As you know, we intend that you become a radio repairman with very good knowledge and all the instruments required to do a good job.
- To test and adjust the device, only a modern radio with FM band and electronic dial with a frequency meter are required.
- A digital or analog tester
- A peak-to-peak value sensing probe
Before entering the construction of the instrument, we will give an explanation of what is FM modulation making a comparison with the already known AM. The frequency bands of both emissions and the different types of propagation that each band has so that the reader knows what type of band to use in each case and the expected coverage radius.
Then we will explain how our circuit is built, how it is simulated in the Multisim and finally how it fits and its range is tested. We will indicate how to radiate music, voice and a calibration tone. Remember that to radiate signals beyond your own home you must have the corresponding authorization from the communications secretary of your country.
Modulation of a carrier
A carrier permanently at the same frequency, amplitude and phase can not transport information. It is only a vehicle that allows sending information such as the postal mail structure of a country that only transports the information. The information must be processed so that it modifies some of the parameters of that carrier.
Strangely, the first radio transmissions were digital because they consisted of sending or cutting a carrier in the form of dashes or dots that was the mode used in the telegraphy that preceded the radio and that was initially called wireless telegraphy. You can consider that this mode of transmission was digitally modulated amplitude because “1s” or “0s” were transmitted through a manipulator.
Note: the filled bars have the carrier frequency, but it is so high that it looks like a blur.
Many years later the real voice and music transmissions were started by amplitude modulation in the OM band from 530 KHz to 1600 KHz with a maximum modulation frequency of 5 KHz.
Note: the previous note is worth
AM (longwave) and OM (medium wave) AM transmissions tend to surround the earth by bouncing partially in the ionosphere. Its scope can be thousands of KM not being very affected by solar activity.
Above the mean wave, different bands of OC (shortwave) are used for commercial activity and radio fiction. These frequencies have the virtue of bouncing completely in the ionosphere and in the earth repeated times so they can go around the world.
AM transmissions are not characterized by their quality because they are very affected by the interference of industrial and household noise such as fluorescent tubes, pulsed sources of computers and TVs and the ignition system of thermal engines.
But due to their scope, they practically have occupied the entire OM band and therefore the transmissions are limited to 5KHz so that they can fit more stations within the band. In this case, the stations can be separated 10 KHz from each other and since the band is practically 1 MHz, it means that 100 stations can be assigned.
To achieve higher quality transmissions, and even transmit in stereophony, a much higher frequency band was created, ranging from 88 to 108 MHz (for America) with 100 or 200 KHz of separation between stations according to the countries. Since the total band is 20 MHz, if the stations are separated 200 KHz, you can assign 100 FM stations.
The ionosphere is not capable of reflecting frequencies as high as those used in FM. The earth is not capable of curving the FM emission. Therefore the emissions have little more than an optical range, which depending on the height of the transmitting and receiving antennas and the emitted power is approximately 50 to 100 km.
The modulation of an FM station is found in the FM frequency, which varies around 75 KHz around the center frequency. This variation is done at the rhythm of audio modulation. The receiver must, therefore, have a frequency detector, instead of the classic diode amplitude detector of the AM radios. Figure 3 shows the corresponding oscillogram but clarifies that the modulation depth should be increased so that the carrier could be observed.
Since the radio detector does not respond to variations in amplitude, it becomes insensitive to all the noises that affect the AM signal and then high-fidelity transmissions can be made.
The complication of designing a frequency modulation detector does not exist today, because it is a stage entirely internal to the only integrated circuit that a modern radio usually has. For that reason, we are going to exempt ourselves from explaining the operation of old and complicated FM detectors with external coils.
The FM transmitter is much simpler than that of AM and that is why we are going to consolidate our knowledge by building one that will also be used to repair FM radios.
The basic oscillator on the FM band
We already know that an oscillator works like an amplifier with positive feedback. There are therefore many types of oscillators suitable for working in different frequency bands. The oscillator that we are going to build is a Colpitts fed by the emitter, built around an MPSH10 transistor that works up to frequencies of 1 GHz (also called Gigantor). The oscillation will be generated with an L1 coil taken from a disused TV (AFT or load coil). The first thing that the reader must do is remove the internal capacitor from this coil since in our circuit the coil is tuned to C2 and C5.
