Transmitter interference
4e.1 Recall that excessive audio amplitude or excessive audio bandwidth
into a modulator can cause excessive bandwidth or excessive FM
deviation. Understand that this may result in interference to adjacent
radio frequencies.
This drawing shows two narrowband FM signals, 25kHz apart
In this drawing, two wideband FM signals occupy the same frequencies,
but this time their signals overlap causing mutual interference.
Wide band signals are caused by either audio that is too strong, or an audio signal that is too wide.
Here an AM signal is modulated
by an audio signal with a wide bandwidth. This causes an excessively
wide signal which can interfere with other signals
When an AM signal is overmodulated it becomes distorted.
When viewed on an oscilloscope it looks like this.
4e.2 Recall that oscillators, mixers and amplifiers can produce harmonics i.e. multiples of the fundamental frequency.
Recall that harmonics can cause interference to other amateur bands and other radio users.
You
build a transmitter for 7MHz and receive reports that you are also
transmitting a signal in the 14MHz and 21MHz bands. Why is this?
The answer is that oscillators, mixers and amplifiers can produce harmonics i.e. multiples of the fundamental frequency.
So, in addition to a 7MHz signal the harmonics will also appear on 14MHz, 21MHz, 28MHz and so on. These
harmonics can cause interference to other amateur bands and other radio
users if the harmonic falls outside of an amateur band.
4e.3 Recall that a filter is a device that blocks some frequencies and passes others.
Understand the effects of low-pass, band-pass and high-pass filters. Interpret their frequency/amplitude diagrams.
Understand that a low-pass filter and a band-pass filter can minimise the radiation of harmonics.
A filter is a device that blocks some frequencies and passes others.
A low pass filter allows low frequencies to pass, but attenuates higher frequencies.
A bandpass filter allows some frequencies to pass through, but attenuates those above and below the pass band
A high pass filter allows high frequencies to pass through, but attenuates low frequencies.
A notch filter produces a high attenuation above and below a certain frequency/ The notch allows signals through.
Low and bandpass filters are used to minimise unwanted harmonic
frequencies. The drawing opposite shows our example of harmonics from a 10MHz transmitter,
a low pass filter could be designed that allowed through the
fundamental 10MHz signal, but attenuates anything above10MHz. As you can see from the drawing a low pass filter will reduce the harmonics from the 10 MHz transmitter to an acceptable level.
4e.4 Understand that too fast a rise and fall time of the transmitted
RF envelope of a CW transmitter may cause excessive bandwidth (key
clicks) and that this can be minimised by suitable filters in the
keying stage.
A final form of interference is the key click.
If a key is used directly with
no filtering the signal goes from zero to maximum in a very short time. This gives a very
sharp signal which produces interference across the band. A CW station
with no filtering can often be heard many kHz above and below the signal.
To remove these clicks the CW
needs a rise and fall time of about a mS. To achieve this a resistor,
capacitor and inductor are used as shown in the diagram.
