Radio Notes 1

Amateur Radio Notes from M0BXR

Electromagnetic Waves

An electromagnetic wave has three components.

1. Magnetic field.

2. Electric field

3. A direction of travel.

The magnetic and electric components are at 90 degrees
to each other and both are at 90 degrees to direction of travel.

spain89ai

Radio

A mode of communication between two or more points using
electromagnetic waves developed in a transmitter, radiated through the
atmosphere from one aerial to another and processed into an
intelligable form by a receiver.

That is to say that alternating RF currents produced by the
transmitter flow in the aerial to produce both an electric and magnetic
field. This might seem quite complex at first, but it is just like
everyday activities like cooking or
poker, once you know the basics the rest comes naturally.


Velocity of Radio Waves in free Space

All electromagnetic waves which include radio, X rays,
infra red waves, visible light and ultra violet rays travel at a velocity
of 300,000,000 metres per sec. in free space. (3 X 108).

Wavelength

The electromagnetic wave is an alternating quantity. The
wavelength λ is the distance between two points
o
f

similar magnitude and sign in the direction of propagation.

 

It follows that velocity      = frequency x wavelength

.^.       λ (metres) =          300
                              frequency(MHz)

Units of frequency

The unit of frequency is the Hertz (Hz) In radio we are
concerned with electrical changes which vary in a repetitive pattern with
time starting from zero going to a peak, falling back to zero then to a
negative peak and finally reaching zero again.

We call this one cycle if it occurs in one second we say
1 Hertz.

Radio waves are very high in frequency and the following
terms are used

Hz Hertz – The basic unit of frequency

kHz one thousand hertz Hertz x 103

MHz one million hertz Hertz x 106

Propagation of  radio waves

Propagation is defined as the travel of waves through or
along a medium.

In practice this medium is the earth’s atmosphere which
is divided up into layers those of main importance being the Troposphere
and the ionosphere.

Troposphere

Layer of the earth’s atmosphere nearest the ground – often
responsible for ducting of VHF signals during periods of settled weather
conditions.

Ionosphere

An upper atmospheric layer which contains belts or layers
of ionised particles which cause refraction of radio waves mainly at high
frequencies.

Frequencies between 3 and 30 MHz are generally considered
high (H.F.).  Above 30 MHz and extending to 300 Mhz is Very High Frequency (V.H.F)

Ionisation

A condition of certain layers in the atmosphere due to
the effects of X Ray and UV radiation which causes
gases to split into ions and free electrons.

Refraction

When an electromagnetic wave goes through a material which
is not of constant density bending occurs and the wave may be subsequently
reflected back down to earth.

Maximum Usable Frequency

This is the maximum frequency which is reflected at a
glancing angle to the ionosphere.  MUF varies with the time of day
and sunspot cycle and is best on HF when the outer F1 and F2 layers have
been ionised by sunspot activity. Often optimum conditions are shortlived
because the inner layers soon become ionised as well.

A high level of ionisation in
the D layer results in absorption of radio waves before they reach the
reflective F layers and results in a Dellinger Fade Out.

This is also known as a Sudden Ionospheric Disturbance
SID.

This may be followed after about two days by a further
fade out due to slower moving particles released at the same time which
concentrate in the D Layer.  This is known as an ionospheric storm.

Critical Frequency

The critical frequency is the highest frequency to be
refracted when radiation is is vertical to a particular reflective layer.

The critical frequency gives an indication that refraction
is occurring and that as a result the wave is “reflected back from the
layer”  It should be noted however that the maximum usable frequency
can be much higher than the critical frequency.  This is because an
antenna is usually arranged to give a low angle of radiation and refraction
occurs much more easily when the angle of incidence to the layer is glancing.
Students requiring a more in depth explanation of refraction will find
it in A level physics books which deal with the refraction of light in
lenses and prisms.  For the purposes of the RAE students need only
know that the process which bends or reflects the path of the waves is
known as refraction and remember the terms Critical Frequency and MUF.

Polarisation

Ths refers to the angle of the electronic component of
the radiation in relation to the earth.

Vertical antenna elements radiate vertically polarised
signals and horizontal element radiate horizontally polarized signals.

By switching rapidly between the two at a controlled
frequency circular polarisation can be produced

Slant polarisation has also been used from time to time.
Usually where long distance propagation of HF is involved the polarisation
is lost and becomes mixed.  In these circumstances horizontal or vertical
aerials can be used for reception.  At VHF / UHF however the polarisation
is much more important and a loss of 6dB occurs if the antenna is cross
polarised.

Skip Distance

Distance over the earths surface from the end of the ground
wave to the receiver which will allow a single skip or reflection from
the layers above.

Types of Propagation

1.  Ground Wave

The radiated wave follows the surface of the earth and
is the major mode of propagation up to 2 Mhz.

At very low frequencies reliable ground wave communication
over long distances is possible.  At high frequencies it may extend
only a few kilometres.

2. Ionospheric Propagation ( Occurs HF 3 – 30Mhz)

Occurs when radio waves are refracted and subsequently
reflected from the upper layers (E, F1 and F2.) of the atmosphere which
contain ionised gases  are received back on earth possibly many miles
from the transmitter.  Many reflections may occur and this is known
as multiple skip.

Skip Zone or Dead Zone

An area which exists between the area served by ground
wave propagation and that served by sky wave propagation where little or
no signal can be received.

Fading of signals

This is particularly associated with ionospheric propagation
-  as the degree of ionisation and its height above the earths surface
is continually changing.  The result is continually changing signal
levels.

Another type of fading is when two signal paths interfere
e.g. single skip and multiple skip.

Due the phase difference between the two signal distortion
often results which is especially noticeable on music.  This type
of distortion is often frequency selective and the distortion effect can
be minimized by using a narrow bandwidth signal such as ssb rather than
am.

