Syllabus Sections:- 3G
Transmitter interference
3G1 24
Understand that over-modulation distorts the
modulating signal resulting in harmonics of the
audio which causes excessive transmitted
bandwidth.
Over modulation causes harmonics
As bandwidth is determined by the modulating frequency
normally maximum of 3kHz, if the modulation overloads a stage
within the transmitter it will clip thus causing distortion of
the modulation This then creates harmonics of the modulation
(the audio signal) at say 6kHz 9kHz or more.
As these are higher in frequency than the intended modulation,
the effect is to modulate the signals with higher frequency
audio, resulting in a higher, and unnecessary bandwidth being
used.
Let's say you are generating a
sine wave and this is fed to an amplifier and the amplifier
clips the signal you will then get a harmonic problem as the
clipped sine wave is now a squarer wave form which is rich
in harmonics.
If
you are using SSB then you need to understand that in an
amateur radio world the generally accepted case is that Upper
Sideband, USB is used on frequencies above 10 MHz and that
Lower Sideband, LSB is used in frequencies below 10 MHz.
As
stated above the normal band width is 3kHz so the distortion
will be greater than 3kHz so any calculation in a question
would yield audio frequency in the LSB more than 3Khz lower
than the operating frequency and if USB more than 3kHz greater
that the operating frequency.
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3G2 24
Understand that over-drive of the RF power amplifier
can also result in excessive transmitted bandwidth.
By
over-driving of the RF Power Amplifier this increases
spurious intermodulation products and harmonics. The
result of this is that your RF amplifier would create
unnecessary bandwidth, and this could give rise to
possible damage to the amplifier due to overheating.
Understand the need to drive external power
amplifiers with the minimum power required for full
output and how overdriving may cause harmonics and/or
spurious intermodulation products.
The generation of unwanted
components, is relatively sudden.
Driving the amplifier with the minimum required output power
results in a "clean" signal i.e. one without harmonics.
There should be a simple linear relationship between input and
output powers for instance :-
input is 10 Watts, output 80 Watts,
input 20 Watts, output 160 Watts, etc.
input 40 Watts, output 320 Watts, etc.
When the linear relationship starts to fail for example :-
input 40 watts gives only an output of 300 watts instead of
the expected 320 watts then you have reached the point of over
driving the amplifier. This may cause harmonics and/or
spurious intermodulation products.
Do not to overdrive but operate below maximum ratings.
For those who know a thing or two about power - and that
should be all readers - a small reduction in output power
level will give a negligible variation to the strength of
received signal. A doubling of power is usually needed to
raise the "S" meter by half a point (but this does depend upon
the calibration of the "S" meter on your rig. So dropping
power to 350W from 400W and having a clean signal will lose
you only a fraction of an "S" point.
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3G3 24
Understand ways to avoid generating harmonics e.g. use
of push-pull amplifiers, avoiding high drive levels.
Push-pull
Amplifier
One way to avoid generating harmonics is to use a push-pull
amplifier, the push-pull amplifier circuit inherently
cancels or minimises harmonics in the 2nd 3rd or 4th
frequencies, however using inductive (see below) or tuned
couplings will also help if used between the amplifier
stages.
Inductive
coupling
The inductive coupling is as shown on an earlier circuit of
the amplifier and an extract is shown below.
L1A is the inductive link coupling into the tuned circuit L1
C1.
This provides basic filtering of the wanted wave form and
not the harmonic wave form being applied to the amplifier.
C1 L1 is tuned to the wanted frequency.
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L1A is the inductive link coupling into the tuned
circuit L1 C1.
This provides basic filtering of the wanted wave
form and not the harmonic wave form being applied
to the amplifier.
C1 L1 is tuned to the wanted frequency.
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Drive
Levels
A very good way to avoid unwanted harmonics is to keep
the drive levels down , By "over driving " an amplifier
you will destroy its linearity and the non linearity
properties and thus it will produce more harmonics, so less
drive equals less harmonic trouble. Be thinking of about 1dB
reduction in your amp drive as this will make a very large
different to the harmonic problem BUT very little difference
to the DX station you are working.
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3G3 24
Recall that transmitters may radiate unwanted mixer
products and identify suitable remedies.
Design
In today's modern
transceiver design the units use a mixing up process to get
the final output frequency, trouble is the output from the
mixer is not just the final frequency (sum) but can often be
any combination of a multiple of either input frequency's.
