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

FM Receiver

One Transistor FM Radio Receiver Circuit

Here’s simple FM receiver circuit for a simple superregenerative FM radio. It is sensitive, selective, and has enough audio drive for an earphone. These designs generally have low component counts, however the design or my construction have been far from simple.

FM Receiver Schematic

FM Radio Receiver Circuit Layout
Because this is a superregenerative design, component layout can be very important. The tuning capacitor, C3, has three leads. Only the outer two leads are used; the middle lead of C3 is not connected. Arrange L1 fairly close to C3, but keep it away from where your hand will be. If your hand is too close to L1 while you tune the radio, it will make tuning very difficult.

Winding L1
L1 sets the frequency of the radio, acts as the antenna, and is the primary adjustment for super-regeneration. Although it has many important jobs, it is easy to construct. Get any cylindrical object that is just under 1/2 inch (13 mm) in diameter. I used a thick pencil from my son’s grade school class, but a magic marker or large drill bit work just fine. #20 bare solid wire works the best, but any wire that holds its shape will do. Wind 6 turns tightly, side-by-side, on the cylinder, then slip the wire off. Spread the windings apart from each other so the whole coil is just under an inch (2.5 cm) long. Find the midpoint and solder a small wire for C2 there. Mount the ends of the wire on your circuit board keeping some clearance between the coil and the circuit board.

A tuning knob for C3

C3 does not come with a knob and I have not found a source. A knob is important to keep your hand away from the capacitor and coil when you tune in stations. The solution is to use a #4 nylon screw. Twist the nylon screw into the threads of the C3 tuning handle. The #4 screw is the wrong thread pitch and will jam (bind) in the threads. This is what you want to happen. Tighten the screw just enough so it stays put as you tune the capacitor. The resulting arrangement works quite well.

FM Radio Receiver Circuit Adjustment
If the radio is wired correctly, there are three possible things you can hear when you turn it on: 1) a radio station, 2) a rushing noise, 3) a squeal, and 4) nothing. If you got a radio station, you are in good shape. Use another FM radio to see where you are on the FM band. You can change the tuning range of C3 by squeezing L1 or change C1. If you hear a rushing noise, you will probably be able to tune in a station.

Try the tuning control and see what you get. If you hear a squeal or hear nothing, then the circuit is oscillating too little or too much. Try spreading or compressing L1. Double check your connections. If you don’t make any progress, then you need to change R4. Replace R4 with a 20K or larger potentiometer (up to 50K). A trimmer potentiometer is best. Adjust R4 until you can reliably tune in stations. Once the circuit is working, you can remove the potentiometer, measure its value, and replace it with a fixed resistor. Some people might want to build the set from the start with a trimmer potentiometer in place (e.g., Mouser 569-72PM-25K).

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Source: Build A One Transistor FM Radio

Emergency Radio

Emergency Crystal Radio

Crystal radio requires no power supply to operate because it uses energy from the signals it receives. Performance is – like one should expect – poor. Nevertheless, with a good antenna and proper ground, it shouldn’t be too difficult to tune to a strong AM station.

Crystal Radio

While this emergency crystal radio circuit is very simple, many points should be observed. The variable capacitor is relatively hard to get in a store these days, so your best bet to find one could be a junked AM receiver. Connect the case to the antenna and the central rod to ground. Otherwise, you’ll de-tune the circuit every time you touch the capacitor.

Next comes the coil. Originally, 60-80 turn air core coils of large diameter (10cm) were used. A less bulky alternative could be 30 turns of enamel-coated magnet wire wound on a ferrite rod. It works for me, at least.

Detection and demodulation of the signal is done by a germanium diode. In the US, 1N34 seems to be very popular. Here in Europe, AA119 is more common and offers similar performance.

Finally, you’ll need high-impedance headphones. These are also quite rare. Ordinary Walkman phones won’t work directly. Add an audio output transformer to drive them or build a small amplifier like shown on the next page.

Problems:
To hear anything, you’ll need a very long antenna. Use any hookup wire at least a few meters in length. Make sure that no power lines are nearby! A good ground is also vital, like a cold water pipe. Otherwise, plant a metal rod as deeply in the ground as possible and it should work well.

Possible uses:
In these times of incredible advance in technology, such a simplistic receiver doesn’t seem to have any real use at all. It’s a nice project for the more nostalgic types, though. Since no power is required for operation, having such a receiver could prove handy in an emergency situation or just to hear the news when you’re out in the woods backpacking.

Amplified Crystal Radio
An improved crystal radio with a two-stage audio amplifier. The 2N3904 (or any other general-purpose NPN) acts as a preamplifier while the LM386 boosts the signal high enough to drive low impedance Walkman headphones or perhaps even a small speaker.

Crystal Radio Amplifier

Problems:
The same rules that we’ve already been through at the previous circuit above.

Possible uses:
Make a real AM receiver! With a decent antenna and good ground you should be able to catch a station or two. You might also experiment with different coils for other band. A 12-turn air core coil 5mm in diameter should get you somewhere between 5 and 15 Mhz, for example.

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Source : Crystal Radio

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

LCD FM Transmitter

Radio Transmitter for FM Broadcast

Here’s easy to build high-quality PLL Radio Transmitter for FM broadcast, with typical output power of 5 W and no-tune design. The transmitter includes RDS/SCA input and Audio/MPX input with optional preemphasis. It can be used with or without stereo encoder. Tuning over the FM band is provided by two buttons that control dual-speed PLL. The transmitter can work also without the LCD display.

