Here's a simple VHF FM transmitter that could be used to play audio files from an MP3 player or computer on a standard VHF FM radio. The 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.
Project image :
USB FM transmitter
To keep the circuit simple as well as compact, it was decided to 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.
t
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.
Circuit diagram:
USB FM transmitter schematics Circuit diagram
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)
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 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.
PCB Layout :
USB FM transmitter Layout PCB
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
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.
Circuit diagram:
Circuit diagram:
