOR.
Radio Building Blocks
• Oscillators are used to generate a pure
sine wave.
– An oscillator must have feedback to drive the
oscillation.
• The frequency of an oscillator is
determined by the combination of
components.
– RC (resistor-capacitor) oscillators
– LC (inductor-capacitor) oscillators
• LC oscillators are more stable than RC oscillators.
– Quartz crystal oscillators provide fixed
frequency operation and are very stable.
• A variable frequency oscillator (VFO) can
be used to tune a radio over different
frequencies.
– LC circuits can be used as a VFO
– Phase Locked Loop (PLL) can be used as a
VFO and has the stability of a crystal.
– Direct Digital Synthesis (DDS) can be used as
a VFO and also has the stability of a crystal.
Mixers
• A mixer combines two signals and creates
sum or difference frequency at the output.
This is called heterodyning.
Multipliers
• Multipliers are used to generate
harmonics, or multiples, of the input
frequency.
• Multipliers are often used at VHF or UHF
where stable oscillators are those high
frequencies are not possible.
Modulators
• Modulators are a part of a transmitter that
encodes the information onto the radio
wave.
• Amplitude modulation (AM) can be
generated by directly varying the output of
the transmitter in step with the audio
signal.
• An AM signal consists of a carrier in the
center and two sidebands.
• A double sideband (DSB) signal consists
of the two sidebands and no carrier.
• SSB (and AM) can be generated using a
balanced modulator.
• The low audio frequencies are closest to
the carrier for both the upper and lower
sidebands.
• In SSB all of the
transmitter’s power is
contained in one sideband
which provides more
effective communications.
• In SSB the displayed
frequency on the
transmitter is the
frequency of the
suppressed carrier. You
should stay 3 kHz from
band edges.
• FM and PM are generated using reactance modulators.
In FM the signal frequency deviates in proportion to the
signal’s amplitude. In PM the deviation is proportional to
the signal’s amplitude and frequency.
• In PM the phase of the carrier will change, but not the
average frequency.
• FM and PM can be demodulated by the same circuit.
PM transmitters are easier to design.
Transmitter Structure
• A simple CW transmitter has a key that
turns on and off the transmitter.
• Adding a mixer and local oscillator (LO)
allows for added bands.
• In a SSB transmitter the VFO output is
modulated by the audio in the balanced
modulator. The output of the balanced
modulator is a DSB signal.
• A linear amplifier must amplify the signal to
higher power with no distortion.
• FM signals are usually generated at a
lower frequency and multiplied up to a
higher frequency.
– Ex: 146.52 / 12 = 12.21 MHz.
– If the deviation is to be no more than 5 kHz,
then the deviation of the 12.21 MHz signal
must be 5 kHz / 12 = 0.417 kHz.
– FM bandwidth, BW = 2 x (peak deviation +
highest modulating frequency)
• For a 3 kHz audio signal, and 5 kHz deviation, the
bandwidth is, BW = 2 x (5 + 3)
– An FM transmitter amplifier does not need to
be linear since the power output is constant
and only the frequency of the signal matters.
Signal Quality
• Overmodulation of
an AM signal will
cause distortion in
the received signal
and generate
spurious signals
(splatter) beyond
the normal
bandwidth.
• Use the automatic level control (ALC)
indicator to set your microphone or audio
level.
• A two-tone test (using 700 and 1900 Hz) is
used to adjust audio gain and level. It only
needs to be done occasionally.
• Flat-topping or clipping occurs when the
drive level of an amplifier causes the
amplifier to exceed its maximum output.
• The average power in a SSB signal can be
quite low. Speech processing increases
the audio level overall and therefore the
average transmit power goes up.
– Compression amplifies the lower audio levels
by as much as 10 dB more and leaves the
gain of the higher audio levels the same.
– Too much processing (overprocessing) can
reduce intelligibility.
• Overmodulation of FM or PM signals
results in overdeviation, which causes
interference in adjacent channels, and
distortion of the audio.
• In CW operation the transmitter is turned
on and off rapidly. If the transmitter is not
properly adjusted then key clicks can be
heard.
• Digital signals can also become distorted if
not adjusted correctly. It is helpful to have
someone listen to your signal to properly
set your audio level.
Amplifiers
• Amplifiers are used to increase the output power
of a transmitter. SSB requires a linear amplifier.
FM, PM, and CW do not require a linear
amplifier.
– Class A – most linear, amplifying device is on all the
time
– Class B – also called push-pull. Uses a pair of
amplifying devices that are only on half the time.
Linearity and gain are good.
– Class AB – midway between classes A and B.
– Class C – highest efficiency but very non-linear. Can
be used for FM, PM , or CW.
– Efficiency of an amplifier is its RF output power
divided by its DC input power.
– A keying circuit is needed to activate the
external amplifier whenever the transmitter is
transmitting. A changeover relay bypasses
the amplifier when receiving. Hot switching is
when the changeover relay is switched while
transmitting. This can destroy the relay and
related electronics. A very short delay in the
changeover can avoid this.
– Tube amplifiers have three controls: band,
tune, and load.
• Adjust the band for the operating band
• Adjust load to maximize the power output
• Adjust tune to so that the maximum power output
occurs without exceeding the maximum plate
current.
– Be careful to not overdrive an amplifier with
too much input power. This can destroy the
transistors (or tubes) in the amplifier.
