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This MP3 Player FM transmitter can be used to listen to your own music throughout your home. The transmitter circuit use no coils that have to be wound. 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. 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.


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


MP3 FM Transmitter Parts List
Resistors (all SMD 0805)
R1,R2 = 22kΩ
R3 = 4kΩ7
R4,R5 = 1kΩ
R6 = 270Ω
P1 = 10kΩ preset, SMD (TS53YJ103MR10 Vishay Sfernice, Farnell # 1557933)
P2 = 100kΩ preset, SMD(TS53YJ104MR10 Vishay Sfernice, Farnell # 1557934)
Capacitors (all SMD 0805)
C1,C2,C5 = 4μF7 10V
C3,C8 = 100nF
C4,C7 = 2nF2
C6 = 470nF
Inductors
L1 = 390nF, SMD 1206 (LQH31HNR39K03L Murata, Farnell # 1515418)
L2 = 2200Ω @ 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)


Notice. The use of a VHF FM transmitter, even a low power device like the one described here, is subject to radio regulations and may not be legal in all countries.

Related Posts : fm, for, mp3, player, transmitter, usb