Production of 5W FM Transmitter

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Veronica FM transmitter is easy to make, stable in performance, pure in signal, does not use professional parts and IC, and has auxiliary test function to make you debug easily without professional equipment. It has two versions, 1 watt and 5 watt. The 1 watt version is suitable for 3 km transmission distance, and the required power supply is 12-16V 200mA; the 5 watt version is suitable for 8 km transmission distance, and the required power supply is 12-16V 900mA. This article introduces the 5 watt version.

Figure 1: 5W Veronica circuit diagram

The transmitter has a built-in mixer, which allows you to transmit audio signals from a CD and a microphone at the same time. Transistor T1 is the microphone amplifier, and variable resistors R1 and R2 adjust the volume (see the debugging section). Between R8 and C21 is the oscillator, which is the component that generates the radio frequency signal. Diode D1 is a so-called "varicap tube", which is equivalent to an adjustable capacitor. It is controlled by the audio signal and changes the oscillation frequency of the oscillator, playing the role of frequency conversion. C12, C13, and L1 determine the frequency of the oscillator. This oscillator is actually composed of two anti-phase oscillators, each running at around 50MHz. When the two signals are combined, they become a 100MHz signal. This circuit is much more stable than a single 100MHz oscillator. The oscillator signal is amplified to 5W by T4 and T6. The circuit to the right of T4 includes antenna impedance matching and low-pass filtering functions. The circuit composed of D2, D3, and T5 is used for auxiliary debugging. It samples the signal output by the RF and controls the light-emitting diode D5. When the output is high, D5 is also brighter.

This circuit does not have a stereo modulator. If you need to play stereo programs, please refer to here to make a stereo modulator.

Parts List

resistance:

R1+2 10k adjustable R3 820k R4 4.7k R5-7 220 R8 1.5k R9 15k R10+11 1k R12 33k R13+14 56 R15+16 68k R17 47 R18 270 R19 10 R20 22 R21 1.5k R22 270

capacitance:

Unless otherwise specified, ceramic or mica capacitors are used.
C1,2,7, 16,17,19, 24,29 & 31 1n C3-5 & 8 10u 16V Electrolytic C6, 18 & 30 220u 16V Electrolytic C9, 10 & 20 10n C11 22p* C12 47p* C13 22p Fine Tuning C14 & 15 15p* C21,25 & 26 65p Fine Tuning C22 100p C23 15p C24 33p C27 1.8p C28 5.6p C32 & 34 47p C33 22p C35 & 38 1n C36 220n C37 100p
*C11, 12, 14 & 15 determine the oscillation frequency, it is best to use high quality mica capacitors.
Coil:

Use a hollow type without a skeleton. Wrap a 1mm diameter wire tightly around a pencil core or other round rod, then carefully stretch it to the correct length and make sure the two ends of the coil are as shown in Figure 2.

L1 6 coils, each with 2 turns, inner diameter 5mm, length 5mm L2 3 turns, inner diameter 7mm, length 7mm L3 3 turns, inner diameter 6mm, length 8mm L4 Wrap 14 turns of 0.2mm diameter enameled wire on a 2.2k carbon rod resistor (diameter about 2mm), and weld the end of the enameled wire to the resistor connector. Put a magnetic bead on each of the two connectors of the resistor, as shown in Figure 2B. L5 5 turns, inner diameter 6mm, length 11mm L6 4 turns, inner diameter 6mm, length 9mm

RF choke:

The choke (H1-4) can be made by winding a 0.5mm diameter enameled wire around a 4mm diameter and 5mm long magnetic bead. Note that the enameled wire should pass through the hole of the magnetic bead, and the magnetic bead should be made of a material with an operating frequency of 100MHz (usually No. 43). If you can't find a magnetic bead, you can also make it by winding a 0.5m long 0.2mm diameter enameled wire around a 33k carbon rod resistor, and weld the end of the enameled wire to the connector of the resistor.

H1 5 turns on the magnetic bead H2 1 turn on the magnetic bead H3 2 turns on the magnetic bead H4 3 turns on the magnetic bead

Diode: D1 is best to use a varactor pair, that is, two symmetrical varactors connected back to back with the negative pole in the middle; but this is not very important, two ordinary varactors will also work.
D1 KV1310 D2+3 1N4148 D4 ordinary light-emitting diode D5 1N4001

Transistor: T1+5 BC548, general small signal transistor T2+3 BF494, high frequency small signal transistor T4 RF power tube 2W, 12V, 10dB@175MHz 2N4427, C2538, C1970 3DA190, 3DA194, etc. T6 RF power tube 4W 18V >=10dB@150MHz MRF237, 2N3926, C1971, C1947, MRF630, BLU99, 3DA21, 3DA106, 3DA56 3DA192, 3DA22, etc.

Note: The pin positions of the power tubes of other signals may be different from those in Figure 8.

