Discrete components Veronica 5W FM transmitter production

Statement: This circuit is for lovers reference, if you need hands-on production experiments, please contact the local radio management approval. Site requirements for radio experiments we must obey the law, any illegal site is not responsible!

Veronica FM transmitter is easy to produce, stable, clean signal, not using professional parts and IC, and a secondary test function in the absence of professional equipment to make your case easy to debug. It has two versions, 1 watt and 5 watts. 1 watt version for 3 km transmission range, the required power is 12-16V 200mA; 5-watt transmitter version for 8 km distance, the required power is 12-16V 900mA. This article describes the 5-watt version.

FM transmitter

Figure 1: 5W Veronica roadmap

The transmitter comes with a mixer, allowing you to simultaneously transmit the audio from a CD and microphone signals. Transistor T1 is the microphone amplifier, variable resistors R1 and R2 to adjust the volume size (see section Debugging). Between R8 and C21 in the oscillator is to produce radio frequency signal components. Diode D1 is a so-called “Varactor”, the equivalent of an adjustable capacitor, which consists of the audio signal control, change the oscillator frequency, frequency play a role. C12, C13, and L1 determine the oscillator frequency. This oscillator is actually two inverted oscillators, each running at 50MHz in the vicinity, when the two signals combine, they become a 100MHz signal. This circuit is much more stable than a single 100MHz oscillator. Oscillator signal by the T4, T6 zoom to 5W. T4 on the right in the circuit including the antenna impedance matching and low pass filtering. D2, D3, T5 auxiliary circuit is composed of debugging, it will sample the RF output signal to control the light emitting diode D5, the output is high, D5 is also brighter.

The circuit itself without stereo modulator, if you need to play back a stereo program, please refer to here making stereo modulator.

Parts list


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


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

Hollow type with no skeleton. 1mm diameter wire to close around the cartridge or other rod, and then carefully stretched to the correct length, and determine the two ends of the coil shown in Figure 2.
FM transmitter

Figure 2A: coil right around the law
FM transmitter

Figure 2B: L4, MRF237 pins and antenna dummy load

L1 6 coils, each 2 turns of diameter 5mm, length 5mm L2 3 turns, inner diameter 7mm, length 7mm L3 3 turns, inner diameter 6mm, length 8mm L4 in the 2.2k resistor carbon rod (diameter 2mm) Shangrao turns 14 The 0.2mm diameter wire, the end of the wire connector on the resistance welding. Resistance of the two sets of connectors on a bead, as shown in Figure 2B. L5 5 turns, inner diameter 6mm, length 11mm L6 4 turns, the inside diameter of 6mm, 9mm long

RF Chokes (RF choke):

Choke (H1-4) available in diameter enameled wire diameter of 0.5mm 4mm, 5mm long beads Shangrao system. Note that the wire from the hole through the bead, beads should be used in the 100MHz frequency material (usually 43). If you can not find the beads, can also be used method of production: in the 33k resistor Shangrao length 0.5m carbon rod diameter 0.2mm enameled wire, resistance welded to the end of the wire connectors.

H2 H1 bead bead Shangrao Shangrao 5 turns 1 turn 2 turn H4 H3 magnetic beads Shangrao Shangrao 3 turns

Diodes: D1 is best to use varactor pairs, ie two symmetrical varactor together back to back, the middle is negative; but this is not very important, two general varactor can.
D1 KV1310 D2 +3 1N4148 D4 D5 1N4001 diodes shine general

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

Note: Other power control signal pin location may be different from Figure 8.
FM transmitter

Figure 3: Top view of three tubes pin regulator:

I1 is a 5 volt regulator to D1 to provide a constant voltage to maintain the transmitter frequency stability.
I1: 78L05 (or 7805) Other: BNC RF output circuit box jack 2 x 3.5mm audio input jack power jack 9-16V power antenna CD player or tape recorder microphone