Note that the C1 trimer is placed between the emitter and collector of Q1 and that the base is at ground potential for the AC due to capacitor C3 and DC conduction potential due to divider R3 / R2 and emitter resistance R1. This means that Q1 can amplify but unlike classic amplifiers the signal enters by the emitter, exits by collector and returns to enter the capacitor C1 / C2 to the emitter (feedback in phase or positive).
In the design, it was taken into account that the gain of the transistor is higher than the attenuation of C1 / C2 and that the phase of the feedback is positive so that the Barkhausen conditions are met and oscillation is generated.
The oscillation frequency will always be the one at which the maximum amplification gain is obtained and this occurs when the capacitor C1 resonates with inductor L1.
In our case, the idea is for the circuit to oscillate within the FM band so that we can hear the signal as silencing of the noise by about half the band.
In the emitter of the transistor, we have an ideal point to take the output signal through capacitor C4 that isolates the direct voltage of the emitter. In that place, you get about 700 mV signal pap that properly connected to an antenna to generate a powerful transmitter. Then an attenuator consisting of R4, R5, R6 and R7 is installed to achieve lower outputs and suitable for use in the repair of equipment.
Finally, the components C6, D1, D2, C5 and R8 are not part of the oscillator but of the pap voltage measuring probe.
Later on, when we have armed the complete circuit, we will indicate the adjustment mode of C1 and L1. For now, we put the trimer in the middle of its travel and the core of L1 with several turns above the reel. Under these conditions, you will obtain a frequency of approximately band center (close to 100 MHz) and the amplitudes indicated in the oscilloscope and the tester. The function generator XFG1 is used for the oscilloscope to start at a frequency similar to the desired one even though it has no physical connection to it. We can say that it is a necessity of the simulation when working with very high frequencies, to place a generator on a frequency of 100 MHz because otherwise, it could not start. In any case, the reader should wait about 30 real seconds for the circuit to start oscillating.
Shifting of the carrier frequency and modulation
Figure 5 shows the aggregate of the carrier frequency adjustment and modulation depth generated with two varicap diodes.
In the figure, we can analyze the complete circuit of the RF section. The XFG2 generator represents the audio section that we will then adapt to our needs to modulate from a microphone, a fixed frequency audio oscillator, a CD player or a CD player. MP3
The circuit is based on two ZC825 varicap diodes or similar connected in parallel. Note that they are now part of the resonant circuit of the oscillator since they are practically in parallel with C1 (since C2 is a high-value capacitor compared to C1).
The capacity of the varicaps is varied with the DC voltage provided by the potentiometer R9, in charge of adjusting the frequency of the oscillator (for now do not know the existence of R13).
This potentiometer must be connected to a 30V source because the diodes chosen are of the high voltage type to obtain a greater frequency variation. That is to say that the project requires two sources, one of 30V and another of 12V whose construction will be faced separately.
The continuous voltage is applied to the union of the varicaps with C7, that is to say, that there is no DC voltage consumption at that point because on one side is the capacitor and on the other the varicaps are inverse. When the equipment is adjusted with the potentiometer at 30V, 108 MHz are generated and with the potentiometer at a minimum of 88 MHz. The resistor R10 is placed to prevent the varicaps from entering directly with the positive peaks of the RF at minimum control voltage.
How does the modulation work?
Now let’s explain how modulation works. When the voltage on the varicaps varies from 3 to 30V the change in frequency is 108 – 88 = 20 MHz. If that same voltage change is made from the collector of Q2 by means of R13 of 1 MOhms, we will get a voltage variation on the varicaps of a value 10 times smaller, because an attenuator is formed that starting from the collector finds R13 / R11 + R9 + R10 that practically equals R13 / R11 since R9 and R10 are negligible compared to R11. This implies an attenuation by ten that is to say that our circuit will have modulation of the maximum frequency of the order of 2 MHz which is much higher than the necessary 75 KHz.
This is graduated with the signal level applied to capacitor C8 and measuring the alternating collector voltage that should be of the order of 1V peak to peak. The reader can adjust this value by measuring the collector with an oscilloscope or with the pico-peak value probe for the tester. This probe can alternatively be connected to C4 to measure the RF output (check the operation of the circuit) or to the collector to adjust the modulation using a one-way key two positions.
