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

iTrip FM Transmitter

iTrip FM Transmitter Review

The iTrip FM transmitter for the iPod can play your music through any FM radio in your car, at a party, wherever the mood strikes you–and you have a radio. It’s clear that the iTrip is made specifically for the iPod. This gives iTrip advantages over similar devices. For example, with the iTrip, you can have the cleanest possible signal–because you can choose any radio station on the dial to tune for the best performance possible. You do this by ‘playing’ special station codes directly from the iPod itself.

Another advantage of the iTrip is that it needs no batteries–it receives its tiny amount of power from the iPod. The original iTrip can even rotate out of the way to charge the iPod while still in use. No more batteries ever again. There’s not even a power switch – just plug it in and go. It shuts off automatically after 60 seconds of silence–just like the iPod. The iTrip is the ultimate accessory for the iPod because it allows you to share the music and share the fun beyond your headphones. Don’t leave home without it.

The next step forward for the FM transmitter that started it all.

ITrip Accesories

iTrip broadcasts the music from your iPod to an open FM frequency so you can listen to your tunes through the nearest FM radio. Use it with car radios, boomboxes, or your home or office entertainment system.

Griffin design and innovation makes iTrip super-easy to set up and use. This design update for iTrip features a new, brighter high-contrast display, easier to read in any light. Griffin-exclusive SmartSound technology optimizes the FM signal sent to your receiver, for the best-quality sound possible.

You get to listen to your music, podcasts, soundtracks, etc., through your own, great-sounding speakers, and share the sound with the whole room (or car).

The iPod is loaded with impressive features, but it’s also an Apple product, which means that looks matter. A lot. And Griffin clearly recognizes this, because the company has released several iTrips designed to match specific iPod models, and each is among the sleekest FM transmitters on the market. Plus, the device measures less than an inch tall and exactly as wide as the iPod it sits on, so it takes up almost no extra space–an important feature to people who value the iPod’s portability.

The iTrip is designed to broadcast to any empty FM frequency on your dial–depending on where you receive the best reception–but it’s only configured to play at 87.9 MHz right out of the box. That means a little setup is required using the included CD-ROM and iTunes. Fortunately, the installation is painless and takes only a few minutes. The installer adds a playlist to your iPod (through iTunes) called iTrip Stations, which consists of a series of short audio files that correspond with all available FM frequencies.

iTrip- FM Receiver

Pros

* Makes iPod music library accessible to anyone with an FM radio
* Simple, easy-to-follow controls using iPod click wheel
* Sound quality is solid, though not terrific

Cons

* Sometimes requires frequent retuning to find best FM frequency
* Hard to find good frequency in crowded urban areas
* iTrip audio files emit unpleasant noise when chosen for shuffle play, though problem is fixable

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See more: Remote Control  Radio FM Transmitter

Website : Griffin Technology

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

Source’s more info : Wireless FM Transmitter

Wireless audio Transmitter

Wireless FM Audio Transmitter Circuit

Here’s is a simple wireless audio transmitter for rebroadcast audio of fm radio or any others audio source. The transmitter circuit consists of a frequency modulated oscillator, an audio preamplifier with pre emphasis to supply the frequency modulating signal, and a buffer amplifier to drive the antenna connector.

Wireless Audio FM Transmitter Circuit

Wireless Audio Transmitter Schematic

Oscillator’s frequency is determined by L1 resonating with the 10 pf capacitor and the total capacitance across it. The collector-base capacitance of the transistors Q3, Q4, and Q5 is a function of their revers bias. This is basically a poor man’s (or lazy man’s) varactor. The voltage across Q3 is set by a voltage divider and is then modulated by an AC coupled audio signal from the pre amplifier, causing the reverse bias to vary with the audio signal, which changes the resonant frequency of L1’s circuit, causing the frequency of the oscillator to vary with the audio signal.

The capacitance of Q4 and Q5 is adjusted by DC bias from the tuning adjustment potentiometer, and this capacitance sets the center frequency of the oscillator.

All of the transistors in the oscillator -Q1 through Q5, are 2N4401.

The purpose of the buffer amplifier is to minimize frequency shift as loading on the antenna is changed. It was specifically designed to reduce the signal amplitude to the antenna. Transmitters should not use any more power than is necessary to achieve the task at hand, and lightly coupling the RF into the buffer’s base with a gimmick capacitor did the trick. The transistor is an MPSH34.

The audio pre amplifier that drives the frequency modulation state is a single inverting transistor. The open loop gain of Q7 is about 150 (mostly set by the voltage across the collector resistor) and the closed loop gain of the stage is about 20 (set by the ratio of the 100k feedback resistor to the 4.7k input resistor). A pre emphasis network is flat from about 3 Hz to 350 Hz, then the response increases at 6 db/octave until it levels off around 3 kHz. This particular network was selected by me to make my CDs sound good on the portable radio I was using at the time. It doesn’t conform to either the British or American FM radio preemphasis curve.

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See more: UHF Audio Video Sender Circuit

Source: FM Broadcast Audio Transmitter

Audio Video Circuit Board

UHF Wireless Audio Video Sender Circuit

This wireless circuit provides you with wireless audio and visual transmission to a TV. The TV acts as a receiver, eliminating the need to buy a separate monitor. You can also hook it up to a VCR or CCD Camera, and even set up a remote CCTV security system!

AV Sender Circuit Schematic

Audio Video Sender Circuit Description

Q3, VC1, C13, C16 and L3 all make up a colpitts oscillator circuit that fluctuates form 220~250 MHz. You can regulate the frequency to any value within this threshold by tuning VC1 or L3. C13 modulates the signal rate. When the capacitance increases, so does the modulation. R9 and C16 bias the local oscillation. If you lower R9’s frequency to 680W the oscillator’s output level will increase.

AV Sender Block Diagram

Q2 and L2 act as a frequency doubler. C7, along with FCZ7S3R5 (IF transformer), the Q4 transistor, C14, C19 and R12 all make up the mixer. This mixer takes both audio and visual signals together and “mix” them into one and passes through RF Amplifier Q1 to transmit the signal to the antenna.

How This Wireless Audio Video Sender Works

1. Turning the blue component’s trimmer on VC1 varies the frequency. When we turn the trimmer, the television’s channel has to be changed accordingly. It is easier to tune the A/V Sender if you have a spectrum analyzer to help you find the correct frequencies. If the frequency is tuned to 474 MHz then this would be the equivalent of your TV’s channel 14 UHF band.
2. The IF transformer is used to synchronize the audio and video frequency’s level radio. If the TV’s image is too blurry then you can adjust the IFT to fine-tune the image.
3. SVR1 controls the video signal input ratio, while SVR2 controls the audio portion. You can tune these components according to your needs.

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Source:  Audio Video Sender