Tropospheric Propagation.

major mode of propagation at VHF  (above 50 MHz).
for long ditances.

Is due to refraction caused by changes in dielectric
constant of the atmosphere which results from changes intemperature and
humidity.

Effect of Seasons on Propagation

Best long distance high frequency (High MUF) occurs when
the upper F1 and F2 layers are well ionised and often combined into a
single layer whilst at the same time the D layer at 80 kilometres i less
ionized.  -  high ionization of the D layers results in absorbtion
rather than reflection of radio waves.  These conditions occur during
the winter daytime when sunspot activity is high.

Sunspot activity follows a pattern and is much higher
about every eleven years with periods of low activity intervening.

Best long distance VHF

  ~  occurs when waves are refracted through
the troposphere due to variation in dielectric constant.

This varies with temperature and humidity.  Generally
warm humid air has a high dielectric constant.

On a hot day when atmospheric pressure is high and falling
a layer of air of high dielectric

may be trapped between layers of lower dielectric especially
as the earth cools towards sunset.  This is known as tropospheric
ducting.  VHF communication can be several hundred miles.

Scatter and Moonbounce

Some operators take particular interest in bouncing signals
off other materials above the earth’s surface.  These include meteor
scatter,  moonbounce,  auroral propagation and troposcatter -
clouds or areas of dust etc.

Generation of radio waves

Radio waves are generated using active electronic devices
such as semiconductors or valves. Semiconductors may be single transistors
or diodes or an integrated circuit which is fabricated onto a single wafer
of silicon. These devices are built into a circuit in association with
various passive devices. In order to understand this it is necessary to
learn about the properties of the components used.

Conductors

Materials are said to be conductors if a flow of electricity
- that is a movement of electrons can occur through them easily.

e.g. most metals.

Insulators

Some materials on the other hand are not able to allow
free movement of electrons and charges exist on the surface which remain
stationary. This is static electricity. Insulators include glass, porcelain,
dry wood and plastics.

Resistors

A component made from materials which are neither good
insulators or good conductors. They are use to help control the flow of
electricity in a circuit.

Resistors vary in there ability to impede electric flow
and the property of resistance is measured in ohms (Ω ).

Even conductors have some measurable resistance but this
is very small typically an ohm or two for several metres of copper wire.

Insulators on the other hand have a resistance of many
millions of ohms between two points quite close together on the surface.

Voltage

In order to cause flow of electricity in a conductor an
electrical pressure has to be produced by a battery or generator.

This pressure or potential is measured in volts and is
often referred to as EMF -electromotive force.

V volt – basic unit of potential

Kv kilovolt – 1 thousand volts 1 x 103 volts

mV millivolt – one thousanth of a volt 1 X 10-3 volts

μV  microvolt – one millionth of a volt 1  x  10-6 volts

Quantity of Electricity

The electron itself is too small an amount of electricity
to be a useful unit so the coulomb is used to measure electricity.

one coulomb = 6 x 1018 (six million million million) electrons

Rate of Flow

The rate of flow of electricity is measured in amperes.
An ampere is defined as one coulomb of electricity flowing in one second.

Total quantity in coulombs Q = It
where   Q = quantity

I = current

t = time
In radio we find that the amp is a fairly large unit for
measurement hence it may be subdivided into smaller units.

ampere basic unit of current

mA ~ milliampere ~ one thousandth of an amp  ~ 1 x 10-3 amp
μA  ~ microamp ~ one millionth of an amp  ~ 1 x 10-6 amp

 

Ohm’s Law

Ohm’s Law states that the current I flowing through a
material of resistance R is proportional to the voltage V applied and inversely
proportional to resistance R

Power

Power is the work done when an electric current flows,
this is often given out as heat.

The power dissipated in a resistance is given by

power (watts) = voltage (volts) x current (amps)

W = V x I

By substituting back in Ohm’s law we have.

Alternating Current

Alternating Currents exist in many areas of electronics.
Our mains supply consists of a low frequency AC at 50 Hz.

High frequencies of alternating current many kHz or MHz
when connected to an aerial produce a field or electromagnetic wave which
can be propagated as radio.

In order to produce a current a voltage or electromotive
force (EMF) is required to make the current flow.

A.C. is no exception but here the E.M.F.  is generated
in such a way that it first causes a current to flow in one direction and
then in another.

~ we have a varying voltage with time.

The time occupied by one complete cycle is the period
T and the number of cycles in unit time is the frequency
ƒ

   
ƒ   =
1
T   =
1        
    ƒ        

Some Terms Associated with AC

1  Peak Value  ~  the maximum +ve. value
of voltage or current reached each cycle.

2. Peak to peak value.

    The difference between the most +ve.
voltage or current value reached each cycle and the most negative.

3.  RMS value.  This is the value of AC voltage
or current which is equivalent in heating effect to a DC voltage or current
of the same value.


R.M.S.  is 0.707 times the peak value.

4. Instantaneous Value.

Voltage or current at a defined instant of time in the
cycle.

Phase difference between two waveforms

If two AC’s are derived from different sources ie. two
generators it is likely that even if they are running at the same speed/frequency
they will not have started at the same instant in time. hence there is
a time or phase difference in there waveforms.

Generator A

Generator B

In the case of a simple alternator we would have one cycle
of AC in each revolution of the alternator.

If one alternator has a red spot painted on the flywheel
we will find that every time this red spot is uppermost the voltage is
the same say zero volts and rising. Another alternator has a red spot in
the same position relative to voltage but is running out of phase with
the first. When A is on top B is at quarter two.


B lags A by 90 Degrees

Now also draw B lags A by 180 degrees

B leads A by 90 degrees.

