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. The wireless FM transmitter can be used to listen to your own music throughout your home. When this wireless 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 wireless 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.

USB Wireless FM Transmitter

Wireless FM Transmitter Circuit Diagram

Based on wireless FM transmitter circuit 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 wireless FM transmitter circuit with power from a USB port.

USB Wireless FM Transmitter Schematic

A common-mode choke is connected in series with the USB connections in order to avoid interference between the wireless FM transmitter 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 frequency setting of the wireless FM transmitter (across the whole VHF FM broadcast band) is done with P2, which is connected to the 5 V supply voltage.

Wireless  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)

USB Powered Wireless FM Transmitter PCB

The wireless FM 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 wireless FM transmitter circuit to be powered independently from the mains supply. Inductor L1 in the prototype is a type made by Murata that has a fairly high Q factor: minimum 60 at 100 MHz.

USB Wireless FM Transmitter PCB

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). Author: Mathieu Coustans, Elektor Magazine, 2009 – USB Wireless FM Transmitter.

See also:  FM Antenna Diagram – RF Wireless Circuit

Here’s is a simple audio wireless transmitter for rebroadcast audio of fm radio or any others audio source. The wireles 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 transmitter is easy to build and use components that are widely available in the market. You just need to tune the working frequency after power supply is installed in accordance with the desired frequency on the VHF band II.

Rebroadcast Audio Wireless Transmitter

Audio Wireless Transmitter Circuit Diagram

Oscillator’s frequency of the FM wireless transmitter 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.

Audio Wireless Transmitter Schematic

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. Wireles transmitter 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 of the FM wireless transmitter  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.  Visit FM Audio Wireless Transmitter

See also: Wireless Video Circuit