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Transmitters that transmit amplitude modulated signals require an AM modulator circuit to produce the required signal.
There are many ways in which amplitude modulation can be generated and a variety of circuits that can be used.
In early AM transmitters the amplitude modulator circuit was a key element of the transmitter. Today with software processes and multi-mode transmitters, the amplitude modulator is likely be contained within an overall modulator circuit that is able to provide a number of different types of modulation.
Nevertheless many transmitters still require AM modulators and the concepts and theory behind them is still relevant.
Many AM modulators were used with valve transmitters – AM was widely used for many communications applications when thermionic valve or vacuum tube technology was used. Accordingly many of the circuits that were developed were for use with these devices. However the principles are the same for both valve and semiconductor technologies.
High and low level AM modulators
AM modulators may be classed as either high or low level dependent upon their level in the overall signal chain.
- High level modulator: A high level modulator is defined as one that modulates a high power section of the circuit, typically the final RF amplifier. It has the advantage that linear amplifiers are not required for the RF amplification stages after AM modulation has been applied. The drawback is that high power audio amplifiers are needed. For broadcast transmitters where very high power levels are used, class D or class E amplifiers may be employed for the audio output.
- Low level modulator: A low level AM modulator would be one where the modulation is applied to low power stage of the transmitter, typically in the RF generation stages, or via the digital signal processing areas. The drawback of this approach is that linear amplification is required for the RF stages.
Anode or plate modulator
This form of AM modulator arrangement required a high power audio amplifier to provide audio to the anode or plate of the vacuum tube / thermionic valve.
In this way the audio modulates the voltage supplied to the final amplifier, and the level of the output signal is modulated in line with the audio. This form of modulation is referred to as high-level modulation, and the audio power level must be 50% of that of the RF amplifier to provide 100% modulation.
Typically a transformer is used to enable the output of the audio amplifier to drive the anode voltage of the final power amplifier within the transmitter. The audio transformer is placed in into the line connecting the anode circuit to the supply. In this way it modulates the anode voltage.
By applying the modulation to the final RF amplifier it meant that all the RF amplifiers could be driven in Class C, making the RF chain more efficient. If the audio was applied to earlier amplifiers, the later ones would all need to be linear.
Semiconductor devices, both bipolar transistors and FETs can be used in the same configuration as well.
Heising or constant-current AM modulator
This form of AM modulator is very similar in many respects to the ordinary plate modulation using a traditional transformer. This type of AM modulator is also known as a choke modulator because of the fact that it uses a choke rather than a transformer.
In the modulator circuit, the RF amplifier anode or plate voltage is fed through a choke. The anode for the audio amplifier is also fed via the same choke and as a result the audio amplifier valve / tube diverts current from the RF amplifier. The choke acts as a constant current source in the audio range.
Although cheaper because audio chokes are cheaper than audio transformers, this type of AM modulator circuit has a comparatively low power efficiency.
Another method of creating an AM modulator is to apply the audio to the grid of the final (or other) amplifier. This has the advantage that a much lower level of modulation is required.
In this type of AM modulator, the input signals including the RF as well as the AF and the DC bias are applied to the grid of the amplifier, which will be running in Class C.
The modulating AF acts on top of the existing DC bias and varies the level of voltage applied to the grid. This changes the operating point for the RF which runs in class C.
The overall effect of the system is to superimpose or modulate the audio onto the RF signal.
Grid based AM modulators were not as successful as anode based ones because unless carefully designed, the distortion levels of grid modulators could be much higher.