| ADVANCED LESSON 41 | |
| LEARNING OBJECTIVES and NOTES | |
| Transmitter Interference 2 | |
| 4h.4
Recall that unwanted emissions may be caused by parasitic oscillation
and/or self oscillation and identify suitable remedies. A further form of unwanted radiation from a transmitter are parasitic or self oscillation or spurious oscillations These are usually above the operating frequency and occur because a length of wire and a capacitor just happen to oscillate, or because there is feedback from one part of a circuit to another. These can be very powerful and cause severe interference. They can be reduced by:
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4h.5 Understand that over modulation causes harmonics (of the modulating signal), which may result in excessive bandwidth. If the audio stage of a transmitter is over driven and we look at the image on an oscilloscope, the sine wave will have flat tops. This in turn will produce audio harmonics. For example if normal speech has an upper limit of 3kHz an overmodulated signal will have harmonics at 6, 9, 12 , 15 kHz etc. When this is used to modulate an RF wave the RF signal will have sidebands of 3, 6, 9, 12, 15 kHz etc. This will cause the signal to be wider and cause interference to adjacent channels. This applies to both AM and SSB transmissions |
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4h.6 Understand how frequency synthesisers may not produce the intended
frequency. Identify remedial measures (out of lock inhibit). In lesson 36 we looked at how a frequency symthesiser works. There may be occasions when the VCO is so far off frequency that it is not possible to lock it on the desired frequency. As a safety precaution frequency synthesisers are fitted with an "out of lock" indicator. This will turn on a warning light and prevent the transmitter working so that transmission does not take place off frequency. One cause of off frequency transmissions is the stability of the reference crystal. This needs to be checked regularly and precautions taken to stop it drifting / shifting off frequency. |
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| External Power Amplifiers and Interference | |
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4i.1 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. External power amplifiers are used to raise the 100Watts output of most transceivers to 400Watts. As with all amplifiers it is important that they are not over driven as this can lead to the production of harmonics and spurious emissions. Here are some tips for setting up and running a valve power amplifier:
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