I1 is a 5V regulator that provides a constant voltage to D1 to keep the transmitter frequency stable.
I1: 78L05 (or 7805) Others: Circuit box BNC RF output jack 2 x 3.5mm audio input jack Power jack 9-16V power supply Antenna Microphone CD player or record player

assembly

RF circuits are quite sensitive to poor circuit boards (including wiring, grounding, component location, etc.). Breadboards should be avoided; double-sided circuit boards with one side grounded are best, but the design in Figure 4 uses a ground conductor to fill the space around the general traces, so this design works well even with a single-sided circuit board. Components should be placed flat on the circuit board with the shortest wires possible. The transmitter should be installed in a metal shielding box (such as a cast aluminum box), and the metal box is connected to the ground electrode of the circuit. 3mm thick bolts and 5-10mm long support columns can be used to achieve a good connection between the metal box and the circuit board components. Transistors T4 and T6 require heat sink cooling. The heat sink for T4 can be made of a metal tube with an inner diameter slightly smaller than the transistor and 2cm long. Cut a slot in the tube so that the hole can be enlarged and fit over the transistor. The heat sink required for output tube T6 can be made of an L-shaped aluminum strip about 14cm long, 2.5cm wide, and 3mm thick (see Figure 10), or a special 5W heat sink can be used. The hole for fixing T6 should be as accurate as possible; you can make a groove on the heat sink according to the diagram, carefully bend the heat sink outward, and insert the transistor. The elasticity of the heat sink will ensure good contact between the transistor and the heat sink. You can apply some thermal conductive glue, such as silicone oil, between the transistor and the heat sink. Fix the heat sink to the PCB with screws, and clamp two gaskets between the PCB and the heat sink. Note: The tube shell and collector of some RF power tubes are connected (related to the model of the three-stage tube). In this case, the heat sink should be insulated from the ground wire or shielding box (about 5mm away). The pin position of other types of power tubes may be different from Figures 2 and 3. Turn some holes on the box cover to ensure air circulation.

The microphone and CD input interfaces can use 3.5mm headphone sockets, and the power supply can also use a similar socket. For the antenna output, we recommend a BNC socket or the F-type socket used for TV sets (the original product uses an N-type socket). The ground pole of the socket should be connected to the metal shielding box, and the internal wire should be as short as possible. You can embed the D5 in the box cover so that you can often check whether the transmitter is working properly.

Power supply The Veronica 5W transmitter uses a DC power supply of 9 to 16 volts; 12V is better, which will give 5W of power and consume about 900mA (related to the RF power amplifier tube T6). If the power supply is of poor quality, the radio's transmission frequency will be unstable or it will emit a "buzzing" AC sound. If you plan to use batteries or a crude power supply, you should add an additional voltage regulator circuit, such as using 7812 or 7815 instead of D4 (see the top of Figure 1). For the 78XX type voltage regulator circuit, XX is the output voltage, such as 15V for 7815, and the parallel capacitor is greater than 10nF.

The transmitting antenna of an antenna station is particularly important, please refer to the special introduction here.

To make the transmitter work properly and efficiently, some simple debugging is required. When debugging, use an antenna "dummy load" instead of the antenna. It can help you distinguish the main transmission signal from the weak harmonic signal, and ensure that you do not transmit the debugging signal to a large area. The dummy load is made by soldering a 47 or 68 ohm carbon rod resistor (corresponding to the antenna impedance you plan to use) to a BNC or N-type antenna socket; make sure this resistor can withstand the power from the transmitter (5W) and is not a wirewound type. If you can't find a 50 ohm 5W carbon rod resistor (wirewound resistors cannot be used), you can use three 150 ohm 2W resistors or five 250 ohm 1W resistors in parallel, as shown in Figure 2B.

Adjust all the trimmer capacitors to the middle position (the upper plate covers half of the lower one), connect the antenna dummy load to the antenna output socket, and connect a CD player to the CD input socket. At this time, turn on the power, LED D5 should be bright (if not, try adjusting C21), and the transmitter should work at about 98MHz. Use a small screwdriver with an insulated handle to adjust C21, 25 and 26 to make the LED brightest. Then adjust the transmission frequency as follows: slowly adjust C13 (towards the direction of the frequency you want to use) until the LED dims, but not completely off; then adjust C21, 25 and 26 until the LED is brightest again; repeat this until you get the frequency you want. Now use an FM radio to check whether you are transmitting on only one frequency. If not, you may have to adjust again from the beginning. If you cannot tune to the end of the FM broadcast band (88-108MHz), you need to change L1: carefully squeeze the coil to lower the frequency, or increase the spacing of the coils to increase the frequency; and try to ensure that the six coils of L1 are the same, otherwise it will affect the purity of the transmitted signal. According to our test results, the transmission frequency of this circuit may change by 50-70KHz during the process of the transmitter being turned on and the internal temperature stabilizing. Therefore, the adjustment of the transmission frequency will not be accurate until the transmitter temperature stabilizes (about 10-30 minutes).

Now adjust R2 until the sound coming from the CD player is as loud as a regular professional radio station. It should be noted that some radio stations use "compression" to make the sound sound louder than it actually is. If you set it that loud, you may cause over-modulation and interference with nearby channels, which should be avoided. You must also be careful not to set the microphone too loud. It is best to use an external sound mixer with automatic gain control.

After the adjustment is completed, replace the dummy load with the transmitting antenna. In general, the transmitter will work normally, but you can also slightly adjust C21, 25 and 26 and change the length, position and angle of the antenna to achieve the maximum transmitting power, and slightly adjust C13 to make the transmitting frequency accurate. In order to avoid being discovered, when testing the antenna, you can connect the headphone output of an FM radio to the CD input of the transmitter, and use the signal of a local FM radio station as the test signal. Do not try to turn on a transmitter without an antenna load, as that will damage the output transistor; when replacing the dummy load with the transmitting antenna, you must also turn off the power first.