A radio receiver uses an aerial to pick up radio waves. Radio waves are high frequency electromagnetic waves that travel at the speed of all electromagnetic radiation i.e. 300000km/s. Radio waves have a relatively long wavelength and so diffract easily around large objects such as buildings and hills. The high speed of radio waves combined with their ability to diffract, and hence reach all areas on the ground, make them idea for communication. There are many different frequencies of radio wave broadcast simultaneously from various transmitting stations and so a radio receiver must be able to select the desired frequency of radio wave, this is achieved with a tuned circuit. The radio waves carry information and are referred to as carrier waves. Encoding information onto radio waves is called modulation. A radio receiver must therefore be able to de-modulate the radio wave to recover the encoded information. Finally the information is processed, in the case of audio signals the information is amplified and played through a loudspeaker. In the case of radio control systems the information is processed to control switches or mechanical devices called servos.
The Aerial picks up electromagnetic radio waves and the radio waves produce a small current in the aerial.
The Tuned Circuit selects one carrier frequency from all the different carrier frequencies picked up by the aerial.
The Demodulator separates the information (audio signal, data etc) from the carrier wave and discards the carrier wave.
In the case of a normal domestic radio receiver, the Audio Amplifier amplifiers the audio frequency signal from the demodulator.
Finally, the Loudspeaker converts the electrical signal from the audio amplifier into a sound wave which we hear.
Sensitivity determines how strong a radio signal has to be to still be received. A very sensitive radio receiver has a tuned circuit with a high gain so that even the weakest signals can be detected. An insensitive receiver can only receive the strongest signals. A good definition of sensitivity would be "the ability to receive a weak signal against background noise" as it is also important that a sensitive receiver can distinguish between the signal being transmitted and the natural background noise.
Selectivity determines how well two signals of similar frequency can be distinguished. A very selective receiver can discriminate between two radio signals that have very similar frequencies. A good definition of selectivity would be "the ability to select one carrier frequency from other that are very similar".
For example modern radio microphones uses an F.M. radio signal to transmit the audio information from the microphone to the receiver at the mixing desk. The transmitter is battery powered and so has a low output power, this means the receiver must be very sensitive if the radio microphones are to have an acceptable range. A standard radio microphone works with a carrier frequency of about 180MHz. The microphone can be set to one of six channels which have a 300kHz difference in their carrier frequencies. Thus the receiver has to be selective enough to distinguish between signals that are 300kHz different i.e. a difference in the carrier frequency of only 0.2%
The function of the aerial is to pick up electromagnetic radio waves from the air. It picks up radio waves of all frequencies. Ideally the aerial is tuned to more effectively receive the frequencies that are wanted, this is achieved by making the aerial a quarter of the wavelength that is to be detected. Electromagnetic radiation is a combination of electric and magnetic fields. The electric field in the radio waves cause the electrons in the aerial to oscillate and so an a.c. current is induced in the aerial. This current is processed by the receiver circuit.
The tuned circuit selects signals of one frequency from all the frequencies picked up by the aerial. The tuned circuit is a circuit with an impedance (similar to resistance) that depends on frequency. The impedance of the circuit is low at all frequencies except the one frequency that is to be selected. At the selected frequency the impedance of the tuned circuit is high. This means that signals of all frequencies are conducted through the tuned circuit straight to ground, except for the one frequency that is to be selected, signals of this frequency are not able to pass through the tuned circuit to ground and so are passed to the demodulator. The impedance of the tuned circuit depends on the values of the inductor (L) and the capacitor (C) which can be varied to alter the frequency received.
The radio receiver detects the carrier wave, a radio frequency signal of a fixed frequency. The carrier wave is not what you hear, it merely carries the audio information or data. The tuned circuit detects the carrier wave. The process of encoding information onto the carrier wave is called modulation. There are two types of modulation, Amplitude Modulation (A.M.) where the amplitude of the carrier wave is changed to represent the audio signal or Frequency Modulation (F.M.) where the frequency of the carrier wave is varied very slightly to represent the audio signal or data.
Amplitude Modulation is achieved using a voltage controlled amplifier. The audio signal controls the gain of the radio frequency (r.f.) amplifier in the transmitter. In the diagram the amplitude of the carrier wave is higher when the amplitude of the audio signal is higher.
The advantage of A.M. is that it is easy to demodulate.
The main disadvantage is that it is prone to interference because environmental effects such as sparks can alter the amplitude of the carrier or produce r.f. that is picked up by the tuned circuit.
Frequency Modulation is achieved using a voltage controlled r.f. oscillator to produce the carrier wave in the transmitter. The frequency of the carrier wave depends on the level of the audio signal - in the diagram the frequency of the carrier is higher when the amplitude of the audio signal is higher.
The advantage of F.M. is that it is much less prone to interference and the audio signal is much higher quality.
The disadvantage is that it is harder to demodulate and you need a higher carrier frequency to make it work well.
The demodulator recovers the audio signal or data from the carrier wave. In the very simple A.M. receiver circuit the demodulator first rectifies the signal using a germanium diode. Germanium diodes have to be used to handle the very small voltages from the tuned circuit. The 0.7V forward bias voltage required by a silicon diode make them useless in a demodulator circuit. For A.M. a simple capacitor circuit is sufficient to act as a low pass filter to allow the low frequency audio signal whilst rejecting the high frequency carrier wave. The capacitor 'smooths' the carrier wave.
The signal from the demodulator is a very small signal (several microvolts) and so must be amplified before it can be used. The audio amplifier must have a very high input impedance so that very little current is required from the tuned circuit and demodulator. The audio amplifier drives a loud speaker. Alternatively the amplifier could drive digital logic circuits etc if the radio was receiving data communication.
The diagram shows a simple crystal receiver, it only receives A.M. radio. The tuned circuit picks up the appropriate carrier signal. The germanium diode demodulates the signal by rectifying it. Germanium diodes must be used as the tuned circuit only generates a few microvolts. A germanium diode has no significant forward bias voltage where as a silicon diode has a forward bias voltage of 0.7V, much higher than the voltage developed by the tuned circuit. The crystal ear-piece requires so little power to operate that it can be driven by the current induced in the aerial. The inertia of the ear-piece means that it cannot oscillate at the radio frequency and so only responds to the average change in amplitude ... the ear-piece is acting as a low pass filter to separate the audio signal from the carrier frequency.
© Paul Nicholls
Electronics Resources by Paul Nicholls is licensed under a Creative Commons Attribution 4.0 International License.