|Radio and Electronics (DED Philippinen, 66 p.)|
|6. RECEPTION OF RADIOSIGNALS (AM - TYPE)|
The origin of any signal processed in a radio receiver is the signal picked up by the aerial.
So the radiowave is an electromagnetic wave the reception (picking up) can be achieved generally in two different ways.
RECEPTION OF THE MAGNETIC PART OF THE RADIOWAVE:
This can be achieved by a so called FERRIT ROD AERIAL. Such an aerial consists of a ferrite rod around which the one or more coils of copperwire are wound. The advantage of this type of aerial is, that it needs only little space and therefore it can be built inside the cabinet of even rather small radios.
But keep in mind: the bigger the ferritrod is, the more powerful is the received signal.
The reception of this aerial is depending very much on its position in relation to the received signal. It is receiving best, if the rod is hit perpendicular by the waves. This fact explains, why a receiver with that kind of aerial can have very different output power at the same spot, if the receiver is turned into another direction.
RECEIVING THE ELECTRICAL PART OF THE RADIOWAVE:
This can be achieved by aerials of different construction.
If the radio is stationary we can improve the reception by a long piece of or just a so called TELESCOP-AERIAL (as you find with most of the small portable radios). In generally one can say: as longer the aerial as better its reception.
While the aerials shown above are mostly used for long- and medium-wavebands, we have to use another type for higher frequencies.
The so called dipol-aerial consists of: either two wires constructed as fig. 36a shows (an OPEN DIPOL) or a loop of a wire constructed as fig. 36b shows (a FOLDED DIPOL).
Both types work best, if they have a length of about one half of the waves which are intended to be received. But even though we talk in such a case of ADJUSTED or TUNED aerials, this does not mean, that this aerial is only able to receive one single radiostation. All aerials are able to receive a rather wide range of frequencies with reasonable results.
This is on the one hand a big advantage (we don't need a special aerial for each different radio station we want to receive). On the other hand it causes problems for the radio technician, because we will always find at the terminals of the aerial a lot of different incoming signals, which are mixed together.
The above stated fact makes already clear what has to be done first with the signals found at the terminals of the aerial: we cannot just amplify the signal coming from the aerial, because under this condition we would hear an awful mixture of sounds of all the radioprograms transmitted in the surrounding of the radio.
We have to make sure first, that only the signal of the desired station will be processed during the next stages of our radio receiver.
This process of barring the other stations out and letting through only one station is called FILTERING. In which block the filtering is achieved, will be explained in the next chapter.
The signal at the terminals of the aerial consists of a mixture of signals of all the surrounding radio stations as shown in fig. 37.
But we want to listen only to one of those stations.
How to sort out one single station?
All of them carry audiosignals, - but this fact does not help us here... (Audiosignals are all of the same frequency spectrum). On the other hand all of them have a carrierfrequency as well. And those carrierfrequencies are - for sure different from each other (otherwise the radiostations would be operated against the law). So we can sort the desired program out, by letting through to the next stages of our receiver only the radiosignal with the carrierfrequency of our desired radio station.
The circuit which will be able to do this filtering is the so called TUNED CIRCUIT and it consists of at least one capacitor and an inductor. How the filtering is achieved really, we will discuss in details later in chapter 6., at this stage of explanation it is just important to keep in mind, that we will find in each radio at least one of those tuned circuits connected right to the terminals of the aerial.
Electronic devices consist nowadays in most cases of a lot of different circuits, each of it having a special purpose playing a special role in the oncert of the whole device. Each of the circuits itself can be very complicated.
To visualize the function of such devices, it would be far too confusing if we would draw all the components and interconnections in those different circuits at once.
Therefore nowadays more and more another method of visualization is used: the so-called BLOCK DIAGRAMM.
Here each different circuit playing a special role is symbolized by only a block (a rectangle carrying a special symbol or a word explaining the function). The blocks are interconnected by lines which show the flow of the signals or of energy from one block to the other.
Using this method of visualization we can draw at this stage of explanation a block diagram of the parts of a radio which we have come to know already.
Fig. 38 shows what we can draw up to now. Additional to the normal blockdiagramm, we find in this drawing the type of signal, appearing between those blocks.
