Build your own fairly simple high quality stereo FM transmitter circuit as shown in the photo. The circuit is based on the BA1404 chip from ROHM Semiconductors and S9018 amplifier for extending tansmitter's range. BA1404 is a monolithic FM stereo modulator that has built in stereo modulator, FM modulator, RF amplifier circuits. The FM modulator can be operated from 76 to 108MHz and the power supply for the circuit can be anything between 6 to 12 volts.
About the FM transmitter circuit with BA1404
The circuit formed by C1, L1, C2, C3 is a filter (optional) for transients that may come from our power supply. Anyway, prefer to use power supplies with good filtering.
R3/P2 and R4/P1 form the limiting circuit for the audio signal level for the integrated circuit. According to the datasheet, this value should be 0.5V (500mV), so adjust the trimpot so that the audio signal is clear when transmitting. You may need to change the 27K resistors at the input.
C8/R5 and C9/R6 form the pre-emphasis circuit, to calculate the value use the following formula Ct = R × C.
Capacitors C10 and C11 with the IC input resistance (impedance) form a high pass filter, allowing the signal to pass above 30Hz. The input impedance is 540 Ohm, for 30Hz frequency, the calculator value for the capacitor is about 10uF.
Capacitors C12 and C13 form a low-pass filter, for FM 15KHz, by calculations we arrive at about 20nF. You can use values of 18nF for the tolerance of the components, thus ensuring that the 15KHz range is passed.
C15 is the bias capacitor of the audio stage signal and C14 the bias capacitor of the 38KHz oscillator, P3 forms the balance circuit and the multiplexed signal.
The audio signal from each channel is amplified separately by 37dB and the signal applied to the multiplexer, the crystal oscillator connected from on pins 5 and 6, create a 38Khz subcarrier and 19KhZ pilot signal with the same phase, but with 1/2 cycle delay. The audio signal and the 38Khz carrier are balanced and modulated in the multiplexer. Thus we will have at the output the L + R signal, the 38Khz subcarrier and two carriers, the L-R. The multiplexed signal is output at pin 14. The 19KHz pilot signal is generated by dividing the crystal frequency by 2, this signal is output on pin 13.
The components connected to pins 13 and 14 mixes the signal, generating the MPX composite signal delivered to pin 12.
The integrated circuit has internally a special transistor to work with high frequencies, forming a Collpits oscillator, the components connected between pins 9 and 10 complete the FM modulator circuit. With the values in the circuit, we have covered the FM range. You may need to change capacitor values to achieve the result. Use ceramic capacitors of type NP0, in order to prevent the frequency from varying due to temperature change.
The FM signal is delivered on pin 7, you may need to change the value of C31 to get better power, values between 4.7pF and 15pF can be used.
The RF coil is usually a critical component in FM transmitter assemblies, the ideal coil for this circuit is a 3.5 turn ferrite core found in FM radios. Below you can see the model of this coil.
However, if you can’t find such a coil, you can make one yourself by winding 4 coils of 22 AWG enameled copper wire on a 5 mm diameter ferrite core. To adjust the operating frequency of the stereo transmitter, use a plastic key to adjust and find an fm-free range.
About Pre-emphasis and De-emphasis
Random noise has a triangular spectral distribution in an FM system, with the effect that the noise occurs predominantly at the higher audio frequencies in the baseband. This can be compensated for, to some extent, by increasing the high frequencies before transmission and reducing them by a corresponding amount at the receiver. Reducing the high audio frequencies at the receiver also reduces the high-frequency noise. These processes of increasing and reducing certain frequencies are known as pre-emphasis and non-emphasis reduction, respectively.
The amount of pre-emphasis and non-emphasis used is defined by the time constant of a simple RC filter circuit. In most parts of the world, a time constant of 50 µs is used. In the Americas and South Korea, 75 µs is used. This applies to both mono and stereo transmissions. For stereo, pre-emphasis is applied to the left and right channels before multiplexing.