The characteristic of PLL Radio FM Transmitter:
Supply voltage: 11-13.8 V (stabilised or from a battery)
Supply current: up to 1.2 A
Standard frequency range: 87.5-107.9 MHz
Audio/MPX input sensitivity: 2 V pp (for 75 kHz freq. deviation)
RDS/SCA input sensitivity: 0.2 V pp (for 7.5 kHz freq. deviation)
Board dimensions: 109 x 54 mm

PLL Radio FM Transmitter Schematic Diagram

Transmitter Schematic

PLL Radio FM Transmitter Printed Circuit Board

Transmitter Component Layout

Transmitter PCB-Top

PLL Radio FM Transmitter Part list

Q1 – BF240
Q2 – BFG135 (BFG235)
Q3 – 2SC1971 (2N3553) + heatsink
Q4 – BC547B
D1 – SB260 (1N5822, 1N581x)
D2, D3 – BBY40 (BBY31)
D4 – LED 5mm

U1 – 78L09
U2 – TSA5511 (TSA5512, SDA3202)
U3 – PIC16F627A (programmed)
U4 – 78L05

R1, R2, R11, R17, R20 – 10k
R3, R21 – 270R
R4, R15 – 33k
R5, R7, R12, R13, R16 – 680R
R6, R14 – 18k
R8 – 47R (33R if Q2 is BFG235)
R9 – 18R
R10 – 4k7
R18 – 3k3
R19 – 100k smd 1206
R22 – 91R
R23 – trimmer 5k mini

C1, C4, C9, C12, C13, C14, C15, C30, C31, C32, C33, C35 – 10n smd 1206 (C)
C2, C17, C20 – 15p (C)
C3 – 10p (C) (15p if the PCB is single-sided)
C5 – 1n (C)
C6, C28, C29, C34 – 100u/10V (E)
C7, C26 – 10u/35V (E)
C8 – 22p (C)
C10 – 47p (C) (33p if Q3 is 2N3553)
C11, C27 – 100n (C)
C16, C36 – 33p (C)
C18 – trimmer 50p
C19 – 470u/16V (E)
C21 – 4u7/50V (E)
C22 – 330p (C)
C23, C24 – 47p (C)
C25 – 3n3 (P)

L1 – 3.5 turns on 7 mm diameter
L2 – 1uH/815mA choke, or about 10 turns of thin wire on mini ferrite core
L3 – 2.5 turns on 6 mm diameter (4.5 turns if Q3 is 2N3553)
L4, L5 – 3.5 turns on 6 mm diameter

Y1 – crystal 6.4 MHz or 3.2 MHz
TR1 – rf ferrite transformer 2:1 (3:1 if Q3 is 2N3553), see text
SW1, SW2 – button mini
J1, J2, J3 – BNC connector 90 deg.
J4 – power supply connector
J5 – HD44780 LCD standard connector, 2×8 or 2×16 characters
J6, J7 – jumper

Additional information for PLL Radio FM Transmitter

For 0.05 MHz step tuning a 3.2 MHz crystal is required. In other case a 6.4 MHz crystal will make good work (step tuning 0.1 MHz).

Wind all coils (except the L2) by a 0.8 mm wire.

The Y1 package must be tied to ground!

Make sure the Q3 terminals are as short as possible (about 2-3 mm above board). The 2N3553 case/heatsink can’t be tied to ground!

To make the TR1 transformer, use specified number of turns on primary side and one turn on secondary side. The secondary wire should be quite thick but the primary can be as thin as you want. Wind on a 2-hole ferrite (material 61 or N1).

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Source’s more for Pira CZ 5W PLL FM Transmitter

VHF Radio Transmitter

VHF Radio FM Transmitter Circuit

Here’s a VHF Radio FM transmitter. This project is a simple VHF FM transmitter using only one crystal and will cover 145.00 to 146.00 MHz. The crystal is a 44.9333 MHz crystal for 145.500 receive, as used in the Trio (Kenwood) 2200, PYE, Motorolla, Tait equipment, to name but four. The frequency of the crystal is not critical as almost any other xtal for the 2-meter band will function.

VHF Radio Transmitter

No provision has been made to tune the vhf radio transmitter to different channels, as this transmitter was first used as a single channel “repeater box”, leaving my main rig free to be used on other channels. The transmitter circuit is given above and simply mixes the output of a (more or less) conventional receiver multiplier (x3) with the output of a 10.7MHz VFO that is modulated with true FM.

Ordinary 1N4001 diodes will function well as varicap diodes, but if true varicap diodes (such as BA102 etc.) are used you will have to reduce the value of the 18pf capacitor coupling D1/D2 to L1. L1 may be a 10.7MHz IF transformer robbed from a domestic receiver, but remove the internal capacitor. Adjust L1 (10.2 – 11.2 MHz) to cover 145-146 MHz.

The transmitter  modulator is a simple circuit which I will post later. Two OP-Amps were used in the prototypes, the first was a MIC amplifier to bring the MIC AF OP up to 500mV RMS. Clamp the AF with a couple of back-to-back diodes (limiter) then the second OP-Amp amplifies the clipped AF to the correct level, (about 1.5v RMS) for 5KHz deviation. Adjust the gain of the first OP-AMP for MIC GAIN and adjust the gain of the second OP-AMP for deviation (with FULL AF).

Audio Modulator

The output of the transmitter amplifier driver will supply about 10-20mW to the PA. I didn’t use a Power Amplifier because I lived so close to the repeater (path loss = -109dB). There are hundreds of VHF QRP PA’s published in SPRAT, INTERNET, RSGB books, RadCom, and PACKET RADIO so I will leave that to your own ingenuity. A single transistor, such as the 2N3866 will be more than adequate to get up to 250mW, but an additional band-pass tuned circuit should be used between them.

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Source: FM Transmitter

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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