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Introduce the principle of WBFM TX V7

2017-08-09 21:01  
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This article is Introduce the principle of WBFM TX V7. The principle of combining text comprehension is a good suggestion. All starts with TR1 (BC547) in an inverted Hartley oscillator configuration. Feedback to the base by a small TR1 4.7 pf capacitor to help keep oscillating as weak as possible while the oscillator is a reliable starter. The oscillator frequency is determined by L1 and 22 pf trimmer capacitor and functional range of about 76 MHz to 119 MHz using the PCB I made. Couple 15 pf capacitor L1 to the top of the varactor diodes is to add additional capacitance to change the frequency of the tuned circuit. R1 added varactor supply voltage, low noise decoupling (0.1 uf capacitor). Audio will be coupled to the BB105 through 47 k resistor. Only a 47 pf decoupling in order not to limit the bandwidth of atrial fibrillation complete transmitters. AF bandwidth is flat from 3 Hz to 72 Hz, but if we go beyond these limits, there is a 6 db in DC. This is because these two 47 k resistor divider AF input voltage by 2, but in the DC 0.1 uf capacitor charging time, two 47 k resistor is not so divided. Collector tomb (2 n4427) has a large (by QRP standard) blocked by the DC supply, but presents a high impedance RF. RF signal is then matched to 50 ohm 15 pence, L5 and 56 p. 1 nf cap only in the supply voltage, or via an antenna. L5 and 56 pf to form a low-pass filter, to help prevent harmonics appear in the output signal. 16 kinds and 47 pf are added to further reduce the level of harmonics. This filter is an absolute must for all transmitters if you do not want to offend all other users of the radio spectrum. L5 and 16 are also placed on the PCB so that the coupling between them a little. This coupling is used to offset any residual signal instead of the filter in the pass band may appear at the input to 15. It is this effect leads to an unexpected clean first prototype, and a small layout of the experiment has now been reduced to -60 in paragraphs 2 and 3 harmonics in all supply voltages dbc. To 150 mw output, which is equivalent to 150 years 3 subharmonic Bonamiwa and 2nd harmonic level was only 50 nanotiles.

 

NOTE -This project is illegal to use or build in many countries. I accept no responsibility what-so-ever for any ilegal use. This circuit is provided solely as an educational project.

The features of this project are:

Higher output power – 150mW min (at 9v) and 300mW (at 12.5v)Very pure output signal due to carefull design and filteringVARICAP modulation – possiblity to add a synthesiserSingle sided Printed Circuit Board, only 40mm x 72mmCovers the domestic FM band – 88MHz to 108MHzEasy to build, but coil winding experience IS required

 

Circuit

TR1 = BC547TR2 = BC547TR3 = 2N3866 or 2N4427

 

 

L2 is wound on small ferrite beads. L2 is placed in series with the emitter of the buffer transistor, TR2. In the interests of stability it is very important that this coil does NOT radiate like a loop antenna. It is composed of 4 turns of 0.15mm Dia. enamelled wire (magnet wire in the US). The grade of ferrite is unimportant, as long as it is a grey one. One complete turn is formed when the wire passes through the hole in the middle once. The ferrite is mounted vertically in the same manner as a resistor. The ferrite is 3mm outside diameter and the assembled coil looks like this:

Coils

L3 is wound using 3 turns of 0.8mm Dia enamelled copper wire with a 6mm inside diameter. Wind it on a drill bit to get the inside diameter correct. The coil is close-wound, that is to say that the turns are just about touching and shall not be spaced. Form the ends of the coil, bending them out then down so that the leads are 5mm apart. The coil should look like this:

Coil

L4 comprises 6 turns of 0.15mm Dia. enamelled wire (magnet wire in the US) wound on TWO ferrite beads, the same beads that were used for L2. The beads are placed side-by-side as a pair of biniculars. One complete turn is when the wire is threaded once though both beads.

Coil

L5 is 5 turns of 0.8mm Dia. enamelled copper wire with a 6mm inside diameter. Just as L3, wind it on a drill bit to get the inside diameter correct. The coil is also close-wound. Form the ends of the coil, bending them out then down so that the leads are 5mm apart.

L6 is 3 turns of 0.8mm Dia. enamelled copper wire with a 6mm inside diameter. Just as L3 and L5 the coil is also close-wound. Form the ends of the coil, bending them out then down so that the leads are 5mm apart. You can get a closer view of L5 and L6 in this picture. The position of the capacitor between them is very important, equal distance from both coils keeps the 2nd harmonic at the lowest level.

