For Assemblying this FM transmitter kit, a beginner will take about 3 hours to tinker the issuer, an electronics hobbyist will have built in about 30 minutes.

FM Transmitter Construction
It is not necessary to drill the transmitter PCB. All components will be soldered to the plate with their legs folded, like this:
The two transistors and the LEDs are polarized:
The transistor has a flat side, the LED a foot longer than the other is the anode (A), the other is the cathode (K). The audio cable (minijack) must be transformed from a stereo cable into a cable.

Mono Sound:
Soldering together the white and red cables, leaving aside the yellow cable (mass). The frequency setting will be turning the variable capacitor gently with a screwdriver or thin cardboard but rigid.

FM Transmitter Parts List

• 1 Ohm resistor 510 (green – brown – brown)
• 100 resistor 1 kOhm (brown – black – yellow)
• 1 MOhm resistors (brown – black – green)
• 1 capacitor 0.1 uF (0.1)
• 1 nF capacitor 47 (0.047)
• 1 capacitor 4.7 pF (479)
• 2 pF capacitors 22 (22)
• 1 variable capacitor 1.5 pF … 15
• 2 transistor BF 246 (F246A)
• 1 red LED
• 1 audio cable (minijack)

Source: Pi-Radio Mini-Shop

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This FM transmitter use a chip made by Maxim Integrated Products, the MAX2606. The VCO (Voltage Controlled Oscillator) in this IC uses a Colpitts oscillator circuit. The variable-capacitance (varicap) diode and feedback capacitors for the tuning have also been integrated on this chip, so that you only need an external inductor to fix the central oscillator frequency.

Download USB FM Transmitter Parts List

The supply voltage to the IC should be between 2.7 and 5.5 V, the current consumption is between 2 and 4 mA. With values like these it seemed a good idea to supply the circuit with power from a USB port. A common-mode choke is connected in series with the USB connections in order to avoid interference between the circuit and the PC supply.

The stereo signal connected to K1 is combined via R1 and R2 and is then passed via volume control P1 to the Tune input of IC1, where it causes the carrier wave to be frequency modulated. Filter R6/C7 is used to restrict the bandwidth of the audio signal. The setting of the frequency (across the whole VHF FM broadcast band) is done with P2, which is connected to the 5 V supply voltage.

The transmitter PCB designed uses resistors and capacitors with 0805 SMD packaging. The size of the board is only 41.2 x 17.9 mm, which is practically dongle-sized. For the aerial an almost straight copper track has been placed at the edge of the board. In practice we achieved a range of about 6 metres (18 feet) with this. There is also room for a 5-way SIL header on the board. Here we find the inputs to the 3.5 mm jack plug, the input to P1 and the supply voltage. The latter permits the circuit to be powered independently from the mains supply, via for example three AA batteries or a Lithium button cell. Inductor L1 in the prototype is a type made by Murata that has a fairly high Q factor: minimum 60 at 100 MHz.

Take care when you solder filter choke L2, since the connections on both sides are very close together. The supply voltage is connected to this, so make sure that you don’t short out the USB supply! Use a resistance meter to check that there is no short between the two supply connectors before connecting the circuit to a USB port on a computer or to the batteries.

P1 has the opposite effect to what you would expect (clockwise reduces the volume), because this made the board layout much easier. The deviation and audio bandwidth varies with the setting of P1. The maximum sensitivity of the audio input is fairly large. With P1 set to its maximum level, a stereo input of 10 mVrms is sufficient for the sound on the radio to remain clear. This also depends on the setting of the VCO. With a higher tuning voltage the input signal may be almost twice as large (see VCO tuning curve in the data sheet). Above that level some audible distortion becomes apparent. If the attenuation can’t be easily set by P1, you can increase the values of R1 and R2 without any problems.

Measurements with an RF analyzer showed that the third harmonic had a strong presence in the transmitted spectrum (about 10 dB below the fundamental frequency). This should really have been much lower. With a low-impedance source connected to both inputs the bandwidth varies from 13.1 kHz (P1 at maximum) to 57 kHz (with the wiper of P1 set to 1/10).

In this circuit the pre-emphasis of the input is missing. Radios in Europe have a built-in de-emphasis network of 50 µs (75 µs in the US). The sound from the radio will therefore sound noticeably muffled. To correct this, and also to stop a stereo receiver from mistakenly reacting to a 19 kHz component in the audio signal, an enhancement circuit is published elsewhere in this issue (Pre-emphasis for FM Transmitter, also with a PCB). Author: Mathieu Coustans, Elektor Magazine, 2009

Source: USB FM Transmitter for MP3 Player

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This FM transmitter design is a result of many hours of testing and tweaking. The goal was simple; to test many existing BA1404 transmitter designs, compare their performance, identify weaknesses and come up with a new BA1404 FM transmitter design that improves sound quality, has very good frequency stability, maximizes transmitter’s range, and is fairly simple for everyone to build. We are happy to announce that this goal and expectations have been met and even exceeded.