Generation of AC at Radio (high) frequency

There is no suitable mechanical means of generating RF
- the most commonly used means is by use of active electronic devices ie
semiconductors (transistors) or vacuum tube devices (valves). Semiconductors
will be described in more detail later but the basic format of an RF oscillator
is an amplifier with positive feedback. Tuned circuits of filters may be
used to enhance oscillation at a particular frequency.


The block diagram shows a simple oscillator. When switched
on random electrical noise at the input is amplified at the output in the
form of a voltage moving in one direction ie. rising or falling.

If this voltage is rising a rising voltage is fed back
into the input causing the output to rise still further. Eventually as
a result of this positive feedback the output has risen to a positive level
which is so high it cannot continue to rise at the same rate – the filter
rejects the DC component of the change in voltage so that when no further
change occurs the voltage at the input tends to fall even though it could
be maintained at the output.

As soon as this happens the output voltage also falls
rapidly the change again being fed to the input until a maximum negative
voltage is reached when again the process is reversed. This gives rise
to the familiar sinusoidal waveform at a frequency which is determined
mainly by the filter.

The AC circuit – Inductance and capacitance

In a DC circuit we have seen that the flow of current
is governed by the resistance of a circuit and the pressure or voltage
applied to it .

At AC two other quantities effect or oppose the flow of
a current in a circuit

~ these are inductance and capacitance.

Inductance

When a steady current flows through a coil there is a
steady magnetic field due to that current. A current change tends to alter
the strength of the field which in turn induces in the coil a voltage or
back EMF tending to appose the change being made. This property is known
as the self inductance or inductance of the coil.

One HENRY is the amount of inductance which has an EMF
of 1 volt produced when the current flow changes at a rate of 1 amp per
second. In radio smaller values of inductance are often required.

H Henry  ~  basic unit of inductance

mH millihenry  ~  one thousandth of a Henry 10-3 Henry

μH microhenry  ~  one millionth of a Henry 10-6 Henry

The inductance of a coil depends upon the square of the
number of turns, the cross-sectional area, a constant called permeability
and its length.

L =
μ AT
   l
where L = inductance
μ= permeability
A = crossectional area
T = no. of turns
 l = length 

Relative Permeability

The density of a magnetic field is greater in some materials
than in others. A vacuum is taken as a standard and is said to have a permeability
of 1 hence relative permeability is the ratio of magnetic flux density
in a material to that produced in a vacuum from the same magnetising force.

see also  RAE Manual “Inductors used in Radio Equipment”

Inductive Reactance

As a result of the back EMF produced when current changes
in a coil – this tends to oppose the AC in the next half cycle, this opposition
is known as reactance.

Inductive Reactance is given by

XL =
2πƒL
where XL = inductive reactance in Ω
ƒ
= frequency
L = inductance in Henry

Capacitance

A capacitor consists of two conductive plates of material
separated by an insulator. When a current flows into a capacitor a charge
or electrostatic field is built up between the two plates.

This charge remains stored and must be overcome before
the capacitor can be charged in the reverse direction by an alternating
current.

Capacitance is measured in Farads

F Farad – basic unit of capacitance

    μF microfarad  ~  one millionth of a Farad ~ 10-6 Farad

    nF nanofarad  ~  one thousandth millionth Farad  ~ 10-9 Farad

    pF picofarad  ~   one million millionth of a Farad  ~ 10-12 Farad

The capacitance of a capacitor is proportional to the
area of the plates and inversely proportional to the distance between them
- it is also dependant on the material between the two plates , this is
said to have a property known as permittivity (the dielectric constant).
As with permeability this is expressed as the ratio compared with a vacuum

Capacitive Reactance

This is the property of slowing down an AC current due
to this stored charge.

XC =
 1 
2πƒC
where XC = capacitive reactance in Ω
ƒ
= frequency
C = capacitance in Farads

 

  The total reactance of a circuit
is the difference between capacitance and inductive reactance.

Hence


X
= XL – XC or
X = XC – XL

Resistors Inductors and Capacitors in Series and
in Parallel

Practical Demonstration using resistors.

Circuit Diagram

Make up the simple circuit using the resistors shown

1.  Measure voltages at points 1, 2, 3, 4 and 5.
If not otherwise stated it is conventional to measure voltages with respect
to the chassis ground or battery negative terminal.

2.  Measure the voltage drop accross all resistors.

3.  Measure resistance of all resistors without any
disconnection other than the battery.

From the above it can be seen that    RT
=  
R1 +   R2  +  R3

In Parallel we have
1 =
  1 +   1  +   1  ———–n
RT
 R1  R2 Rn

 

For two resistors this is simplified as

 

    1 =
  1    +   1 
RT
R1 R2
From this we get  RT =
R1  x R2
R1 +  R2

Total Inductance in series and parallel can be calculated
in the same manner as resistance.

Hence in series we have      

LT   =   L1 + L2  +  L3

 

and for parallel we have   1  =   1  +   1  +   1  + —n
LT L1  L2 Ln

 

Capacitors do not add together in series and parallel
circuits in the same way as inductors.

Series and parallel circuits are calculated in the exact
opposite way.

Try writing down the formulae you  would need to
calculate series and parallel capacitors.

Impedance

If a circuit has resistance as well as capacitance or
inductance the total opposition to current flow is known as impedance.

Resistance, reactance and impedance are all measured in
ohms however the impedance is obtained from the square root of the sum
of the squares of the reactance and resistance.

ie.
Z = √ (R2 + X2)

Time Constant of an RC
Circuit

The charge Q (in coulombs) in a capacitor is the product
of the voltage accross the capacitor and the capacitance

ie    Q  =  C  x  V

If a Capacitance of C Farads is connected to a DC source
of V volts via R ohms at switch on the current is given by
I   =   V
         R

The capacitor charges exponentially and the resistor
/ capacitor network has a time constant which is defined as the time taken
for the capacitor to charge to 63% of the supply voltage.