The use of pre-emphasis becomes a problem because many forms of contemporary music contain more high-frequency energy than the styles of music that were prevalent at the birth of FM broadcasting. Pre-emphasis of these high-frequency sounds would cause excessive deviation of the FM carrier. Modulation control (limiter) devices are used to prevent this. Systems more modern than FM transmission tend to use program-dependent variable pre-emphasis; for example, dbx in the BTSC TV sound system, or none.
The 38KHz crystal guarantees stereo transmission, in case you can’t find this component for your circuit assembly, you will get a high quality mono transmission.
The high-frequency signal follows from pin 7 to transistor T1 to guarantee a longer transmission distance, capacitor C9 couples the antenna that can be a telescopic or a piece of hard-wire.
The power supply should be a 6 to 12V source or battery, the higher voltage will feed the power transistor, but for the BA 1404 integrated circuit it will be necessary below 3 Volts, or it may burn out. To power the integrated circuit we use an adjustable voltage regulator IC LM317L set to about 2.4V. If you prefer, you can change the resistors to have a higher voltage. For example, by changing R1 to 330 ohms, we get 2.5V at the output. If you prefer, use this calculator for LM317.
When assembling this circuit, make sure all components are positioned in the correct place and only then connect the battery, then connect an audio source such as a cell phone, for example, and with a nearby FM receiver tuned to a free FM frequency, adjust the coil, using a plastic or wooden “key”, until you hear the signal on the radio. If you can’t adjust it, try other values for C13, and then retune the settings.
If you cannot tune in the FM range 88 to 108 MHZ, try changing the value of the tuning capacitor. In the last case the coil can be 4 coils of enameled wire with an air core and the capacitor a variable capacitor from 6 to 30pF. The reason for not using an air-core coil is that with changes in ambient temperature, there is a variation in the size of this coil that will interfere with frequency stability, which is not ideal in good quality FM generators.
Suggested PCB mounting stereo FM transmitter with integrated BA 1404 and crystal 38KHZ.
Part Value Description Quantity
R1 300 Orange, black, brown, gold 1
R2 330 Orange, orange, brown, gold 1
R3, R4 27K Red, violet, orange, gold 2
R5 68K Blue, gray, orange, gold 1
R6 47K Yellow, violet, orange, gold 1
R7 100K Brown, black, yellow, gold 1
R8 2.7K Red, violet, red, gold 1
R9 10 Brown, black, black, gold 1
R10, R12 220 Red, red, brown, gold 2
R11 10K Red, red, brown, gold 1
C1, C2, C6, C12, C13, C16, C21, C26, C27, C31 10nF (103) Ceramic Capacitor 10
C3 100µF/25V Electrolytic Capacitor 1
C5 100µF/12V Electrolytic Capacitor 1
C4, C10, C11, C15, C19 10µF/12V Electrolytic Capacitor 5
C7 100nF (104) Ceramic Capacitor 1
C8, C9, C14, C22, C28, C30 1nF (102) Ceramic Capacitor 4
C17, C23 10pF (10p) Ceramic Capacitor NP0 2
C18 220pF (221) Ceramic Capacitor 1
C20 470pF (471) Ceramic Capacitor 1
C24, C25, C29 15pF (15p) Ceramic Capacitor NP0 3
IC1 LM317L Adjustable voltage regulator integrated circuit. 1
IC2 BA1404 FM transmitter integrated circuit 1
T1 SS9018 (S9018) or equivalent NPN transistor for RF 1
Q1 38KHz Quartz crystal 1
J1 DC (Power Supply) Terminal block 2 pin 5.08 mm 1
J2 IN (audio input) Stereo P2 connector. 1
L1, L3 10µH Brown, black, black, silver 2
L2, L4 Adjustable coil Adjustable coil, 3.5 turns ferrite core. 1
P1, P2 22K (223) Trimpot 2
P3 100K (104) Trimpot 1
Solder, Wires, PCB, case, font, etc.