FM Transmitter

USB Powered Wireless FM Transmitter Circuit

Here’s a Wireless FM transmitter circuit powered from USB ports that could be used to play audio files on a standard VHF FM radio. The transmitter circuit use no coils that have to be wound. This FM transmitter can be used to listen to your own music throughout your home. When this FM transmitter used in the car, there is no need for a separate input to the car stereo to play back the music files from your MP3 player.

This FM transmitter use a chip made by Maxim Integrated Products, the MAX2606 [1]. This IC from the MAX2605-MAX2609 series has been specifically designed for low-noise RF applications with a fixed frequency. 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.

Transmitter Schematic

It is possible to fine-tune the frequency by varying the voltage to the varicap. Not much is demanded of the inductor, a type with a relatively low Q factor (35 to 40) is sufficient according to Maxim. 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. There is not much else to the circuit. 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.

Transmitter PCB

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 ?s (75 ?s 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

MP3 FM Transmitter Parts List
Resistors (all SMD 0805)
R1,R2 = 22k?
R3 = 4k?7
R4,R5 = 1k?
R6 = 270?
P1 = 10k? preset, SMD (TS53YJ103MR10 Vishay Sfernice, Farnell # 1557933)
P2 = 100k? preset, SMD(TS53YJ104MR10 Vishay Sfernice, Farnell # 1557934)
Capacitors (all SMD 0805)
C1,C2,C5 = 4?F7 10V
C3,C8 = 100nF
C4,C7 = 2nF2
C6 = 470nF
Inductors
L1 = 390nF, SMD 1206 (LQH31HNR39K03L Murata, Farnell # 1515418)
L2 = 2200? @ 100MHz, SMD, common-mode choke, 1206 type(DLW31SN222SQ2L Murata, Farnell #1515599)
Semiconductors
IC1 = MAX2606EUT+, SMD SOT23-6 (Maxim Integrated Products)
Miscellaneous
K1 = 3.5mm stereo audio jack SMD (SJ1-3513-SMT
CUI Inc, DIGI-Key # CP1-3513SJCT-ND)
K2 = 5-pin header (only required in combination with 090305-I pre-emphasis circuit)
K3 = USB connector type A, SMD (2410 07 Lumberg, Farnell # 1308875)

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See more:  FM Transmitter Antenna

Source: FM Transmitter for MP3 Player Powered from USB

RF Wireless FM

RF Wireless FM Transmitter

Here’s a rf wireless FM transmitter can be built with simple, affordable and widely available parts. Construction is fun and much can be learned although performance is modest; for example, your voice gets difficult to hear at distances greater than 25 feet.

This FM transmitter is far from perfect offering only modest performance. First, tuning the transmitter can be frustrating. Even slight turns in the variable capacitor can result in large frequency changes. Second, transmitter tuning often resulted in a harmonic frequency. Instead of the intended 108 MHz for example, capacitor tuning yielded a 216 MHz transmitter frequency. In addition to hearing your voice one could slightly hear radio station broadcasts.

If performance is modest, why would I build this transmitter?

One answer is that much can be learned and this tutorial is is appendixed with the underlying mathematics to calculate parameters like (1) transmitter frequency, power output and range (2) antenna length and (3) required coil winding. Often on the web, one just finds a schematic. By adding the analysis (with high school level math), one can conceive improvements on transmitter performance.

RF Wireless Circuit

Construction
A combination of wire wrapping and soldering was used to construct the FM transmitter. Jameco’s prototyping card provides enough room for (non-critical) part placement. You should try to keep all parts close together and keep wire leads short.

RF Wireless-Inductor

RF Wireless-Bottom

RF Wireless FM Transmitter Operation
First, use a battery-powered pocket radio as a receiver. AC powered boom-boxes and home stereos (110 or 220 V) are not recommended; battery-powered radios are much better at receiving transmissions than AC-powered units.

1. Tune your radio to dead air, i.e. frequencies within the FM radio band that are silent or only have some hiss. Frequencies near 108 MHz are typically dead air. The Radio-Locator web page lists local radio stations in your area. This can help you identify dead air frequencies.
2. Turn on your RF wirelessFM transmitter, extend its antenna and keep the transmitter approximately 2 feet away from your FM radio. Speak into the mic while slowly adjusting the variable cap. Use your fingernail or non-metallic screwdriver until you hear yourself over the radio. This process is frustratingly tedious, requiring careful capacitor tuning. You are tuned once you hear howling (also known as a hot mic) which indicates transmitter-receiver feedback.
3. Increase the transmitter-to-radio distance. Congratulations – you have a rf wireless microphone!

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See more : Wireless FM Audio Transmitter

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BA1404 FM Transmitter

FM Transmitter for MP3 Player

Do you have an idea to create your own radio station, transmit the music around the house, or simply create a wireless link between your iPod and a receiver in your car, this transmitter will let you do these things easily. With BA1404 HI-FI Stereo transmitter you will be able to transmit MP3 music from your iPod, computer, discman, walkman, and many other audio sources. 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 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.

FM Transmitter Schematic

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.

FM Transmitter Printed Circuit Board
This a suggested high-resolution PCB layout for BA1404 Transmitter. It is ready for printing and no further adjustments are necessary. Dimensions of the PCB should be 57 mm x 35 mm (W x H).

FM Transmitter PCB

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Source: BA1404 Stereo FM Transmitter