Time Constant in Seconds = Resistance in ohms
X     Capacitance in Farads.

T = RC

Resonance

Resonance is a phenomenon that
occurs with sound, mechanics and electronics. It is best understood by
considering various physical objects.

Every object has its own natural
frequency of vibrations depending on its size and mass.

eg. when a certain note is struck
on an instrument a nearby vase may vibrate. This means that the natural
frequency of oscillation of the vase has been excited by a note of that
frequency.

and energy is absorbed by the
vase to sustain vibration.

Similarly soldiers marching across
a bridge in step could cause it to vibrate at its natural frequency. If
the constant small impulses from the marching soldiers take place at the
same frequency as the natural frequency of oscillation of the bridge resonance
occurs and the bridge vibrates. This effect is cumulative and the vibrations
could become so large as to destroy the bridge.

Most musical instruments depend
on resonance effects to create sounds.

Although mechanical resonace is
used in radio electromagnetic resonance which occurs at radio frequencies
in LC tuned circuits is more common and forms the basis of tuning in many
receivers.

In many ways it is similar in
behaviour to mechanical resonance.

We have already met the two formula
XL = 2πƒL  
where

Xl = inductive reactance in Ω 
ƒ
= frequency
L = inductance in Henry

        XC = 1
               2πƒC

where

XC = capacitive reactance in Ω 
ƒ
= frequency
 C = capacitance in Farads

It can be seen that as frequency rises XL increases whereas XC
falls. At a certain frequency XL becomes equal to XC

Since  X= XL – XC or  X = XC – XL

at this point the reactance will become zero hence the only impedance will be the circuit resistance.

 

If XL = XC
it follows that   
2πƒL =
      1   
 2πƒC


.^.
ƒL=         1
          (2π)2ƒC

.^. ƒ2L      1
          (2π)2C


.^.
ƒ2       1
         (2π)2LC


.^.
ƒ       1
       (2π)√ LC

The above relates to a series tuned circuit known as an acceptor circuit .

Parallel tuned circuits on the other hand are known as rejector circuits and have maximum impedance at resonance. for which the above formula still holds good.

Piezo Electric Effect and
Resonance

A small quartz crystal will resonate mechanically at R.F.

and quartz has the ability to produce a voltage when pressure is applied to it or conversely will change in shape with voltage.

Hence if an AC of a particular frequency is applied to it or if it is used as a filter in an oscillator it will resonate.

Semiconductors

Most substances are either conductors
or insulators. Generally conductors have a surplus of free electrons which
move about in a piece of material and electricity is said to flow.

Insulators – there electrons are
closely bound in the atomic structure and are unable to move hence electricity
cannot flow.

Resistors lie somewhere between
these two in that they allow limited current flow .

Semiconductors are materials which
sometimes can allow flow – become conductors but under some conditions
are very good insulators.

In there pure state they have
some conducting property known as intrinsic conduction.

Holes

It is generally accepted that
conduction is due to the movement of electrons. If however we look at the
structure of semiconductors such as silicon and germanium we find a regular
crystalline structure with few free electrons

Where an electron has become detached
from the structure due to energy within the crystal (perhaps due to light
or heat) we say that a hole remains. If the hole is filled by movement
of a nearby electron we say that the hole has moved. Movement of holes
results in current flow.

Doping

The number of holes or electrons
available for conduction can be increased by adding small amounts of impurities
to the silicon or germanium.

Phosphorous or arsenic impurities
have excess electrons and are said to produce n type material .

Gallium and Boron on the other
hand are deficient in electrons (only three in the outer shell ) therefore
holes are produced- P type material.

These impurities increase the
conductivity of the material many times and are said to be responsible
for extrinsic conductivity.

The simplest semiconductor device
is the PN junction diode

In semiconductor material when
a charge is applied the electrons which are said to be -ve charged flow
towards the positive side – some current is said to flow.

A diode consists of a small piece
of P type material fused to a small piece of N type material. Because the
N type and P type material have different characteristics as described
holes diffuse towards the N type – deficient in holes. and electrons towards
the P type – deficient in electrons.

As a result of this diffusion
the n type builds up an excess positive charge and is depleted of its free
electrons. The P type builds up an excess negative charge and is depleted
of holes.

As a result a depletion zone is
said to exist and no current can flow

The potential difference across
the junction opposes further diffusion and is referred to as the barrier
potential

When the + ve. pole of a battery
is connected to the P type side of the diode electrons are attracted out
of the material and holes exist once more at the junction These electrons
flow through the battery into the n type material where they make good
the depletion and current flows.

The voltage of the battery must
be sufficient to overcome the barrier potential of the junction which is
about 0.6 volts for silicon and varies with the temperature.

Reversing the battery causes holes
to flow towards the n type material where they again neutralise the electrons
which are flowing towards the P type material and the depletion zone is
now reformed – no current flows through the reverse biased diode.

Specialist Diodes

Varactor or varicap Diode

We have seen that a diode consistes of two conducting
areas of P and N type materials respectively.

and that unless the diode is forward biassed there is
an insulating layer between – the depletion Zone

It can be seen that the structure is similar to that of
a capacitor which has two metal plates separated by an insulator.

A diode does in fact have the property of capacitance.
The thickness of the delpletion zone varies with the potential applied
to it becoming very wide under high reverse voltge conditions and very
narrow around zero volts up to 0.6 volts (for silicon) when it breaks down
completely.

Consequently as C the capacitance is inversely proportional
to the spacing d it is therefore possible to vary the capacitance by varying
the voltage applied.

Resonant circuits in receivers are often tuned this way.

Zener Diode

in normal diodes large voltages may be applied to reverse
bias the diode junction and very little current flows.

If certain impurities are added to the semiconductor
material it is possible to consruct a diode in which this smal current
will flow only up to a certain voltage but if this amount of reverse bias
is exceeded a large current will flow.

