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.

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.

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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