IAN LANG ELECTRONICS
No5. Modulated Amplifier
At this stage we have now generated and amplified both the carrier and the audio signal that will modulate it and we can now marry (or more formally heterodyne) them together. This is the process of modulation whereby the low frequency audio signal is mixed to a high frequency RF carrier which can then radiate. At this point in our model high-power system we will have very large voltage values for both the carrier and the signal and will have to use valves as semiconductors will not withstand the power dissipated. A common circuit, known as the plate modulation system, is shown below.
The drawing at the left shows a high level plate modulator. As can be seen, it consists of the modulator, here shown in red, and the final stage of the class C RF amplifier, with the addition of the modulating transformer . The tank output can be attached either to the antenna or to a linear amplifier, which we shall discuss next. The advantage of doing it this way is that modulator, RF amp and antenna are all electrically isolated from each other.
The mechanics are: V1 is biased at cutoff, and when a positive pulse comes in at the grid from the isolating transformer , the tank output circuit receives a shock pulse that sends it into operation and it oscillates. Both plate and tank receive a DC supply from the secondary coil of the modulating transformer, which is coupled on the primary coil to a varying amplitude generated by the audio signal coming in to the modulator. This either adds to or subtracts (depending upon where it is in its cycle) from the supply going to V1 and tank, and thus causes a variation of the plate current of V1, in turn causing a variation of the amplitude of the signal given out by the tank output circuit, and causes wave fronts to be radiated from the antenna if it is connected, or to pulse into the linear amp if that is the next stage. V1 needs to be rated at four times minimum the supplied DC voltage to be able to operate with the oscillating tank circuit as it reaches null impedance, and heterodyning this way requires a lot of audio power which comes from the modulator. Hence the term high level modulation. When the value of the audio signal is equal to the value of the supply, 100% modulation occurs.