These diodes are called Zener or avalanche diodes and
are used as a voltage reference in power supplies which are designed to
give a constant voltage whatever current

is drawn.

Transistors

A device where a current flows
between two electrodes which can be controlled by a third is a transistor.
The depletion zone is broken down by forward biasing the base emitter junction
and current then flows between collector and emitter.

Two pieces of N type material
are separated by thin layer of P type material. – two diode junctions exist
with a common P type electrode. If a voltage is applied between collector
and emitter no current will flow because the central junction area of the
device will be depleted. If the base is forward biased the depletion zone
will break down and current will flow not only from base to emitter but
also from collector to emitter

Because the collector voltage
is normally larger than the base voltage a heavier current will flow through
the collector than through the base furthermore a small change in base
current will rapidly cause an effect on the amount of depletion which will
cause a large effect on collector current hence we have current amplification.
- this is the main use of the transistor.

The two resistors R1 and R2 set
the base bias potential at around 0.7 volts. The base electrode is thus
forward biased in respect to the emitter and a current flows through R3.
Changing R1 and R2 slightly would cause a large change in current through
R3.

If an AC signal is applied to
C1 this also is amplified and appears at C2. If further amplification is
required C2 can go to the base of TR2.

Biasing of Transistors
- Class A

A silicon transistor requires
a forward bias of about 0.7 volt between base and emitter. Below is the
relationship between IC

the collector current an
VB the base voltage


The line AB is not perfectly
straight ie the collector current is not perfectly proportional to the
base current. As a result some distortion is poduced.  Distortion
of this type can largely be removed by negative feedback but we do not
need to detail this here.

Cut off and bottoming.

If too large a signal is
applied between the base and emitter of the small signal amplifier then
the output will be very distorted. On the negative half cycle the base
current will cease to flow because the base emitter voltage will fall below
0.5 volt.

On the +ve. half cycle input
the collector current may increase to such a level that a very large drop
in voltage occurs across R3 this drop in voltage is limited eventually
by the voltage of the supply and bottoming occurs

The two may occur together
to produce a square wave which is rich in harmonics.

Class B Bias

If the fixed DC bias at the input
of the transistor is such that the signal excursions are either side of
the turn on point, ie. 0.5volts, conduction will only occur on positive
hard cycles and the output is grossly distorted.


A single transistor in Class
B is rarely used as the the output is distorted however we can combine
two transistors together in Class B push pull.

This circuit uses less current
than Class A and is widely used in the audio output stage of portable transceivers.

Class C

No forward bias is used in a class
C circuit therefore the transistor only conducts when the signal is above
.5 volts on +ve half cycles. The main use of Class C in radio is for RF
amplification and frequency multiplication in transmitters.

The circuit produces gross distortion
but this is removed in RF circuits by a tuned circuit.

The circuits we have discussed
so far have in the main been common emitter circuits .

In this context the word common
means used for both input and output.

Common base and common collector
circuits are also used.


In common emitter above left in input and output impedance
is usually between 1 and 10 Kohms.

In common base above right there is low input inpedance
and high output impedance.  Common base is often seen used at the
input of a receiver where the 50 ohms or thereabouts input impedance gives
best signal match with the antenna.

Input and output inpedances for
various circuits are shown below as it is this characteristic which governs
the use of the various circuits.


Common collector often looks like
common emitter note however that the collector is grounded to signal via
capacitor CG – check for this if in doubt.

Transmitters

Oscillators

The basis of all transmitters
is an oscillator at RF.

Variable Frequency Oscillator
V. F. O.

An oscillator the frequency which
can be varied between predetermined limits. If manually controlled this
can facilitate netting to a calibrated receiver so as to transmit on an
exact frequency. A factor of prime importance in Amateur radio is frequency
stabilty.

This is effected by.

1.) The careful choice of components
to minimise the effect of temperature changes.

2.) Sturdy construction to prevent
the effects of pressure or vibration. 25

3.) Stable voltage supply.

If the output of a transmitter
is high in frequency or if the transmitter is required for several harmonically
related bands doubler or tripler stages are used for instance an 8 MHz
VFO or crystal is much easier to construct than one at 144 MHz.

However if a doubler and two triplers
are used this frequency is attained
.
The doubling or tripling is achieved
by running the stage in a non linear state usually Class C bias. In the
output circuit is a tuned circuit resonant at the desired harmonic, the
fundamentals and other harmonics are very much reduced by this and subsequent
tuned circuits.  These stages are known as multipliers.

Power amplifiers

This is the final tuned stage
of the transmitter coupled to the aerial and often working at high power.

Mixer

We have already mentioned that if a stage is non linea
an output may be obtained which is a multiple of the input frequency.

If however we have two input frequencies there are then
several possibilities in terms of output frequency.

It is usual to use a tuned circuit to select the desired
frequency providing that the output frequencies are not too close.
The main outputs from
f1    +   fare

         f1   +  f2,     f1 -   fand f2   -   f1
Multiples such as 2(
f1    +   f2 )  and     2( f1   -   f2 )

Balanced Mixer

If F1 is very close to F 2 ,within
a few % say, it may be difficult to remove the unwanted F1 from the desired
F1 + F2 .

by means of a tuned circuit. A
balanced mixer is therefore used.

In the above diagram F1 is applied
 to the base of the two transistors via preset VR1. The signal appearing
at the collectors is set to be exactly equal and no current from F1 flows
through T1.

F2 is fed in at Tr1 emitter and
is not balanced at the collector – as a result F1 + F2 is obtained at T2
but F1 cancels and is not available.

Modulation

The following types are in common
use and should be learnt.

1) A1A – Telegraphy (morse) by
on – off keying without the use of a modulating audio frequency.

2) A2A – Telegraphy using an amplitude
modulating audio frequency.

