IAN LANG ELECTRONICS
Block Diagram Explanation No.1 :The Master Oscillator
Certain materials, when a current is passed through them periodically, have a natural frequency at which they prefer to resonate. The result of this is that they take a pulse put through them at slightly erratic time divisions and stabilise it to a remarkably accurate degree. The preferred frequency is dependent upon the size of the slice and the way the material is cut, and amongst the naturally occurring materials quartz is the most commonly used. If the material is used in just a resonant mode, some 4 to 10 MHz are possible, if in overtone up to 100 MHz.
Crystals do suffer from temperature instability which may take them out of tolerance and to combat this an oven is used which is precisely calibrated to keep the correct temperature. Some off-the-peg crystals have them built-in and a small current is passed between two terminals to heat the casing. Clever thermistor design helps to vary the current and thus the temperature. In this way a highly accurate frequency can be obtained.
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.