FM Transmitter DIY KitFM Transmitter DIY KitFM Transmitter DIY KitFM Transmitter DIY Kit

"Little Beacon""Little Beacon""Little Beacon""Little Beacon"

http://radiohobbystore.com

SchematicSchematicSchematicSchematic

Components List:Components List:Components List:Components List:

Resistors:Resistors:Resistors:Resistors:

R1 – 5% Carbon Film Resistor 36 Ohm (orange, blue, black, gold)

R2 – 5% Carbon Film Resistor 56 Ohm (green, blue, black, gold)

R3 – 5% Carbon Film Resistor 2.2K Ohm (red, red, red, gold)

R4 – 5% Carbon Film Resistor 100 Ohm (brown, black, brown, gold)

R5, R8 – 5% Carbon Film Resistor 470 Ohm (yellow, violet, brown, gold)

R6 – 5% Carbon Film Resistor 15K Ohm (brown, green, orange, gold))

R7 – 5% Carbon Film Resistor 1K Ohm (brown, black, red, gold)

R9 – 5% Carbon Film Resistor 1 Ohm (brown, black, gold, gold)

R10 – Multiturn Potentiometer 3296W 10K Ohm (103)

Capacitors:Capacitors:Capacitors:Capacitors:

C1 – 5% 1.5nF Mylar Film Capacitor (152)

C2, C10 – Ceramic Disc Capacitor 1.5nF (152)

C3, C6, C7 – Ceramic Disc Capacitor 3.3pF (3.3)

C5 – Ceramic Disc Capacitor 15pF (15)

C11 – Ceramic Disc Capacitor 22pF (22)

C4, C12 – Ceramic Disc Capacitor 100nF (104)

C9, C13 – Ceramic Disc Capacitor 1nF (102)

C8 – Electrolytic Capacitor 10uF 25V

Semiconductors:Semiconductors:Semiconductors:Semiconductors:

T1, T2 – N-Channel RF Transistor J310

VD1 – BB910 VHF Varicap Diode

Other:Other:Other:Other:

Male Header Pin 2.54mm Connectors x10Pcs

L1 – Handmade Inductor RF Coil

w=1mm; t=10; D=5mm; L=12mm

• R10 multiturn potentiometer can be any value from 5K to 50K. R9 value depend on your PLL.

Technical Specifications:Technical Specifications:Technical Specifications:Technical Specifications:

• Supply Voltage: 8-12V Regulated

• Supply Current: 12.5mA at 12V

• PCB Size: 4 x 3.5 cm

• Frequency: 97MHz – 102MHz with default L1 Coil

• Modulation: Wide Band FM

• Range: up to 25 Meters in open field condition

• PLL Compatibility

• PC Line out sound-card Compatibility

• Easy DIY FM Kit for the beginner

• Easy tuning process with variable resistor

Introduction.Introduction.Introduction.Introduction.

This is education purpose DIY module project of FM Transmitter 97-102MHz.

The original circuit of the Voltage Controlled Oscillator (VCO) was taken from the

famous russian radiostation “Mayak” (Beacon). That’s why we called our project

“Little Beacon”. The circuit contain J-Fet based VCO and Buffer Stage. It do not

include RF amplifier stages because in many countries is illegal to broadcast

with RF Power more than several milli-watts. The project was developed ONLY

for education purpose and you have to understand it clearly. Even it have

compatibility with PLL, it still stay amateur DIY device and does not fit into

technical standards of the legal FM Broadcasting.

When you study the circuit and components list you are certainly noticed that

the components are very common. Except varicap diode and j-fet transistor. The

varicap diode is voltage controlled element which acts like variable capacitor.

When you change the bias voltage with R10 potentiometer, the capacitance of

the BB910 diode is changes, hence the frequency of the transmitter is also

changes. With the default L1 coil and 12V power supply the frequency can be

tuned to 97-102MHz, but if you need another frequency's just add or remove one

turn of L1 coil. You can also gently push or compress the L1 to reach the desired

frequency. RF coil winding technology and calibration process will be discussed

later in the manual.

J-Fet Transistors J310 is very sensitive to static electricity. We suggest you to

wear antistatic wrist when you are soldering the kit.

To receive the transmitter you can use a standard FM Radio Receiver 88-

108MHz.

