Tuesday, August 4, 2009

Transmitters - let's start with the basics

Right, time to knuckle down for some more study. We've covered receivers quite well, so time to look at the transmitter. It's worth bearing in mind that of the 60 questions in the exam, 35 are in section A, and of those 35, a total of 10 are about transmitters and receivers. In other words, this accounts for one sixth of the questions in the entire exam, so these are two modules worth covering in detail and learning inside out. Remember to answer 30 questions in the QADV program when you've done your study and keep doing so until you get a score of 90% or better.

The IRTS disk starts with the different modulation modes, and there are five of these, as follows:


CW is Continuous Wave, and occupies a narrow bandwidth. It is transmitted through an on-off Morse Key

AM stands for Amplitude Modulation. The output amplitude varies according to the amplitude of the modulating signal. The original carrier and two side bands are transmitted. AM occupies a bandwidth which is the equivalent of twice the modulating frequency. There is a lot of power in the carrier.

SSB is (Amplitude Modulation) Single Side Band - where the carrier is suppressed, in most cases by a balanced modulator. One sideband is suppressed by a filter. SSB occupies half the bandwidth of AM. As there is no carrier, all the power is in the side band, making SSB more efficient.

FM stands for Frequency Modulation. The deviation of the output frequency is proportional to the amplitude of the modulating signal. The amplitude of the carrier is constant.

You can read a bit more about modulating methods on this website.

Just a little bit about bandwidth which is skipping a bit but you will definitely be asked at least one, possibly two questions about this. In the June exam, candidates were asked which bandwidth was best for CW and also, I think, a question asking how much wider an AM bandwidth was compared to CW.

IRTS says: "The bandwidth of a signal is determined by mode and by audio"

FM is widest.
AM is next
SSB is next
CW is narrowest.

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