Electronics/Transmitter design

  
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For a fixed frequency transmitter one commonly used method is to use a resonant quartz crystal in a Crystal oscillator to fix the frequency. For transmitter where the frequency has to be able to be varied then several options can be used. It is often the case for VHF transmitters that it is not possible to operate the crystal controlled or variabl
Electronics/Transmitter design - schematic

e frequency oscillator at the frequency of the final output. Also, for reasons including frequency stability, it is better to multiply the frequency of the free running oscillator up to the final frequency which is required. If the output of a amplifier stage is tuned to a multiple of the frequency which the stage is driven with, the stage is optimised to give a larger harmonic output than that found in a linear amplifier. In a push-push stage, the output will only contain the even harmonics. This is because the currents which would generate the fundamental and the odd harmonics in this circuit (if one valve was removed) are canceled out by the second valve. Note that in these diagrams that the bias supplies and the neutralization have been omitted for clarity. In a real system it is likely that tetrodes would be used as plate to grid capacitance in a tetrode is lower so making the stage less likely to be unstable. The task of many transmitters is to transmit some form of information using a carrier wave. This process is called modulation. There are many types of RF modulation, and the choice of modulation often depends on the type of information being transmitted. For instance, audio information is continuous in time and value, and scaling by a constant (i. e. signal inversion, volume control) is acceptable, so AM and FM transmission work. But for digital communications, the signal is discrete in time and discrete in...



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