“Little Beacon” DIY FM Kit has compatibility with external PLL. You can connect,

for example, SAA1057 PLL by http://pira.cz or any other 88-108MHz FM PLL

devices. In case you are going to use external PLL then remove R10

potentiometer because it control the bias varactor voltage manually!

If you are planning to reach more RF power for your device, please connect

external RF Amplifier stage to ANT output of the PCB with short coaxial cable.

Dipole antenna is the best and simple technical solution for this DIY FM Kit and

we advice you to build the Dipole if you need more distance and quality

broadcast. But any wire approximately 75-100 cm length will do the work of the

antenna in case you want just test your device. Antenna will be discussed later in

the manual.

The circuit and the PCB do not contain audio volume or modulation control. The

device was developed in compatibility with Personal Computer (PC) Line OUT

level. It mean you need to control volume directly in your software (WinAmp,

Windows Media, AIMP, Foobar2000 or other Audio Player). To improve audio

transmission quality we suggest you to use Stereo Tool WinAmp Plugin. You can

download free version from: http://www.stereotool.com

Stereo Tool include Pre-emphasis, Compressor/Limiter and even Software

Stereo and RDS coding. For more technical information about StereoTool please

visit it website.

Of course, you can use external hardware pre-emphasis, stereocoder or RDS if

you have enough knowledge and experience to build it. But we think for the

beginners it will be easy to use only software audio compressor. StereoTool has

ready to use presets and you can choose which one you like.

As you can notice, there is no decoupling capacitor in audio stage. The

modulation AC voltage from your sound-card enter directly to the anode of the

varactor diode BB910. This method allow to exclude decoupling capacitor and

separate modulation AC from Bias DC voltage of the varicap. However input

impedance of the modulator is low. That's why the bass in sound may lack.

The FM DIY Transmitter kit require Regulated Power Supply. LM317 or LM7812

simple power supply can be used. The PCB has reserved place for LM78L12 IC

installation in case your power supply is noisy.

Assembling and Soldering.Assembling and Soldering.Assembling and Soldering.Assembling and Soldering.

1. Solder all carbon film resistors. Solder 78LXX link, use resistors leads for

creating the link. If you have noisy power supply you can install your own

78L12 voltage regulator.

2. Solder all ceramic capacitors. Install capacitors body close to the pcb.

3. Install and solder male header pins.

4. Solder L1 coil.

5. Solder transistors and varicap diode. Put attention to cathode/anode

direction. You can refer the photo for proper transistors and diode

installation.

6. Solder C1, C8 and R10 potentiometer.

The module is ready!

Testing and Tuning.Testing and Tuning.Testing and Tuning.Testing and Tuning.

Before the first power-up:Before the first power-up:Before the first power-up:Before the first power-up:

Check your soldering for short circuits. Check all parts installation regards PCB

Layout. Trim R10 potentiometer to the middle position. Solder wire 75-100 cm

to ANT pin. Connect 12V regulated power supply to psu pins with proper

polarity. Tune your FM Radio Receiver to 100MHz. If you do not have a digital FM

tuner, just tune it somewhere close to 100MHz position. You should hear the

hissing noise from the receiver. Place the radio receiver in distance about one or

two meters from your antenna.

Tuning the Transmitter:Tuning the Transmitter:Tuning the Transmitter:Tuning the Transmitter:

Power up the transmitter. With small screwdriver trim R10 potentiometer for

frequency of 100MHz. When you are tuned to the desired frequency there will be

complete silent in the receiver, no any hissing sounds! VHF devices is very

sensitive to many factors and you'll need a patience to calibrate your first VCO.

Even your hand over the VCO can change it frequency, so do not be discouraged if

at first time you cannot tune it. After you are tuned to 100MHz please measure

the bias varicap voltage with the multimeter on testing point A.

The voltage should be somewhere 1.5V-9V, but for better quality and modulation

linearity it should be in 2V-6V diapason. If it not, your should trim L1 a little,

gentle push out or compress the turns of the coil. Please do it very gentle,

otherwise the frequency can be changed dramatically. Just keep in mind every

time: VHF devices is very- very sensitive!