3) A3E – Telephony double sideband.
ie. amplitude modulation

4) J3E – Telephony – single sideband
supressed carrier

5) F2A – Telegraphy – On/Off keying
using a frequency modulated audio tone.

6) F3E – Telephony using frequency
modulated carrier.

Telephony and Keying – See also
Rae Manual.

In order to minimise interference
without upsetting frequency stability the signal is modulated at the lowest
power level possible but not the oscillator. eg a low current buffer or
multiplier stage after the oscillator.

A M modulation and bandwidth

In all modulation processes frequencies above and below
the carrier wave are produced called side frequencies.  The bands
of side frequencies are called sidebands.  With A.M. modulation the
highest side frequency is the sum of the carrier frequency 
fc
and
the highest modulating frequency
fm.

ie on top band 160 meters

if    fc = 1.950  kHz

and fm =  5 kHz   the highest and lowest frequencies are 1.955
and 1.945 respectively.
The bandwidth is then 10 kHz

In some amatuer transmissions a filter may be used
to cut off audio information above 3kHz.  The bandwidth will then
be some 6kHz which allows more stations on the congested bands.

Modulation depth

With AM modulation an envelope of RF is shaped by the
audio speech pattern.

Here we show how RF and audio combine.

The amplitude of the RF varies with the audio speech signal
imposed on it.

Setting Up amplitude Modulation

A similar pattern to that observed above is observed in
the oscilloscope or modulation monitor.  If the modulation is increased
to an excessive level flattening of the trace occurs at X  ie the
RF goes to zero each cycle – over modulation is said to occur.  This
is very bad as it results in distortion and serious interference to ajacent
receivers.

It is therefore good practice to aim for a modulation
level of about 80-90 % of the maximum on speech peaks.

More may be obtained with telegraphy as the audio tone
does not vary and overmodulatioon is less likely to occur.

A.M. is usually obtained by applying A.F. to the supply
of the P.A. and sometimes in addition to its driver.

Amplitude Modulation In transmitters ~ A3E

A.M. is usually obtained by applying
A.F. to the supply of the P.A. and sometimes in addition its driver.

Single Sideband Operation ~ J3E

  Above is an A.M. carrier
with its bands of side frequencies.

The information is carried in
the side frequencies which are a mirror image below and above the carrier.

The carrier wave itself does not
contain any information so we could suppress this and would then be transmitting
double sideband. To demodulate the signal the carrier is reinserted in
the receiver. The signal can then be demodulated with a detector diode
in a similar manner to A.M.

To save on power and bandwidth
still further one set of sidebands can be filtered away to give an S.S.B
suppressed carrier transmission J3E.

S.S.B. is usually more conveniently
generated by a balanced mixer which suppresses the carrier in conjunction
with a crystal filter which removes the unwanted sideband. It is conventional
on the amateur bands to use lower sideband on 7.O MHz and below with upper
side band used on the higher frequencies.


Modulation Level of SSB

In order to ensure that the modulation
level on sideband is not excessive a two tone test should be performed.
Two audio tones are fed into the mike input and the modulation is viewed
on a monitor. If the modulation is excessive or if the bias is incorrect
a distorted waveform will be seen. Things to note regarding this test are.

1. Two tones are essential one
tone produces a constant level of signal at ssb so modulation cannot be
checked that way.

2. In all modulation checks the
oscilloscope speed is set slightly faster than the audio modulation frequency
not at radio frequency.

The following shows the effect
on modulation of excessive drive or incorrect bias.


 

Frequency modulation F3E

Frequency modulation differs from
A.M. and S.S.B. in that the power radiated from the transmitter on F.M.
is constant. There is however a change in frequency which can be quite
small say
±
3kHz or quite large say
±
80kHz. On the amateur bands narrow band F.M. is used the deviation being
about 3kHz. on most bands.

The louder the speech peaks the
more the carrier is deviated.

Method of obtaining F.M.

1) By varying the frequency of
the voltage controlled oscillator by applying a modulating (audio signal)
to the voltage control point of the oscillator. This would normally be
a varicap diode.

2.) A varicap diode can similarly
be applied directly to a crystal oscillator at a low level of deviation.
Here the crystal and circuit parameters must be chosen carefully to ensure
freedom from distortion.

3.) A valve or transistor may
be used to alter the reactance of a tuned circuit ~ not common in modern
equipment.

4.) Phase modulation may be used
with a filtered A.F. response. ~ See also phase modulation.

Sidebands and bandwidth

With F.M. it can be proved mathematically
that an infinite number of side frequencies exist i.e.. the bandwidth is
infinite. In practice the bandwidth depends on both the level of deviation
and the modulating frequency. 2 – 3 KHz of deviation requires 10 – 12 kHz
of bandwidth for normal speech a on amateur N.B.F.M 12.5 kHz spacing. This
is can be compared with quality Band II stereo broadcast where a station
will occupy a bandwidth of up to 200 kHz with a deviation of 75 kHz.

Due to the low level of deviation
with N.B.F.M the level of recovered audio is low. As a result there is
a tendency for people to over deviate the signal to make it sound louder.
The result of this will be interference to adjacent frequencies or distortion
due to the signal being too wide to pass through the filters in the receiver.

An important point to note with
FM is that if frequency multiplier stages are used the deviation is also
multiplied hence in order to get 4 kHz deviation on 145 Mhz with X 18 multiplication
from an FM crystal at 8 MHz it is necessary to apply sufficient audio signal
to deviate the frequency by

4000 = 222 Hz ~ quite small
18

see later notes on deviation measurement.

Phase Modulation

If the phase of the current in a circuit is changed there
is an instantaneous frequency change during the time the phase is being
shifted.

The amount of frequency change or deviation depends on
how rapidly the phase shift is accomplished. It is also dependant on the
total amount of the phase shift. The rapidity of the phase shift is proportional
to the frequency of the modulating signal. Hence the deviation with phase
modulation is proportional to both the amplitude and the frequency of the
modulating signal.

