Oscillators

Electronic circuits that generate voltages of sinusoidal waveform are called"sinusoidal oscillators". There are also oscillators generating periodic signals of other waveforms, among which square wave generators are most popular. Square waveoscillators are predominantly used in digital circuits to produce time references,synchronization, etc.

We use oscillators in communication circuits for variety of reasons. There are twooscillators in TRC-10. The first one is an oscillator that generates a signal at 16 MHzfixed frequency. This oscillator is a square wavecrystal oscillator module.

A square wave of 2 volts peak to peak amplitude is depicted in this Figure.


In fact, such a square wave can be represented in terms of sinusoids as a linear combination.

We refer to the sinusoids with frequencies 2ω, 3ω, 4ω,…, nωasharmonicsof thefundamental component , sinωt.

We can obtain an approximation to a square wave by taking ao, fundamental, and onlyfew harmonics into the summation. As we increase the number of harmonics in thesummation, the constructed waveform becomes a better representative of squarewave. This successive construction of a square wave is shown inFigure 1.3.



Even with only 3 terms the square wave is reasonably well delineated, although itsshape looks rather corrugated.

A common graphical representation of a signal with many sinusoidal components is toplot the line graph of the amplitude of each component versus frequency (either f or ω). This is called thespectrumof the square wave or itsfrequency domainrepresentation. Spectrum of this square wave is given in Figure, which clearlyillustrates the frequency components of the square wave.

We need sinusoidal voltages in TRC-10, not square waves. Indeed, we must avoid theharmonics of our signals to be emitted from our transceiver, because such harmonicswill interfere with other communication systems operating at that frequency. We usethis fixed frequency square wave oscillator module, because such modules provide a very accurate and stable frequency of oscillation and can be obtained at a low cost.We first filter out the harmonics of the waveform, when we use this module in our circuit.

The other oscillator is aVariable Frequency Oscillator (VFO). This oscillator produces a sinusoid of frequency that varies between 12 MHz and 13.7 MHz. Thisfrequency is controlled by a d.c. voltage.