If you need different than 100MHz frequency in range 97-102MHz, the tuning

process is the same like was described above. The point is when you are close to

97MHz or 102MHz there will be more actions with the coil to reach 2V-6V on the

varicap.

Other Frequency Tuning:Other Frequency Tuning:Other Frequency Tuning:Other Frequency Tuning:

If you need to tune to any other frequency's than is declared with default L1 coil,

you should make a new coil.

Before processing L1 coil rewind, you should clearly understand that you have

tools and skills for desoldering the previously L1 from the PCB .

If you going down to 88MHz add one turn, and if you are going high to 108MHz

remove one turn of the coil. Maybe just pushing out or compressing the coil

turns will be enough. Try it before re-wind.

My coil winding technology is very simple and I'm going to share it with you. I

use 0.8-1 mm enameled wire, some 5-6mm diameter bolt, knife and solder iron.

1. Cut 20-25 cm of enameled wire and wind it over the bolt former.

2. Cut the coil leads and scrape off the insulation with knife. After that tin the

leads of the coil with a solder. Screw off the coil from the bolt former.

Connecting Audio:Connecting Audio:Connecting Audio:Connecting Audio:

Connect audio signal from your PC to audio pins on the PCB. Before you are

starting to transmit any music please trim the volume less than ¼ of the volume

bar. Start transmit music and adjust volume in the PC regards your FM radio

receiver. Do not overmodulate the transmitter with audio! If the sound start

wheezing lower the volume. In any case “your station” will be quieter than

commercial radio station. Using compressor/limiter can raise the volume and

improve sound quality.

50Hz interference noise (hum):50Hz interference noise (hum):50Hz interference noise (hum):50Hz interference noise (hum):

There is several possible reason for hum.

• Noisy power supply

• Poor screening / long connection cables

• Antenna system

Power Supply Requirements:Power Supply Requirements:Power Supply Requirements:Power Supply Requirements:

Wide range of regulated power supply's can be used to feed the transmitter.

Toroidal power transformers is better than regular power transformer because

toroidal transformer has less radiation.

Your power supply unit have to be able to produce a current at least two times

more than all your project modules needs.

Main filter capacitor with the minimum capacitance of 3300uF. More capacitance

is better but not always possible because of high cost.

Enclosure arrangement advice:Enclosure arrangement advice:Enclosure arrangement advice:Enclosure arrangement advice:

Metal made project box enclosure will mate this project. Connect enclosure body

to Ground – minus of your power supply on filter capacitor. Screening will

improve stability and will reduce the impact of the environment. All connecting

wires and cables inside the case should be as short as it possible. You can twist

supply wires or use shielded cables for the supply and signal connections. The

shield of the cables should be grounded from the two sides of the line. Wrap

power supply line wires on the ferrite toroid for EMI suppression. Solder

RG178U (or similar) coaxial cable to BNC chassis for antenna connection.

One of the possible enclosure arrangement is shown in the picture. This

arrangement will reduce hum and other interferences between supply and signal

lines. BNC and PLL IN lines should be connected with coaxial cables. Audio cable

is mono shielded wire. All other lines is twisted or shielded wires. EMI

suppression toroid diameter can be from 1cm to 2cm. Do several turns around

the toroid with supply wires.

Dipole Antenna:Dipole Antenna:Dipole Antenna:Dipole Antenna:

Using a simple wire or single telescope antenna, instead of the balanced dipole

antenna, can cause 50Hz hum or other interference noises in your receiver.

That’s why we suggest you to build a simple Dipole.

Use two telescopic antennas to create the dipole. You can mount them on a

wooden construction. The upper telescope is soldered to central coaxial wire

and the lower telescope is soldered to coaxial shield. Balun is just 5-6 turns of

the coaxial cable around wooden construction.

Technical support:Technical support:Technical support:Technical support:

support@radiohobbystore.com

Technical support can be given on multi-language: english, russian and hebrew.

Respond time can vary, but usually during 24-36 hours you'll receive the answer.

Copyrights:Copyrights:Copyrights:Copyrights:

You cannot duplicate PCB design for commercial use! If you are going to use

manual text or pictures from this pdf please provide original link to the store.

PCB designer, manufacturer and Kit distributor:

Alex Boguslavsky – RadioHobbyStore ©

http://www.radiohobbystore.com