NOTE -L5 and L6 MUST be wound in the same direction.

Construction

When all the components have been fitted, check your work thoroughly. I reccomend you shine a strong lamp behind the board component side and compare the tracks with the PCB foil pattern. This will allow you to check for solder bridges between tracks. Assuming all is well, connect a 50-Ohm resistor to the antenna (ANT) terminals. Two 100-Ohms in parallel will be fine. Now connect the board to a 9v supply in series with a 12v 3W torch lamp. If the lamp glows brightly then switch OFF and check your wiring because you have a fault. If there is no fault then the lamp should only glow dimly, if it glows at all. The complete transmitter should draw less than 100mA.If all is well, switch ON an FM radio set tuned to somewhere around 108MHz. Adjust the tuning capacitor on the board so the plates are at around minimum capacitance and you will hear the transmitter on the radio. With the capacitor plates near maximum capacitance you should be able to tune the transmitter to 88MHz.

 

 The vertical scale is 10dB per division and the horisontal scale is 50MHz per division:

As you can see, the worst case is the 3rd harmonic at -60dBc. The carrier level was 23dBm (200mW) at 10v supply. This falls off a little to about 160mW at the ends of the bands. Brief specifications are given below. I have not been all that meticulous with the figures. When I got a reading of 73mA I rounded it up to 75mA to keep the figures simple. The figures are only a guide anyway.

ParameterSupply=9vSupply=12.5vSupply=13.8v
Freq range76 – 116MHz77 – 119MHz78 – 121MHz
Supply Current (98MHz)75mA85mA95mA
Output power (88MHz)160mW310mW370mW
Output power (98MHz)180mW360mW420mW
Output power (108MHz)165mW320mW380mW
Spurious Outputs (DC – 1GHz)-60dBc-60dBc-60dBc
RMS AF for /-75KHz deviation210mV200mV195mV
AF response 0/-3dB3Hz – 70KHz3Hz – 70KHz3Hz – 70KHz

Let us now cover these items, beginning with a recap of the circuit diagram:

Circuit

As you can see from the original circuit, the varicap voltage is kept high by R1. Without this resistor the DC voltage on the diode will be zero, causing the oscillator to stop. R1 shall be removed if using external synthesiser control. We can, however, use the CTRL terminal to have a preset “frequency” potentiometer on the outside of the box. All we need is a 500K Linear potentiometer. Nothing else! no capacitors, nothing. This will give typically 10MHz tuning range

 

Given that R1 is connected directly to the battery supply voltage, if the battery voltage were to vary then so would the TX frequency. You should really be using a stabilised power supply, or a high-current battery that has a fairly constant supply voltage. If this is NOT the case then you can use the CTRL terminal to bypass R1, without making any modifications to the TX. All we need is an external zener diode and a 6K8 resistor. The Zener diode should be as high as possible. If you have a 12.5v supply, for example, then a 10v diode would be great. With a 9v battery then a 6v8 diode is about the maximum practical. 8v2 would be Ok until the battery voltage went down a bit. We will asume that VE is a 9v battery.

If you are using DC modulation then the AF input will allow this. This can be used to give low frequency Frequency Shift Keying. This should only be done in conjunction with the voltage regulator above. If you are NOT wanting to have a DC shift, and your input source has a “DC Continuity” (resistance) then there is problem. If you were to connect a magnetic microphone or CD player, for example, to the AF input then the TX frequency would jump. You therefore need to add a capacitor to block the DC shift at the input. 10uF will do nicely. A 4K7 resistor should also be added to give a load to the audio source and to help prevent “hum” or “pickup” from 50Hz (60Hz) wiring. The value of the resistor should be selected to match the audio source impedance of the device. 4K7 is normal for computer and CD LINE-OUT signals.

If you wished to use audio directly from a PC or CD-Player, then you should add some form of pre-emphasis. If you are using a stereo encoder or FSK applications then you should NOT use pre-emphasis. Pre-emphasis increases treble a little so that when it is received and turned down again, added noise is also turned down. Select Cx(nf) to be 10x the number of Microseconds of pre-emphasis you want. If you wanted 50uS pre-emphasis then Cx = 50 x 10 nf = 500nf.

 


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