If you have a closer look to the output signal of the tuned circuit you will find, that it is exactly the signal which would leave the transmitter of the radiostation we want to listen to.
But you should know: this drawing showing an ideal situation Normally you will have a huge attenuation on the way from the radiostation to the receiver, and the signal leaving the tuned circuit is very small (often less than a milli Volt).
If we receive the signal of a radiostation very near by (let us say a few hundred meters) this signal would be - with some luck - a few hundred milli Volts.
A sensitive earphone can produce sound with such a low voltage, but KEEP IN MIND: EVEN IF THE OUTPUT SIGNAL OF THE TUNED CIRCUIT IS POWERFUL ENOUGH YOU CANNOT LISTEN TO IT BY CONNECTING AN EARPHONE DIRECTLY.
The reason for the effect stated at the end of the last chapter, can be explained very easily, if we have a closer look to the signal produced by the tuned circuit: This signal is actually a mixture of two signals - the carrierfrequency modulated by the audiofrequency.
- The earphone - when connected to the terminals of the tuned circuit - will be passed by a current with a frequency which is the carrierfrequency. If the diaphragm would be able to follow this carrierfrequency it would produce air pressure oscillations of a frequency far above the range of audiofrequencies, therefore we would not hear anything.
So the diaphragm cannot follow these high frequencies, it will be at rest. Therefore we cannot hear anything at all.
In order to be able to listen to anything, we have to remove the carrierfrequency from the modulated radiosignal (to discharge the audio signal).
The process of removing the carrierwave is called DEMODULATION and it is carried out by a circuit called DEMODULATOR or DETECTOR.
If we assemble the blocks which we came to know up to hear, we would be able to hear at least a strong radiostation which is near to our receiver.
Fig. 39 shows the system which can be achieved if we do so. By doing so we have got a very simple kind of radioreceiver called a CRYSTAL RADIO.
This was the type of radio which was used first during the first days of radio-technology.
But you can very easily imagine, why this kind of radio was not a satisfying one: The sound produced was very weak and only one person could hear something if he was lucky enough to receive a station which could deliver enough energy for his earphones.
How could this receiver be improved? Of course have you have heard something about the law of CONSERVATION OF ENERGY.
If you apply this law on our crystal receiver, you will find out very easily, that the transducer (the earphone) can only produce sound energy with a maximum which is limitted by the input energy of the aerial.
This means as well: If we want to increase the sound energy we have to add energy to the energy from the aerial. This energy must be supplied within our radio by a so called poer supply.
This part of the radio has to deliver a certain amount of a rather constant dc-voltage.
But this power supply alone will not helps us, because we cannot connect it directly to the transducer for example. If we would do so, we would hear only one crack and nothing else, because the current flowing through the coil of the earphone or loudspeaker would be constant and therefore the diaphragm would be at rest afterwards (not producing any sound).
This means: the current flowing from the power supply to the transducer has to be controlled in a way that the diaphragm of the transducer will oscillate with the frequency of the AF-signal but whit a stronger amplitude than before.
This function: the control of the current from the supply to the transducer by the rythm of the AF-signal is done by a so called amplifier.
The inputsignal connected to the so called amplifier has a very tiny power compared with the output power. The amplifier has two terminals for the powersupply where the additional energy is delivered into the circuit and an output where the signal is produced which has the same shape but a bigger size (energy) than the input signal.
Using all the blocks which we came up to here we can now achieve a radio which would give us a reasonable sound also for stations which are not very near.
The result of the construction of a radio in that way would be a very simple radio receiver like it was built soon after the technology for amplifiers had been invented (the basics for this technology were the VALVES).
But very soon it was found that this simple construction had always a very high distortion, which was due to two reasons: So there was only one amplifier it was necessary in order to achieve a signal which was strong enough to have a very high AMPLIFICATION (the signal had to be enlarged very much in a simple amplifier circuit - called STAGE). Therefore it was necessary to have a signal as big s possible coming from the tuned circuits with a bad SELECTIVITY, which means they could not filter very exactly.
