Modulators & Transmitters
Radio transmitter design is a complex topic which can be broken down into a series of smaller topics. more...
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Frequency synthesis and frequency multiplication
Synthesis
Fixed frequency systems
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 a transmitter where the frequency has to be able to be varied then several options can be used.
Variable frequency systems
An array of crystals - this has been used to enable a transmitter to be used on several different frequencies, rather than being a truly variable frequency system, it is a system which is fixed to several different frequencies (a subset of the above).;
Variable frequency oscillator (VFO);
Phase locked loop frequency synthesiser;
Multiplication
It is often the case for VHF transmitters that it is not possible to operate the crystal-controlled or variable frequency oscillator at the frequency of the final output. In such cases, for reasons including frequency stability, it is better to multiply the frequency of the free running oscillator up to the final, required frequency.
If the output of an amplifier stage is tuned to a multiple of the frequency with which the stage is driven, 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 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, thereby reducing stage instability.
 Here in the push-pull stage the output will only contain the odd harmonics because of the cancelling effect.
Frequency mixing and modulation
The task of many transmitters is to transmit some form of information using a radio signal (carrier wave) which has been modulated to carry the intelligence. One example of the rare type of transmitter which does not need to carry information is the RF generator in a microwave oven.
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