This effect will produce multiple harmonics .However if the
transceiver uses a balanced mixer set up then this will
suppress or "balance out" many of the harmonic products.
If you are to design a circuit and you have a VFO and
crystal mixer circuit, giving you the output frequency you
would have to observe the amplitude levels of the VFO input
and amplitude
level of the crystal oscillator input into the mixer and
reduce them to such
amplitude levels to give you just the output of the wanted
frequency. If you have either
amplitude input too high then unwanted harmonics will be
present on the output.
Also see fig 8.4
page 52 in the RSGB full licence manual for another version
of a similar diagram
3G3
24 Understand the use of low pass,
band pass and band stop (notch) filters in minimizing the
radiation of unwanted harmonics and mixer products.
When using
an HF transceiver it is good practice to use low pass
filters in the antenna cable after all other items eg ATU
power amplifier etc, this will minimise any left over
harmonics or unwanted mixer products.
When using
a VHF transceiver, due to interference caused to TV and
similar higher frequencies apparatus, the use of a band pass
filters is more preferable as only the frequencies of the band
of operation are transmitted.
At VHF
unwanted mixer products will normally be found LOWER than the
wanted frequency as opposed to higher in HF radios.
The greatest interference will occur at the 2nd harmonic and
this is where it is best to use a notch filter.
Mixers produce the sum and difference frequencies so should
always be designed with filters to select the wanted signals
and remove the unwanted signals.
However in summery:-
In general try to use a bandpass filter rather than low
or high filters because:-
1. An FM transmitter operating in class C is rich in
harmonics, if uncontrolled a 144mhz versions would have
outputs on 288, 432, 720 ect.
2. The mixer and multiplication of frequencies for say a
145mhz rig can produces outputs above and below the band
if uncontrolled.
3. Two transmitters operating on the same mast at the same
time can produces intermodulation products by one transmitter
getting into the others PA.
3G4
24 Recall that unwanted
emissions may be caused by parasitic oscillation
and/or self oscillation and identify suitable
remedies.
Unwanted emissions may be caused by parasitic
oscillation and/or self oscillation within equipment
being built and these unwanted frequencies must be
prevented from being radiated.
Normal
filtering etc will do much to control the
problem,however for self oscillation problems that are
generally at or near the working frequency look towards
the feed back path and check any screening and closeness
of both input and output wiring.Make sure the wires are
separated to different stages and make sure they have
good decoupling properties.
If
parasitic oscillations are operational higher than your
wanted frequency's ,using a resistor choke or radio
frequency choke will help bring it under control,
But if you get it below your wanted frequency then best
to look towards the microphone input amplifier leads and
rectify using as short as possible leads.
Much as a flea is a parasite (unwanted, and unintended)
on a cat, a parasitic oscillation must be investigated,
and "killed" by suitable circuit design.
Please note an alternative diagram is in page 53 fig
8.6 in the RSGB Full manual
Coil wound on the resistor
These unwanted frequencies often occur with badly
designed decoupling. Elimination of some of these
frequencies can be achieved by the use of low value
series resistor and a coil wound on the resistor. (This
functions by introducing a slight "loss" in a circuit,
preventing sufficient gain to cause oscillation in the
first place.)
Parasitic STOPPER resistor
A 22ohm resistor connected directly in the collector of
a LOW power common Emitter amplifier is a parasitic
STOPPER resistor.
3G5 24
Understand how frequency synthesizers may not
produce the intended frequency. Identify appropriate
measures to prevent off-frequency transmissions.
Below is a block diagram of a frequency
synthesizer.
Please note in this diagram the phase detector can also be
called the phase comparator and alternative diagram can be
found on page 54 fig 8.8 in the RSGB full manual
It is quite possible for the frequency synthesiser not to
produce the correct frequency in-band or close to the band
edge. Filtering of such frequencies will not help as
filtering is normally a broad band approach.
It is very desirable that there is no output from the
transmitter whilst the synthesiser is "unlocked", and it is
usual to take a signal from the synthesiser which inhibits
the transmitter until "lock" has occurred.
It should be realised that the accuracy of a synthesiser
depends on the accuracy of the crystals employed and even
small inaccuracy could result in inadvertent transmission
outside of the band edge, whilst believing you are "just
inside".
The synthesiser will be "locked" under these circumstances
hence the "lock inhibit" circuit will NOT prevent
transmission
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