Method of obtaining Phase Modulation

Modulation can be applied to a transmitter as phase modulation
but if an audio filter is inserted before the modulating stage which corrects
for the increase in frequency the net effect is the same as frequency modulation.

Synthesisers

Over the last decade there has
been increasing use of frequency synthesis in rigs. These circuits which
often work in conjunction with microprocessors and digital frequency displays
are based on a circuit called the phase locked loop or p.l.l..

A phase locked loop circuit uses
a crystal as a reference oscillator in conjunction with a programmable
divider and comparator to provide a range of stable frequencies which can
be used in a transmitter or in a receiver circuit. Its advantages are cost
compactness and convenience.

The use of valves in Amateur
Radio

Valves are regarded by many as
an obsolete device. To a great extent this is true however there are a
couple of notable exceptions to this.

Most high power bipolar transistors
do not perform well at high frequency those that do especially at VHF and
above are expensive to buy. Valves have a much better gain. At VHF it is
easily possible to get 100 watts output power from a single stage valve
amplifier with an input of 2 watts or less. Most transistors which are
available at an affordable price tend to require 2 watts drive for an output
of 10 watts at VHF with 12 volts on the collector.

Better results can be obtained
with 24 volts in theory but in practice few people have used this in design.

Another disadvantage of transistors
is intolerance to overload. They usually fail completely if affected by
transients or excessive voltages and currents which occur if the SWR is
poor.

Valves will overheat under such
circumstances but when switched off they will often recover to work again
later.

Amateurs wishing to run high power
on VHF or UHF will build valve based power stages. Many commercial HF rigs
which are still in use today also use valve output stages although it is
fair to say that there is an increasing trend nowadays to use power FET’s
in PA’s. as these devices are becoming more reliable and less expensive.

Notes on Safety

Whereas valve equipment uses higher voltages and possible
areas of high R.F. power are more likely to be encountered these notes
suggest points of good practice which should still be followed
in a non valve environment.

Low voltage power supplies are often capable of delivering
high currents.

Although DC voltages less than 50 volts are unlikely to
cause serious electric shock the high currents involved can cause metallic
objects to get very hot if a short circuit occurs. Always switch off the
power before using metal tools in equipment and take care not to wear rings
when working on equipment where there is a risk from exposed terminals.

All equipment in the radio shack should be connected to
a common switch. Other members of the household should be aware of its
position so that it can be turned off quickly if a problem occurs.

The earthing of the mains outlets should be checked to
ensure that it conforms to IEE regulations.

All wiring should be properly insulated and high voltage
connections must not be exposed.

Capacitors in power packs should have a suitable bleeder
resistor across there terminals so that they do not become a shock hazard
when the equipment is serviced. The RSGB recommends that this applies to
high voltage capacitors over 0.01μF. The size
of the bleeder resistor should be 1/C megohms.

Indicator lamps showing when mains is on should be installed
and maintained on equipment.

Double pole mains switches should always be used with
the correct type of fuses. Switches should be off when fuses are changed.

If metal cased equipment has inspection covers which can
be easily opened the use of micro switches which turn off the power when
opening is recommended.

Test prods and lamps should be insulated.

Attention to floor coverings is important. Rubber or suitable
insulating materials help prevent serious shocks. Damp increases the likelihood
of electric shock.

It is always best to switch off before making adjusments.
If live adjustments to equipment cannot be avoided always use one hand
and keep the other in your pocket. Always use tools with insulated handles.

Do not wear headphones when making internal adjustments
to live equipment.

Ensure metal cases of microphones; Morse keys; are properly
connected to the chassis of equipment.

Do not use meters with metal adjusting screws or control
knobs with metal grub screws and shafts on high voltage equipment.

Mains and other high voltages should be avoided on antennas.
An Rf choke will provide a suitable DC path to earth.

RF voltages can be a hazard if a person comes in contact
with a high voltages node on an antenna. Transmitting antennas should be
suitable sited and all cables insulated.

RECEIVERS

There are four main types of processing required in receivers.

1.  Amplification  -  The final sound or vision signal is always large compared with the very small voltage induced into the aerial.

2. Selection and filtering.  -  the receiver is designed to select a signal at a particular frequency – tuned circuits are used for this.  If the bandwidth of the signal is very narrow crystal filters ae also used.

3.  Demodulation  -  Various techniques will be described depending on the type of signal.

4.  Frequency Conversion  -  One or more frequency conversions take place in a receiver this may simply be from RF to AF as in direct conversion or TRF receiver or to an Intermediate frequency in the case of a superhet.

Two main types are used these are the T.R.F. (tuned radio frequency) and the superhet (superheterodyne)

Both types consist of a number of stages of amplification
at radio frequency followed by a detector which recovers the modulation
and further amplification at audio frequency.

In the TRF set however the RF is kept at the frequency
of the transmission and all the RF amplifiers are tuned to it usually by
LC resonant circuits. This method has several disadvantages.

a) When the tuning is altered all the tuned circuits have
to be exactly in step. This is achieved by setting up trimmers in parallel
with the main tuning capacitor and padders in series with it and adjusting
at different points in the tuning range for best performance. This is time
consuming as it has to be repeated several times for good results.

b) The selectivity and gain of a tuned circuit varies
with the frequency so that performance is not consistent on different bands.
Poor selectivity especially at higher frequencies  -  as we increase
in frequency the narrowest bandwidth that can be acheived with an LC circuit
becomes wider.

c) Because there are more variable tuned circuits the
receiver will be bulky and more expensive.

 

Block Diagram of TRF Reciever

In a superhet frequency conversion is used to obtain
an IF frequency which is used for all signals. Once the IF has been produced
it can easily be amplified many times by using circuits tuned to a single
frequency.