All the components used are easy to achieve, except for two of them. Regarding the varicaps ask to see what is achieved in your favorite trade, download the specification on the Internet that you offer to see what is your maximum capacity. The ZC825 is 100 pF to 1V and if for example, you get one of 25 pF you must use four in parallel. We advise you to use the Internet to locate in which stores in Argentina you get the varicap and the transistor (MPSH10). Note: Resistors R6 and R7 are not part of the oscillator itself, They are placed to obtain high signals on R5 and attenuated on R7.
The audio section – The modulation oscillator
There are many ways to generate a fixed frequency audio signal at 1 KHz that can be used to modulate our RF generator in frequency. These oscillators are known by the name of the scientist who studied them.
The Multisim has a complete help system for the design of different devices. This is how they are predesigned circuits with the 555, active and passive filters, amplifiers of a transistor, and Wien bridge oscillators.
Entering “Tools> Samples> Wien-Bridge oscillator” we obtain a 1Khz bridge-type generator from Wien. The original design uses two sources of +15 and -15V. We modified it to use a single 30V source and we added a potentiometer to attenuate and adjust the output required for the Q2 base of the oscillator-modulator section design.
Control the operation of the audio generator with the oscilloscope, with the pap value probe, or by entering the input of a CD player. Remember to set the gain preset R1 to a minimum to force the oscillation and then increase its value until obtaining an output of 15V pap by measuring with the peak-to-peak probe connected to R7. audio pap.enerator with the oscilloscope or with the value probe pap.a transistor, and oscillators p
Temporarily connect the output of the audio generator to the capacitor C8 of the oscillator and modulator.
Connect a 75 cm cable on R5, to act as an antenna and bring it close to the telescopic antenna of the radio with a frequency meter.
Place trimmer C1 in its central position. Place the potentiometer R9 in the middle of its travel. In these conditions, our generator should produce a tone of approximately 1 KHz in the center of the FM band. Look for the tone with a radio that has a frequency counter and adjusts the L1 coil accordingly so that the tone falls near 98 MHz.
Then bring the frequency adjustment potentiometer R9 to its maximum value. Bring the radio to 108 MHz and adjust trimmer C1 until the tone is heard. Now take the potentiometer to the minimum setting the radio at 108 MHz and adjust the coil core so that the tone is heard. Repeat the 108 and 88 MHz settings several times until they are perfectly adjusted.
In this way, the RF section is adjusted.
Now you have to adjust the modulation depth. Connect the pap value probe between the collector and ground of Q2 and set the R8 preset of the audio oscillator to 200 mV pap.
As the reader can see our voltage controlled oscillator is a complete instrument armed with materials that are surely available in our workshop.
Although it is built mainly to adjust an FM radio, it is easy to modify it to cover the FI of a TV.
The FM station
Whether our device is an FM radio adjustment generator or an FM station, everything depends on the audio source used as a modulator. Using the 1 KHz generator, look for an area of the band that is relatively empty as the frequency of your home station.
Without authorization from the communications secretary, you can not place an aerial antenna, but you can place a transmitting antenna inside your house or in your park or yard at ground level.
The polarization of the FM waves is vertical, so the transmitting antenna must be a ¼ wavelength dipole with at least four radials, used as a ground plane with an angle of 120º concerning the antenna.
Note: in the figure, the radials were left at 90º, but the reader should fold them further down.
The irradiating rod must pass through a hole in the printed circuit to give the assembly greater mechanical strength. All rods should be welded to the copper with a welder of 100 or 150 W.
The live and mass conductors of the antenna can be made of 2 mm copper and their length for both the irradiator and the ground plane must be calculated according to the formula
L = c / 4F
Where c is the speed of light of 300,000 km/sec and F the frequency chosen for its station. For example, if it is in the center of the band, the frequency is 98 MHz and the calculation gives 75 cm. The central support is made on a printed circuit.
