Syllabus Sections:-

Transmitter interference

4h.1 Recall the effect and the importance of minimizing drift.

When any piece of equipment, using an oscillator warms up, the frequency of oscillation changes. In the design stage of the equipment such variation will have been taken into account.

Should the frequency change after the warm up period then the effect of the change in frequency is called "drift".

You can sometime hear this drift of signals on the HF bands when older equipment is being used and to keep track of the frequency you would need to re-tune using the RIT. Then if you were to work the station your TX would go out on the original frequency and you would be working on different frequencies.

Should you own old equipment then you must keep the amount of the "drift" to a minimum less than 3kHz else you could move in frequency out of the pass band of the station that is tuned to your signal. This drift would cause interference to other stations using nearby frequencies and could result in out of band transmissions - hence the reason to minimize drift.

4h.2 Recall the cause and effect of 'chirp' and identify suitable remedies. Recall the cause and effect of 'key clicks' and the shaping of Morse keying waveforms.

Chirp and Key clicks are associated with CW or Morse code transmissions.

This topic is associated with CW or Morse code which is still much in use on amateur bands - even though it is not now a requirement in UK to pass a "Morse Test" ( not to be confused with the Morse Appreciation given to you in the Foundation Licence practical assessment)

Please note that Chirp and Key clicks are two entirely different factors.


"Chirp" is a change in frequency when the transmitter is keyed on ( by pressing the Morse key) and keyed off ( the release of the Morse key ) when the Morse characters are being formed. - thus the change in frequency would be heard on a receiver at the start and end of each keying stroke.

Chirp Causes

There are three main causes of chirp :-

Poor design of the transmitter

  1. DC Instability caused by poor voltage regulation.

  2. Pulling

RF Feedback

  1. RF Feedback getting back into the frequency determining stage (oscillators)

DC Instability - changes in voltage of the supply rail going to the oscillator of the transmitter when the transmitter is keyed. This particularly happens when the power supply to the oscillator is the same as the power supply to the PA.

REMEDY Use separate power supplies for the oscillator and PA and ensure high level of regulation and stabilisation to power supply of the oscillator.

Pulling This is where the frequency of other stages changes due to the keying of the transmitter.

REMEDY Provide better isolation of the oscillator from the other stages .

RF Feedback This is where stray RF signals are finding there way back into earlier stages. Such feed back might be by poor constructional layout.

REMEDY Greater attention to the detail of screening leads carrying RF and keeping leads as short as possible and the use of decoupling capacitors on the power lines.


When you key the letter A dit dah you have a very sharp leading and trailing edges to the wave form, if you looked at it on an oscilloscope as shown below (without the annotation !!)

The sharp edge occurs as the carrier goes from zero to maximum in the shortest possible time and is very much like an AM signal that has maximum modulation applied. With the very sharp edge and very short rise time it generates noisy sidebands which are the clicks that can be heard.

It is therefore the transition from zero to maximum carrier in the shortest possible time that causes the key clicks.

The way to remove the key clicks to the "soften the keying." This is done by having a less sharp and some what more rounded wave form.

The leading and trailing edges are softened / more rounded - if you make it too soft you would loose intelligibility of the Morse character.

A rise and fall time of about 1mS (1 milli second) might be considered a reasonable average but it all depends upon the equipment, as the softening and hardening of characters also depends upon things like decoupling in the bias circuit.

If you wanted to construct a transmitter and avoid key clicks you would need to look at the wave form on an oscilloscope with the scope connectors clipped across a dummy load being fed from the transmitter - on very low power - you can observe the wave form and then change the keying characteristics by using capacitors or a key click filter of a resistor and capacitor or a choke resistor and capacitor. There are various combinations depending upon the amount of current that you are keying.

Here is an example of a Key Click filter circuit


The sharper or more rapid the transition from zero to maximum the wider the bandwidth the signal will be as the click is generated.

4h.3 Understand ways to avoid generating harmonics (e.g. use of push-pull amplifiers, use of inductive coupling between stages, avoiding high drive levels).

Recall that transmitters may radiate unwanted mixer products and identify suitable remedies.


The way to avoid generating harmonics is to use push pull amplifiers. The push-pull amplifier circuit inherently cancels even harmonics.

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.

Drive Levels

You want to keep the drive levels down. If you "over drive" an amplifier you will destroy its linearity and the non linearity will produce more harmonics.


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 levels of the VFO input and crystal oscillator input into the mixer and reduce them to such a level to give you just the output of the wanted frequency. If you have either input too high then unwanted harmonics will be present on the output.

Understand the use of low and band pass filters in minimizing the radiation of unwanted harmonics and mixer products.

When using an HF rig it is good practices to use low pass filters in the antenna cable after all other items eg ATU power amplifier etc.

When using a VHF rig, due to interference caused to TV and similar higher frequencies apparatus, the use of a band pass filters so that only the frequencies of the band of operation are transmitted.

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.

Also a high pass filter could be used at VHF and it will help reduce emissions generated from mixer products at lower than VHF frequencies. A band pass filter would be better as it reduces both the mixer products and harmonics.

However in summery:-

It is thought that it should be bandpass filter rather than low or high and here is the reasons why:-

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 2 metre 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.

Now it might seem over the top but a bandpass / bandstop filter will go a great way to controlling this.

4h.4 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.

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.

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.

4h.5 Understand that over modulation causes harmonics (of the modulating signal) which may result in excessive bandwidth.

Over modulation causes harmonics

As bandwidth is determined by the modulating frequency, if the modulation overloads a stage to the point of clipping (distortion of the modulation envelope), then harmonics of the modulation (the audio signal) are produced. 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.

4h.6 Understand how frequency synthesizers may not produce the intended frequency. Identify remedial measures (out of lock inhibit).

If you are not familiar with the diagram below then please refer back to the text explanation before proceeding.

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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