IAN LANG ELECTRONICS

Otherwise known simply as the oscillator or frequency generator this is the module that provides the carrier frequency familiar to us. Let us say we want to listen to Absolute Radio which has a frequency of 1215 kHz, then to broadcast this station the master oscillator needs to be providing an oscillation either at or at an integral dividend of 1215 kHz to provide the correct carrier wave upon which we can modulate a signal. If we wanted to listen to talk sport, the frequency of the carrier generated would be 1089 or 1053 kHz depending on where you live, and for Radio 5 Live 909 or 693 kHz.
The carrier has to be exceptionally stable for these purposes and an LC tank such as one would find in the receiver will not be of sufficient stability to maintain the nominal frequency at the required tolerances. A piezoelectric crystal would be used instead. (See the sidebar on the left).

A tank with an inductor and capacitor as shown left will not be stable enough for this application.

In the early days of radio Marconi used a spark-gap transmitter to do the same job. Although effective, it causes a transmission to be made across an extremely wide band, and it is illegal to operate such a device today. Fessenden used a Westinghouse HF alternator, which was better, but incredibly difficult and expensive to make, run and maintain. A small crystal from Maplin costs £1:59.
We have to bear in mind that we will not get full transmitting power from our oscillator: its job is to provide a relatively small AC voltage of continuous and precisely varying frequency from an applied DC voltage. The AC voltage provided is sent for amplification, and this is discussed in Explanation No. 2.