USB FM Transmitter

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.

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.

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 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.

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

Read more »

Guitar and Bass Sustain Unit

We have all heard that wonderful sound of a guitar, where the note just hangs there seemingly forever (or at least until next Thursday).  Sustain can be obtained by turning the amp up full, but the rest of the band will just kill you - they need to be able to hear themselves too!  This little project is best used in the effects loop of a guitar amp (if it has one - not all do).  It can be used direct from the guitar, but the effect is not as good, since it is designed for relatively high levels (around 1 Volt).

The circuit is very simple to build, and mine is on a piece of Veroboard.  Because it can easily be built as a pedal or even into a guitar amp (such as that described in Project 27), I do expect to make PCBs available in the not too distant future depending on demand, and these may have a few additional functions as well. 

Here is not a lot to it, but the LED and LDR (Light Dependent Resistor) are critical - they must be completely enclosed in a light proof enclosure of some kind.  Vactrol make some very nice little LDR opto-isolators, but unfortunately they are not easy to get, and are fairly expensive.  The next best thing is a couple of pieces of black heatshrink tubing.  The LED and LDR must be as close to each other as possible, and a flat topped LED is recommended if you can get one. 

Note the rather unusual earth (ground) connection.  This is not a mistake in the drawing.  U2A is used to buffer the 1/2 supply voltage created by R3 and R4, and instead of using the 12V supply negative as earth, the output of U2A is used instead.  This gives a balanced supply from a single voltage source.  Note that the AC/DC adapter (plug pack or wall wart - select the term you are most comfortable with :-) must not be used to power other equipment as well, since this may cause problems.  If you wish, a conventional +/-15V supply may be used instead, and U2A will not be used.  Note that if a +/-15V supply is used, you must increase the value of R13 to about 3.3k to limit LED current to around 10mA.

All resistors are 1/4 or 1/2 Watt, and may be 1% or 5%.  R1 and R2 should be metal film for lowest noise.  Although the TL072 is suggested for the audio path, other opamps may be used as well.  Likewise, the LM1458 can also be substituted if you like.  Caps are 16V types, but higher voltage units can be used if desired.  D7 is a power on indicator, and D6 is there to prevent damage to the circuit if the polarity of the applied 12V DC is incorrect.  Be warned that the AC/ DC adapter will be damaged if the polarity is wrong, and it is left connected for any length of time.

VR1 is a simple volume control, and is used to set the output level.  VR2 is the limiting threshold control - as it is adjusted to a higher setting, the volume will decrease.  You may wish to wire the pot "backwards", so that maximum output is obtained when VR2 is set to the fully clockwise position.

U1A is the gain control stage.  Maximum gain as shown is unity, but R2 can be increased if you find that the gain is too low.  When the signal level is high enough for D1 - D4 to conduct, the LED illuminates, and reduces the gain of the input stage.  Any further increase of input voltage will just cause the LED to glow brighter, which reduces the gain.  In this way, a constant output level is maintained, since as the input signal reduces, so does the LED brightness and the stage gain increases again.

The connections shown will be fine for most purposes, but some LDRs may give distortion at low frequencies.  A 100uF cap in parallel with the LED will probably help if this is a problem.  LDRs typically have a slow "release" time.  After illumination, they take some time to return to their full dark resistance.  This characteristic is exploited here, to allow a very simple circuit with an almost perfect attack and release time for musical instrument use.

http://sound.westhost.com/

Read more »