CONSEQUENCE: there are always radio stations transmitting on a carrierwave whose frequency is near to the frequency of the desired station, which reached the level of AUDIBILITY.
If a single amplifier stage has to amplify which very high amplification, it tends to produce oscillations itself, therefore it produces distortion itself. So the next step was to built in a second amplifier, but now one which was amplifying the still modulated signal appearing just out of the tuned circuit. This amplifier was called the RF-amplifier. The result was the a so-called TUNED FREQUENCY RADIO RECEIVER or TRF-receiver which was used from about 1930 till 1950.
The problem arising with RF-amplifiers in the TRF was: the RF-amplifier did not only amplify the desired signal of selected radiostation, but also to each additional signal, passing the tuned circuit.
So, by inserting the RF-amplifier the SENSITIVITY of the radio was improved (it could respond to weaker incoming signals too) but at the same time the SELECTIVITY (the ability to filter out a single radiostation only) was decreased, because now it could happen, that the speaker gave the sound of more than one radio-station at the same time.
The question was now: How is it possible to amplify (to add energy) the signal of the desired radiostation exclusively?
The answer was found in the beginning of the 40ties of this century. It was in physical sense the effect of SUPERHETERODYINING.
The function is in short like shown in fig. 42. The signal coming from the tuned circuit - from now on the RADIOSIGNAL (a) is mixed in a MIXER-STAGE with a RF-frequency signal supplied from the OSCILLATOR - from now on called the OSCILLATORSIGNAL (b) resulting at the end to a frequency f3 called the INTERMEDIATEFREQUENCY SIGNAL (c)
The oscillatorsignal is a signal with a constant frequency and a constant amplitude. You might assume that the oscillatorfrequency is desired to be exactly equal to the radio frequency in order to add energy to the incoming radiosignal.
For two reasons this is not true:
1. it would be very difficult to make sure that the oscillatorfrequency is exactly equal and in phase with the radiofrequency (if not phase, it would diminish the radiosignal).
2. It has a very big advantage to mix with a frequency distant from the radio frequency but to keep the distance constant. This advantage will be cleared during the next chapter.
If two signals with different frequencies are mixed in a MIXERSTAGE there will appear several new signals at the output: beside the two original frequencies we will find two new frequencies
fa = foscillator + fradio
fb = foscillator - fradio
In case of a wanted RF-signal of 1.52 MHz and an oscillator frequency of 1.976 MHz the frequencies fa and fb would be:
fa = 1.976MHz + 1.52MHz = 3.496MHz
fb = 1.976MHz - 1.52MHz = 0.456MHz
If we make sure that the distance between the frequency of the tuned circuit and the frequency of the oscillator is kept constant, we will find for every radiofrequency the same frequency at the output.
Therefore we will have at the output of the mixerstage always a constant frequency which is called now the INTERMEDIATE FREQUENCY which is derived from the oscillatorfrequency and the radiosignal and which is therefore still modulated with the audiosignal carried by the radiosignal. Figure 44. shows how this always constant intermediate frequency is achieved.
It is done by coupling two portions of a variable capacitor together. One of the portions is the part determining the frequency of the tuned circuit. The second portion is determining the oscillatorfrequency.
So both portions are changed strictly synchronically (they are turned by the same shaft) the frequency difference can be kept constant. This construction of a radio has several big advantages
1. The amplifiers amplifying the INTERMEDIATE FREQUENCY (called IF-amplifiers) can be tuned amplifiers which are tuned only to the very IF of this radio.
2. The energy of the desired radiosignal is increased very much, so that other stations fail the energy of calling through (being heard even though not desired).
We came to know now a series of different constructions of radios which have been invented step by step through the history of radio technology.
From step to step there was a technical improvement, always heading in direction to more sensitivity (being able to listen to distant stations) and more selectivity (being able to select only one station even if there are stronger stations near to the receiver) radios.
Fig. 45 to 47 should give you a general view about this development
1. What is the function of the different blocks of a radio!
2. Explain how a tuned frequency radio receiver is made up!
3. How is a superhetradio working?
4. What is the advantage of a superhetradio compared with a TRF radio?
5. What is the IF? Where and how is it produced?