This generally avoids the use of expensive and bulky ganged
tuning capacitors.

The frequency conversion is acheived by mixing the the
received signal wit a locally generated one.

By varying the frequency of the local oscillator tuning
is acheived and the IF remains constant.

Choice of Intermediate Frequency and Double
Conversion

The lower the frequency of an I.F. strip using LC tuning
the smaller will be the bandwidth.  Hence for narrow bandwidth we
use a low final I. F.

In a single conversion superhet this results in image
interference.

This can be seen by looking at the figures below.

28MHz  +  455kHz  =  28.455
MHz  -  This would be the local oscillator frequency.

28.455  +  .455  = 28.910MHz.  This frequency if incoming from the antenna is also mixed with the oscillator to produce 455kHz  which would interfere with
the wanted frequency.

Remember that this unwanted signal is called the image frequency.

A higher IF improves the image performance because the
image is further away from the wanted signal and is more easily removed
by tuned circuits at the input.

In some designs two I.F.’s are used.  This is acheived
using a second frequency conversion and combines the narrow selectivity
of a low I.F. with the image rejection of a high I.F.

This is known as double conversion and gives the benefit
of a low I.F. for sharp tuning and a high I.F. at the first conversion
to help remove the image.

Single conversion using high quality crystal filters
at a high I.F. is expensive but works well.


Detectors

Three main detectors are used in Amateur Radio.

1). An envelope detector for A.M.
2).A product detector for S.S.B.

An envelope detector will work with S.S.B. but a product
detector allows for easy insertion of the C.I.O. and gives better recovered
audio.

3). For F.M. an IC discriminator circuit is often used
or a ratio detector can be used. Both these circuits produce good quality
recovered audio but also produce a high noise level when there is no carrier.
A squelch circuit is used to overcome this. The squelch circuit can be
noise driven or can take the output from the am envelope detector to switch
a gate which only allows the audio through when a signal is present. This
removes much of the unwanted noise on F.M.. A squelch control varies the
level at which is muted thus allowing weaker signals to be received.

These are discussed in more detail below.

The A.M. envelope detector is a simple circuit which
takes an R.F. envelope and removes the R.F. component from it using an
R.F. diode and a resistor and capacitor with a suitable time constant.

R1 and C1 remove R.F. from from the circuit as C1 is
relatively small in value.  The lower audio frequencies from the envelope
pass through the larger capacitor C2 via the volume control VR1.


A1 Telegraphy using keyed carrier – An A.M. detector
may be used but no audio tone will be produced unless a beat frequency
oscillator is used.  This may be introduced at any stage

i.e. the antenna

1st I.F.

2nd I.F.

However it must be of the correct frequency.

e.g.  Antenna 28MHz +x

I.F. 1    10.7MHz  +  x

I.F. 2 450 KHz + x

x is a small difference in frequency which becomes the
resultant audio tone when the two beat together.

There is usually some facility for varying the frequency
of the beat fequency oscillator till the tone is comfortable to the listener.

 Single Sideband Reception

In order to recover audio from a single sideband transmission
it is necessary to reinsert the the carrier which was removed at the transmitter.
This is done in a similar way to that above where an oscillator is mixed
into the I.F.

The term carrier reinsertion oscillator is used but in
actual fact there is usually only one oscillator which is used for both
C.W. and s.s.b. reception.

The terms “carrier insertion” oscillator and “beat frequency
oscillator” are often interchanged.

An ordinary signal diode as used to demodulate A.M. will
resolve s.s.b once the carrier is reinserted but a more favourable performance
is obtained using a product detector.

Here the signals are mixed in a more linea device than
a diode resulting in purer more linea audio output.

Detection of an F.M. Signal

A frequency modulated signal can be resolved by the A.M
envelope detector by detuning the signal from the peak of the response
to a point half way down the curve a shown in the fig.

As the frequency varies it is attenuated to a varying
degree by its position in the passband.  Although this works it is
a rough and ready method.  It has the disadvantage that if the passband
is narrow distortion occurs and if it is wide poor selectivity results.
-  even the best compromise between the two does not give good results.
This method effectively converts the F.M. signal to A.M. and uses the slope
of the passband to modulate the amplitude of the carrier.  It is often
referred to as slope detection.

Other circuits have been developed however which give
better results with F.M.

These are:-

1.  Foster Seeley discriminator – used for high quality
F.M.  systems but has poor A.M. rejection  and is rarely used
in Amateur equipment.

2.  Ratio detector.

3.  Various I.C.’s have been developed which use
a balanced mixer and descriminator.

e.g.  SO41P  CA3089  TBA120S

Ratio Detector Circuit

The signal from the input develops an R.F. voltage accross
the coil L1 which is fed into the bridge consisting of L2 and C2 ,
L3 and C3.

L2 and L3 are resonant at the centre frequency of the
signal.

F.M. Deviation Measurement

Section 4 of the Amateur Licence A states that the emitted
frequency of the apparatus comprised in the station is as stable and as
free from unwanted emissions as the state of technical development for
amateur radio apparatus reasonably permits; and

b) whatever class of emission is in use, the bandwidth
occupied by the emission is such that not more than 1% of the mean power
of the transmission (not including the power contained in spurious emissions)
falls outside the frequency band.

In order to comply it is necessary along with other requirements
to have a fairly accurate measurement capability for max. deviation of
the carrier at speech frequencies. A peak deviation of 3 kHz. is acceptable
(±1.5kHz). Commercial instruments are available but these are expensive
for the individual but are often available via Amateur Radio Clubs.

It is however possible to construct an FM discriminator
measure its audio output on an oscilloscope and calibrate this against
a standard signal.


Measurement of
RF power ~ Dummy Loads and Modulation Monitors.



Add Comment Register



Leave a Reply