This antenna has a characteristic impedance Z0 of 50 Ohms and can be connected to the RF oscillator with a coaxial cable of the same characteristic impedance. The live connect to C4 with a resistor of 68 Ohms and the other side to the vertical irradiator. The Maya is connected to the mass of the transmitter and on the other side to the mass radials.
Test the range with the audio tone of 1 kHz and when you have everything in conditions add to the design the audio section with the microphone and the audio input for the CD player or MP3, which can be seen in the following figure.
This circuit is based on a two-terminal electret microphone. An electret microphone works by the very principle of the flat capacitor. If a capacitor is made with a fixed plate and another with a tense membrane shape and is spoken about for the dielectric to change thickness, a variable capacitor is obtained by the voice. This capacitor is charged from a source with a resistor (R4) and after the charge is left with some fixed electrons inside given by the formula
Q = C / V where V = C / Q
Since Q does not change when changing C due to vibrations, you must change V and that is exactly what you are looking for. A tension that varies according to the vibrations of the air. Since the capacitor is of small value, the sensitivity of the microphone is low and a transistor amplifier must be added as close as possible to the electret. That is to say that it is perhaps convenient to assemble the microphone and the transistor Q1 immediately above it and then a three-conductor cable with the source, ground and collector of the transistor. In this case, we work at an impedance of 10K and the buzz pickup is much lower. To improve the pickup problem, you can use a shielded stereo audio cable and use one live for the source the other for the collector and the ma as a mass.
The reader can see that the collector output is connected to a potentiometer that operates as a microphone volume control. Separately an input is provided for an MP3 or an auxiliary output of a CD player that is connected to another potentiometer that operates as an auxiliary volume control. This system is what is called a mixer and allows the compensated operation of our home station and perform fades of music and voice.
We leave our readers the possibility of combining the three sources of sound with a mixer with three potentiometers to combine a 1KHz tone with the two sound sources that we have just explained.
With the due authorization of the communications secretary, a transmitter of this type, with an antenna at the height of about 10 meters can reach scopes of 1Km or more. The use of commercial antennas can extend the scope even more, but the type of irradiation that is desired must be taken into account. A point-to-point transmission is not the same as a transmission of the circular type towards the four cardinal points.
Coaxial cables are widely used in electronics to connect radio-frequency devices and are built to prevent the energy transported by the cable from being irradiated directly.
The coaxial or coax cable was created in the 30s of the last century; It has two concentric conductors, a central one, called a live one, in charge of carrying the signal, and an outer one, with a tubular aspect, called mesh or shield, which serves as a reference for earth and return of the currents. Between both is an insulating layer called a dielectric, whose characteristics will depend mainly on the quality of the cable. An insulating cover usually protects the whole set.
The central conductor may be constituted by a solid wire or by several twisted copper wires; while the outside can be a braided mesh, a rolled sheet, or corrugated copper or aluminum tube. In the latter case, a semi-rigid cable will result.
The construction of coaxial cables varies greatly depending on the type. The choice of design affects its size, flexibility and properties. A coaxial cable consists of a core of copper wire surrounded by an insulator, a braided metal shield and an outer cover.
Surrounding the core is a dielectric insulating layer that separates it from the wire mesh. The braided wire mesh acts as a mass and protects the core from electrical noise and interference from the adjacent coax. The core and the mesh must be separated from each other. If they touch each other, the signal will short-circuit. A non-conductive outer cover (usually made of rubber, Teflon or plastic) surrounds the entire cable, to avoid possible electrical discharges of the mesh. In coaxial cables, the fields due to the currents circulating in the internal and external conductor cancel each other out.
Most coaxial cables have a characteristic impedance of 50, 52, 75, or 93 Ω. The electronics industry uses standard type names for coaxial cables.
|Type of cable||Diameter in mm||Impedance in Ohms||Speed factor||DB attenuation every 100m as a function of frequency|
Note: remember that 20 dB equals ten times.
In this lesson, we present a test instrument for FM radios, a fixed frequency audio oscillator and an FM transmitting station in a single device. We recommend that the student approaches his arm without hesitation because for very little money he will have the possibility of performing a very useful practice for his development as a repairman.
At the same time, we applied very important components, previously studied, such as the varicap diode and the more zen diode, and we knew others that it was essential to know as the electret microphone and the coax cable.