Electronics Principles V11

Clive W. Humphris

RADIO AND COMMUNICATION: AM Radio Transmitter Block Diagram.  


AM is an abbreviation for Amplitude Modulation. The AM radio transmitter is the collection of circuits that generate the electromagnetic waves that convey our messages to receivers. The simple diagram contains all the blocks necessary to make this happen. Beginning with the input as a tiny signal from the microphone this is first amplified and then mixed with the output from the Variable Frequency Oscillator (VFO). This combined voltage signal must then be amplified and sent as much more powerful signal with an output impedance suitably matched to drive the antenna.

The output is always a sine wave with a constant frequency for that transmission, called a carrier. Onto this is modulated our audio signal. The purpose of the carrier is to provide a means of transferring audio frequency signals which by themselves do not radiate as electromagnetic waves, whereas the higher frequency of the carrier does, extremely effectively.

There are several methods of modulating this carrier, but for now we will only consider amplitude modulation. This means the carrier frequency remains constant, but the mixed audio signal causes this to be varied in amplitude. This is the loudness of the received speech. The frequency of the audio signal continually varies as we speak and this shows as longer or shorter duration of the modulation envelope, where higher speech frequencies cause the modulation to occur much faster than lower frequencies.

Microphone is a device that outputs electrical energy from input sound energy.

The force of the speech causes a transducer i.e. moving coil or crystal to output a small voltage that varies in both amplitude and frequency in direct relation to the level and frequency of the speech.

The output from the microphone is at a very low level, no more than a volt in amplitude.

The audio amplifier increases that amplitude to a sufficient level suitable for modulating the carrier waveform. A low pass audio filter is required as part of this circuit to severely limit any frequencies above 3kHz. This narrow range of audio frequencies is sufficient for intelligible speech.

The modulator has two inputs, one the carrier which is a sine wave of relatively high frequency (1 to 500MHz) and generated by the Variable Frequency Oscillator and the other the amplified audio (speech) signal.

The modulator is a circuit that outputs a signal that is a combination of the two. This means the high frequency carrier now includes the audio information (having a ride).

Variable Frequency Oscillator is a frequency generator and produces a carrier at the desired wavelength for transmitted signal, between 1MHz and 500MHz. It's always a pure sine wave. Any errors here mean the signal transmitted could fall outside the allocated radio bands. In practice this method of an amplitude modulated output is not actually used, because it is extremely inefficient, but forms basis of all radio transmissions and so its important to understand the principle of using one carrier to transport the wanted audio signal to the receiver(s) antenna.

It is extremely important that the output frequency, which is continuously variable by the user controls, once selected remains stable and various methods are used to ensure this. These range from switched crystal oscillators to digital frequency synthesisers. Accurate calibration of the VFO is an important part of using your rig. This is one area of the transceiver circuit where the difference between the budget and top of the range rigs will be clearly identified.

The input i.e. output from the modulator (audio modulated carrier) will usually be a voltage waveform with very little current (fairly high impedance), however the signal for driving the antenna must contain electrical power, most likely a low voltage with lots of current (low impedance), depending upon the design of the power amplifier.

Radio Frequency (RF) amplifier is similar to an audio amplifier where a small voltage drives the cone of a heavy loudspeaker via a powerful amplifier capable of developing lots of watts, except in this case its at a much higher RF (Radio Frequency). To limit the transmitter output to those frequencies in the desired transmission band, a low pass filter is incorporated, thereby cutting off any frequencies outside those permitted.

Transmitter Antenna provides the electromagnetic interface between your rig and the outside world. To be effective it has to be designed to match the output impedance of the transmitter to maximise on the efficiency of the power output and also physically constructed so as not cause obstructions or interference with other electrical equipment.

This is greatly simplified here, but helps to explain the operation of an antenna. When transmitting, consider the antenna as a transformer operating at a high frequency. The current changes in the carrier sine wave will cause a 'magnetic' field to build up around the dipole that will radiate outwards. This magnetic radiation can be picked up by ferrite rod aerials tuned to the same frequency as the ferrite provides an easy path for the magnetic flux which is then converted to a weak voltage input signal at the receiver through the aerial coil winding. The voltage (electrical) fluctuations will be similarly collected by any tuned dipole. An antenna tuned to the same frequency will cause tiny currents to travel down the feeder to the RF input stages of the receiver, thereby developing the voltage input signals, which are then amplified and demodulated.

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