Crystal-Controlled Fm Stereo Transmitter Circuit

In this application, a BA1404 is utilized to generate an FM MPX baseband signal. This signal modulates a crystal oscillator (Q3) through a dual varactor series modulator. This transmitter can be used to play CD audio on an existing FM auto radio.

The circuit utilizes the BA1404 integrated circuit, which is designed specifically for generating FM multiplex (MPX) signals. The BA1404 is capable of processing audio signals and converting them into a form suitable for FM transmission. The crystal oscillator (Q3) serves as the frequency reference for the modulation process, ensuring that the transmitted signal is stable and within the appropriate frequency range for FM broadcasting.

The dual varactor series modulator is a critical component of this design. Varactor diodes are voltage-controlled capacitors that allow for the modulation of the oscillator frequency in response to the audio input. By varying the reverse bias voltage on the varactor diodes, the capacitance changes, which in turn alters the oscillation frequency of Q3. This modulation creates an FM signal that carries the audio information.

The transmitter's design aims to interface with existing FM auto radios, making it possible to broadcast CD audio without requiring extensive modifications to the vehicle's audio system. The output of the BA1404 can be directly fed into the antenna input of an FM receiver, allowing users to enjoy high-quality audio playback from CDs through their car's speakers.

Additional considerations for this circuit include ensuring proper power supply decoupling and filtering to minimize noise, as well as implementing appropriate shielding to prevent interference with other electronic devices. The layout of the circuit should also prioritize short signal paths to maintain signal integrity and reduce distortion. Overall, this design offers a practical solution for transmitting audio signals over the FM band, leveraging the capabilities of the BA1404 and the characteristics of varactor modulation. In This application, a BA1404 is used to generate an FM MPX baseband signal. This modulates a crystal oscillator (Q3) via a dual varactor series modulator. This transmitter can be to play CD audio on an existing FM auto radio. 🔗 External reference