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     FM Transmitter USB pictureHere is a simple USB FM transmitter that could be used to play audio files from an MP3 player or computer on a standard VHF FM radio by connecting it to an USB port. The circuit use no coils that have to be wound. This USB transmitter can be used to listen to your own music throughout your home.

    USB Transmitter Circuit Schematic

    usb fm transmitter circuit diagram

    To keep the fm transmitter circuit simple as well as compact, it was decided to use a chip made by Maxim Integrated Products, the MAX2606. 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.

    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.

    USB FM transmitter PCB Layout

    usb Transmitter PCB Layout
    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 fm transmitter usb. 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 usb fm transmitter 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).

    usb transmitter components list

    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
    L1 = 390nF, SMD 1206 (LQH31HNR39K03L Murata, Farnell # 1515418)
    L2 = 2200Ω @ 100MHz, SMD, common-mode choke, 1206 type(DLW31SN222SQ2L Murata, Farnell #1515599)
    IC1 = MAX2606EUT+, SMD SOT23-6 (Maxim Integrated Products)
    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)

    Source: http://fmtvguide.blogspot.com/2009/07/mp3-fm-transmitter-circuit.html

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    12 Responses to "USB FM Transmitter Circuit"

    1. l am a hobbist based in mozambique.l want to know if you sell various electronic kits by mail. please if so send me the details of how l can make orders for pcbs and components

    2. how to build a pcb without track, please provide a picture of that, so that some one could make one………….

    3. step1:buy all components
      step2:go electronic shop & give this pics & components

    4. sir hwz u? its me eng owais ahmed from pakistan (peshawar). sir plz text me about a simple circuits which i can construct it. coz i m feeling good if i can do a new somthings. i shall be very thankful to u. my contact number is +923339052571 and +920916010246

    5. i need to built this circuit what is the operating frequency of this ciccuit can i add some software to it

    6. sir can u tell me if this circuit will work in the Himalayas.. eg. Mt.Everest????

    7. For @Krishna…. Go & build with imaginary components & PCB that will be the best…. Do imagine testing it & post d result…. Thank u!

    8. I want to know where can i get all the components required at a cheap price. please reply

    9. Hay can you help me i want 2 knw if i have VHF-FM Transmitter what else to i need to build my own community radio station

    10. hoy sor, I need to know if this circuit can be powered with potatoes? once finished, can I use it to order a pizza?

    11. job de nooijer says: on May 7, 2014 at 12:35 pm


      thank you for posting this circuit. I am doing a project for school, and I need to know if its just powerd over usb, or does it also make a connection with the pc I am using. I want to power it over usb, but i can’t make a connection because of a usb flash memory.

      thank u for helping me.

    12. Nice circuit. Where can one get the PCB or do I have to breadboard it?

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