– ALC from the amplifier can be connected to
the transmitter to avoid overdriving. Check for
compatibility between the two ALC signals.
– Self-oscillation can occur in a vacuum tube
amplifier. A neutralizing (variable) capacitor
between the amplifiers output and input will
“neutralize” the oscillation. Adjust the
neutralizing capacitor whenever the tube is
replaced.
Receiver Structure
• Receivers must be able to select out very
tiny signals (nW or pW) from a mix of all
the radio signals present. A receiver
needs to be selective and sensitive.
• Superheterodyne receivers are both
selective and sensitive.
• A basic superheterodyne receiver consists
of a mixer connected to the antenna, a
local oscillator (LO), and a detector.
• AM, SSB, and CW signals are detected
using a product detector. AM can be
detected with an envelope detector (a
diode).
• An FM (or PM) receiver uses a
discriminator or quadrature detector to
demodulate the FM signal.
• The limiter is a non-linear amplifier for
improved efficiency.
• The mixer in a superhet receiver produces
both sum and difference frequencies.
– If the IF is 455 kHz and the LO is 13.800, then
the superhet receiver can pick up 13.800 +
.455 = 14.255 MHz and 13.800 - .455 =
13.345 MHz. The later is an image. Filters at
the receiver front end remove images.
• The LO, and its harmonics, can leak into
the signal path and created unwanted
steady signals which are called birdies.
• Using two (double conversion) or three
(triple conversion) IF stages can improve
sensitivity, selectivity, and frequency
coverage.
• The receiver’s bandwidth should match
the transmitted bandwidth for the best
signal to noise ratio (SNR).
• Notch filters can remove a very narrow
band of audio signals, such as an
interfering whistle.
• Passband or IF shift adjusts the receiver’s
passband away from the displayed carrier
frequency. This can reduce adjacent
interference.
• In CW, switching between USB or LSB
can get move your reception to the other
sideband away from an interfering station.
Digital Signal Processing
• Digital signal processing (DSP) converts
the signal from analog to digital for
processing and then back again to digital.
• Typical applications of DSP are
– Signal filtering with selectable
(programmable) filters.
– Noise can be reduced by recognizing noise
sources and removing them from a signal.
– Notch filtering can remove whistles (carriers
from other stations).
– Audio frequency equalization adjusts the
frequency response of the receiver or
transmitted audio.
• Software defined radios (SDR) convert the
RF directly to digital and perform all the
functions of the receiver within the
software.
Managing Receiver Gain
• RF gain should be set to maximum for
weak signals, and to a lower setting for
stronger signals. Lowering the RF gain
will lower the noise level.
• The automatic gain control (AGC) adjusts
the gain to get a constant audio volume.
– Use fast AGC for CW and slow AGC for voice.
– The AGC circuit controls the gain of the IF
amplifier by changing the voltage. This same
voltage drives the signal strength meter (Smeter).
– The more that the AGC circuit must reduce
the gain to keep the volume constant, the
higher the S-meter reading.
– Reducing the RF gain also causes the Smeter reading to increase.
– S-meters are calibrated in S-units (0 to 9).
Each S-unit represents a 6 dB (4x) change in
signal strength. After S9 are usually numbers
like 20, 40, 60, which represent 20 dB over
S9, etc.
• Receivers respond linearly to signals. But
a strong signal can overload the receiver
and cause non-linearity (distortion).
– Attenuators reduce the power coming in to the
receiver and therefore eliminate the overload.
– RF gain can also be adjusted to reduce
overload.
HF Station Installation
• Mobile operation is easier with today’s
solid state radios that run on 13.8 V.
– HF radios typically transmit 100 W, which
requires 20 A. The best power connection is
directly to the battery (not the cigarette lighter
or auxiliary power) with two fused wires.
– Antennas are the most significant limitation to
HF mobile operation due to their small size
(by necessity).
– Sources of noise/interference from the car
include ignition, onboard computers, and
electric motors (fuel pump, electric windows).
RF Grounding and Ground Loops
• All equipment should be grounded to a
common point (star ground).
• Ground connections should be as short as
possible to avoid resonances. Odd
multiples of a quarter wavelength look like
high impedances to RR, which can allow
large RF voltages to exist on the
equipment.
• Ground loops occur when you ground
equipment in “series.” The created loop
can have an induced current from
magnetic fields from the AC wiring causing
a 60 Hz hum in receivers or transmitted
audio.
• Ground loops can be avoided by using a star
ground. If equipment must be connected in
“series”, then bundle the cables to reduce the
area of the loop.
RF Interference
• Fundamental overload occurs when a radio or
TV cannot reject a strong signal. Try filters in
the signal path.
– CW or FM signals produce on-and-off clicking or
humming
– AM and SSB will come through as the actual audio,
but SSB will sound garbled.
• Common-mode occurs when signals are picked
up as common-mode currents on a cable shield
or on unshielded cables. Use RFI filters, RF
chokes, ferrite beads, or bypass capacitors.
• Direct pickup is similar to common-mode, but
gets directly into the electronics. Better
shielding of the device can eliminate this.
• Harmonics are spurious emissions from a
transmitter. A low pass filter can reduce
harmonics.
• Rectification can occur with poor or corroded
contacts. Repair the bad contact.
• Arcing in AC power lines or with high power
equipment can cause interference over a broad
spectrum.