Veronica transmitter used in printed circuit board (PCB) as shown in Figure 4.
FM transmitter
Poor RF circuit board (including wiring, grounding, component location, etc.) is very sensitive. Should avoid the use of bread board; use of double-sided circuit board side of the ground the best, but 4 was designed with a general grounding conductor traces filled the surrounding air when such a design that uses single-sided circuit board was also very effective. Components should use the shortest possible wire placed flat on the board. Transmitter should be mounted in a metal shielded box (such as cast aluminum box), while the metal box to connect the circuit of the earth. 3mm thick bolts can be used with 5-10mm long support columns, to achieve the metal box on the circuit board pieces of good connections. Transistors T4, T6 radiator cooling needs. T4 radiator can be slightly smaller diameter than the transistor, 2cm long metal tube to do. Cut a slot in the tube, so that holes can be larger and cover the transistor. Output tube T6 needs a radiator can be about 14cm long, 2.5cm wide, 3mm thick L-shaped aluminum production (see Figure 10), can also be a special 5W radiator. T6 holes should be fixed as accurately as possible; in accordance with the icon you can open a slot on the radiator, carefully turn out some of the heat sink, insert the transistor, the transistor heat sink and radiator flexibility will ensure good contact. In the middle of the transistor and heat sink can be painted a number of thermal plastic, such as silicone oil. Heat sink screwed to the PCB and the PCB sandwiched between two gaskets and radiator. Note: Some RF power tube and shell collector is connected (with three tube model), in this case, the radiator and the ground should be insulated or shielded box (about 5mm from the distance). Other types of power control pin location may Figures 2 and 3 are different. Turn on some holes in the lid to ensure air circulation.

Microphone and CD inputs are available in 3.5mm headphone jack, power can also use a similar outlet. For the antenna output, we recommend use BNC sockets, or the kind of F-type TV socket (formerly with the N-type socket). Socket of the earth should be connected with the metal shield case, and the internal wires should be as short as possible. D5 can be embedded in the lid, so you can always check the transmitter is working properly.
FM transmitter
Figure 5: Component Assembly Location

Veronica 5W transmitter power used by a 9-16 volt DC power supply; better with 12V, 5W will be the power consumption of about 900mA (with the RF power amplifier tube T6). If the poor quality of power supply, radio transmitter frequency instability may launch a “buzzing” hum. If you intend to use battery power or poor, should add an additional voltage regulator circuit, such as 7812 or 7815 instead of using D4 (see Figure 1 above). Regulator circuit for 78XX, XX is the output voltage, such as 7815 for the 15V, the capacitor in parallel can be greater than 10nF.

Antenna radio transmitting antenna is particularly important, see here devoted.

In order to properly debug transmitter work efficiently, you need to do some simple debugging. Debugging antenna with a “dummy load” instead of the antenna, it can help you distinguish between the main transmitter signal and the weak harmonic signals, while ensuring that you do not debug a wide range of the signal to launch out. Pre-load production methods are: a carbon rod 47 or 68 ohm resistor (and you intend to use the antenna impedance corresponding to) soldered to a BNC or N-type antenna socket; determine resistance to withstand the power from the transmitter (5W), and not wound type. If you can not find a carbon rod 50 ohm 5W resistor (not wire wound resistor) can be used three 150 ohm 2W resistor or five parallel 250 ohm 1W resistor, as shown in Figure 2B.

All the tuning capacitor to the middle position (the upper half of the Department of board stay covered), the antenna receiving the dummy load antenna output jack, CD player will receive a CD input jack. Then boot, light-emitting diode D5 is bright (if not, try to adjust C21), and the transmitter should be operating in 98MHz or so. Insulated with a small screwdriver to adjust the C21, 25 and 26, the light-emitting diodes to achieve the best light. Then adjust the transmission frequency as follows: slowly adjust C13 (close to you towards the direction of the frequency to be used) until the LED dim, but not completely exterminate; then adjust C21, 25 and 26 until the LED to the brightest; Repeat this until you get the frequency you want. Now with an FM radio to check you in only one frequency transmit signal, if not, you may have to re-adjust from scratch. If you are not transferred to the FM radio band (88-108MHz) at the end, you need to change the L1: carefully to reduce the frequency of coil compaction, or increase the spacing to increase the frequency of the coil; as much as possible to ensure the six coil L1 is the same, otherwise it will affect the purity of the transmitted signal. According to our results, the transmission frequency of the circuit in the transmitter power to a stable internal temperature may change during the 50-70KHz, therefore, adjust the transmit frequency transmitter to wait until the temperature stabilized (takes about 10-30 minutes) to accurate.

Now adjust R2 until the launch from the CD player sound like a professional radio station as big as normal. Should be noted that some stations use “compression” technology to achieve the sound sounds better than it actually sounds great effect, if you set a big voice, you may cause over-modulation and interference to a nearby channel, which should be avoided . You must also be careful not to set the microphone too loud, preferably with automatic gain control with an external audio mixer.

After adjustment, the false load into the transmitting antenna, in general, the transmitter will work, but can also be minor adjustments to C21, 25 and 26 and change the antenna length, position, angle in order to achieve maximum transmit power, the small size adjust the transmission frequency accuracy C13. To avoid detection, the test is available when an FM radio antenna, headphone output to the transmitter of the CD input, an FM radio station with a local signal for the test signal. Do not try to open a load of the transmitter antenna is not connected, that would damage the output transistor; will leave when the load into the transmitting antenna should first turn off the power

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1 Response

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