The transmitter can work from a single 1.5V cell battery and provide excellent crystal clear stereo sound. It can also be supplied from two 1.5V battery cells to provide the maximum range.

One of the qualities of BA1404 Stereo FM transmitter is excellent frequency stability. This is mainly due to a use of high quality 3.5 turn variable coil. Tunable RF coils are ideal for precise frequency tuning because their magnet wire is halfway embedded within the plastic, which minimizes frequency drifts. Regular air coils are not preferred for professional broadcasting because the coil expands and contracts with temperature changes. That’s the very reason why variable coil was chosen as a substitution for an air coil and a variable capacitor.

Another quality of the presented BA1404 stereo fm transmitter is a crystal clear stereo sound and improved sound separation. There are several factors that account for improved sound quality and a separation. First reason is the use of 38 KHz crystal which provides rock solid frequency for stereo encoder. Another reason is the use of two 1nF decoupling capacitors one for BA1404 chip and another for 3.5 variable coil. These capacitors have to be as close as possible to a BA1404 chip and a variable coil because this will GREATLY improve the sound quality, sound separation and even frequency stability as well. What they do is filter out the noise in the incoming DC voltage. If the noise enters BA1404 chip stereo generator will include it in a transmitted sound affecting both the sound and multiplex signal that is responsible for generation of the clear stereo signal. If that noise enters it will also be included in a generation of subcarrier frequency affecting the frequency stability. Most people are not aware of how important this is and might place them in a wrong location, away from the target components which provides no use, or worse decide not to use these capacitors at all.

Another factor that is extremely important and which improves overall quality of the whole BA1404 transmitter including frequency stability, sound quality and sound separation is the use of the ground plane on the transmitter’s PCB. It is recommended that ground plane should always be used in circuits that deal with higher frequencies.

Source: BA1404 HI-FI Stereo FM Transmitter

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Features

• Power supply: 8-15 V stabilized, 40 mA
• Frequency range: 82,5-108 MHz
• Step frequency: 100 kHz
• RF input voltage range: 10-500 mV
• RF input impedance: 135 ohm

PLL Synthesizer Schematic Diagram

Pin meaning:
1 – Tuning voltage (output)
3 – RF signal from oscillator (input)
2, 4 – Ground
Tl1 – Reset

Download : Parts Lists – PLL Frequency DIP Switch – User Friendly Program

Software for the IC2:
pll16f84.asm – pll16f84.hex. Oscillator type: RC, watchdog: ON.

Instructions
The Tl1 button resets the unit. Press it after frequency set. The unit provides a reset on power-up, so you don’t need to use the button.
The D1 LED indicates the tuning cycle is done (after one second from reset). It’s not needed to use it and the R6 resistor.
The R8 resistor provides a minimal voltage of about 2 V on the PLL output. Use this resistor if the transmitter’s oscillator doesn’t work if the tunning voltage is below this value (mainly after power-up). Place the resistor over the R3.

Notes
Output of the PLL should not be directly loaded with big capacities (over about 0,3 uF). The PLL loop may be unstable. This applies to some unknown transmitters.

Printed Circuit Board (PCB)

Source : PLL-Digital Tuning

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Background
Today it is very popular to connect the Ipod, MP3 or CD player to some kind of FM transmitter. The transmitting signal can the be picked up by any FM radio receiver. Most often used in cars. The problem with most FM transmitter is that they have very weak signal and short transmitting range. Some units are so bad that even when they are placed really close to the receiver, you barely receive the signal.

Ipod FM Transmitter Amplifier Schematic
The schematic show you a RF amplifier with very high gain. The feeding RF signal enter C9 to transistor Q1 which has a self biased working point. The gain and working point is set with the two resistors R1 and R2.

FB1, C5, C6 works as filter for rejecting RF to power line. Q1 has a gain about 15dBm. The output signal can be found a the collector which then enter a second amplifier stage Q2. This stage also has a self biased working point.

The gain is set by the resistors R3//R4 and R5//R6.
Why do I have 2 parallel resistors like that?

It is because I want to be able to change the gain of the amplifier. On the PCB below you will see that I only have 2 pads for the resistors. When I want to resistors I solder the two resistors R5 and R6 on top of each other and the same with R3 and R4.

I advice you to start building without R3 and R5 and test the unit. If you want you can then add R3 and R5 later to obtain max gain of this stage. Q2 has a gain of 12 dBm. FB2, C7, C8 works as filter for rejecting RF to power line.

The last amplifier stage is based around the transistor 2N3866. This transistor has low input impedance.
I match it by using 2 capacitors (C11, C12) and the inductor L1 to about 50 ohm. The transistor has an output impedance match, (C13, C14, and L3) to get best performance for an 50-75 ohm antenna.

# The inductor L1 is made by a wire 2 turns with 5mm diameter.
# The inductor L2 is made by a wire 7-9 turns with 6.5mm diameter.
# The inductor L3 is made by a wire 4 turns with 6.5mm diameter.

Source: Ipod Stereo FM transmitter with 1W output power

Read more: Mini FM Transmitter