Each radio receiver must have a reception antenna. It is an electrical conductor, where voltages of various frequencies and amplitudes are being induced, under the influence of electromagnetic fields from various radio transmitters. Besides these voltages, those induced by EM fields that are created by various disturbance sources (such as electrical motors, various household appliances spark-plugs of an automobile and all other devices where electrical current is being switched on/off during work) are also present in the antenna, as well as those from fields originating from outer space or the Earth’s atmosphere. Basic roles that a radio receiver has are:
a. To separate the signal (voltage) of the radio station that it is tuned at from the multitude of other voltages, whilst suppressing (weakening) all other signals as much as possible,
b. amplifies the extrapolated signal and take out information from it and
c. reproduces that information, i.e. restores it into its’ original shape.
Even the simplest radio, the one we are discussing in this chapter, must be able to accomplish all these tasks. The electronic diagram of one such device is given on Pic.3.1. It is the famous (years ago) Detector Radio Receiver or shortly, Detector. The signal selection (separation) and voltage amplification are performed in the oscillatory circuit that is
made of the capacitor C and coil L, the separation of information (speech or music) from the AM station signal in the detector that comprises the diode D, capacitor C2 and resistance of the headphones, and information restoring in the very headphones.
Main advantages of this device lie in its extreme simplicity and the fact that it requires no additional energy sources for its’ operation. All the energy required it draws from the antenna, which therefore has to be at least a dosen metres long for proper operation. It is also useful to have a good ground. One can do without it but the reception with it is truly better, especially considering the distant and small-power transmitters.
3.1.1 Input Circuit
The capacitor that takes the signal from the antenna (so-called coupling capacitor) C1, variable capacitor C and coil L form the input circuit of the radio receiver. Its main role is to separate the signal of station the receiver is tuned at from multitude of voltages (having various frequencies and amplitudes) existing in the antenna, amplify that signal and turns it over to the detector.
In order to better understand the requests that are to be fulfilled during the practical realization of input circuit, it is necessary to know basic characteristics of circuit made of capacitor C and coil L. It is called ‘The oscillatory circuit’ and is shown on pic.3.2-a. The amount of its impedance (resistance to AC current) between points A and B, which is marked with , depends on the frequency, as it is shown on the diagram on pic.3.2-b. The most important characteristic of this circuit is its resonance frequency, being given by the Thomson’s formula:
As one may notice, the resonance frequency depends on the capacitance of the capacitor C and inductivity of coil L, and changes if one of them change. In our receiver, a variable capacitor is used, that can change its capacitance from Cmax to Cmin, therefore changing the resonance frequency in boundaries from
The area between fd (lower boundary frequency) and fg (upper boundary frequency) is the reception area of the input circuit, as shown on pic.3.2-b. On this picture, carrier frequencies of four radio transmitters are being marked with fs1, fs2, fs3 and fs4. The resonance frequency of the oscillatory circuit is set (by means of C) to be equal to the carrier frequency of the second station: fs2. In that case, the impedance ZAB – frequency dependance is shown in continuous line. As one can see, the impedance ZAB for all received signals whose carriers have frequencies less than fs1 and greater than fs3 is less than 20 kOhms, while for the station that is tuned it is equal to 200 kOhms. Let us now imagine that the parallel oscillatory circuit is connected with the antenna and ground, as shown on pic.3.1-b. Imagine, also, that there are (only) four voltages in the antenna, that have the same amplitude and are created by four radio
transmitters, having carrier frequencies of fs1, fs2, fs2 and fs4. Since these voltages spread between the antenna and the ground, four currents will flow through the oscillatory circuit: Is1, Is2, Is3 and Is4. The voltages that are created by them in the oscillatory circuit, between points A and B, are equal, acc. to Ohm’s Law, to the product of current and impedance: UAB=I*ZAB. Acc. to pic.3.2-b, for Is2, impedance of the circuit is ZAB=200 kOhms, and for currents Is1 and Is3 it is 10x smaller. That means that the voltage that is being created in the oscillatory circuit by the station that transmits on frequency fs2 will be ten times greater than the voltages being created by stations transmitting on frequencies fs1 and fs3. This is how selection of one station is performed, by means of the oscillatory circuit. Transition to some other station is performed by changing the capacitance of capacitor C, as long as the resonance frequency of the oscillatory circuit does not become equal to the carrier frequency of that station. If its frequency happens to be fs4 (acc. to pic.3.2-b), the impedance of the oscillatory circuit for that case is shown in dashed line, which causes that on the circuit output voltage of the station that transmits on frequency fs4 is acquired, while other stations’ signals are suppressed.
At first glance, everything is just the way it should be: Parallel oscillatory circuit extrapolates one and suppresses all other stations. Unfortunately, the reality isn’t so simple. First of all, radio transmitters operate with various output (emission) powers and on various geographic distances from the receiver, therefore making the voltages that their signals create in the reception antenna very different in amplitude. It is clear that stronger signals will “cover” the weak ones, thus disabling their reception. E.g. if radio transmitter that emits on the frequency fs1 is geographically much closer to our radio receiver that the transmitter operating on fs2, the voltage the former creates in the reception antenna can be even 200 times greater than the one created by the latter. The oscillatory circuit will do its job as previously described, but on its ends the voltage of the first transmitter will still be greater (20x) than that of the transmitter the receiver is tuned at, and normal reception won’t be possible. There are also other problems whose solving will not be discussed herein, and readers that are interested in those can read a book “Radio Receivers”, written by Momir Filipovic, issued by the National Textbook Publishing Company from Belgrade, Yugoslavia.
To conclude this chapter, we may say that the simplest radio receiver can cover only signals of the local and powerful radio transmitters.