Fiber-optic-transmitter

Description: An alternate LED driver utilizing the 74F3040 line driver has been implemented in this application example. The 74F3040 offers a higher current rating but lacks the variable threshold capabilities found in the NE522. The LED diode operates in a saturated on-off mode to achieve optimal signal-to-noise performance. This receiver circuit comprises a wideband differential amplifier (NE592), a voltage-controlled oscillator (VCO) (NE564), and an LED driver (NE522), which functions as a high-speed comparator. The video signal is AC coupled into the modulator preamplifier, followed by a sync tip clamp that restores the composite video signal and prevents modulation deviation variations with changing picture content. A video signal level of 250 to 300 mV peak is necessary to maintain optimal picture modulation. Frequency compensation (preemphasis) is introduced through a passive RC lead network at the input of the NE592 differential amplifier. The main FM modulator consists of the NE564, which is employed solely as a linear wideband VCO, while the other sections of the device remain unused. Differential decoupling to the VCO terminals is achieved via the loop filter terminals, specifically pins 4 and 5.

The circuit design incorporates a robust LED driver configuration using the 74F3040 to enhance current handling capabilities. This choice is particularly suitable for applications requiring high drive strength, despite the absence of variable threshold functionality. Operating the LED diode in a saturated on-off mode ensures maximum signal clarity and minimizes noise interference, which is critical for high-fidelity signal transmission.

The NE592 wideband differential amplifier plays a pivotal role in signal processing, providing the necessary gain and bandwidth to accommodate the video signal requirements. By utilizing AC coupling, the circuit effectively eliminates DC offsets, ensuring that the video signal remains centered around zero volts, which is essential for proper modulation. The inclusion of a sync tip clamp further stabilizes the composite video signal, mitigating variations in modulation deviation that can arise from dynamic changes in picture content.

To maintain optimal picture modulation, a specific input video signal level of 250 to 300 mV peak is essential. This requirement is crucial for ensuring that the modulation index remains within the desired range, thereby preserving the integrity of the transmitted image. The frequency compensation implemented through a passive RC network at the NE592 input serves to enhance transient response and reduce distortion, ensuring that the amplifier can accurately reproduce fast-changing video signals.

The NE564 serves as the primary FM modulator in this configuration, functioning exclusively as a linear wideband VCO. This approach allows for precise frequency modulation of the input signal while ensuring that the other sections of the device are not utilized, simplifying the design and improving reliability. The differential decoupling to the VCO terminals through the loop filter terminals (pins 4 and 5) ensures stable operation and minimizes noise coupling, further enhancing the performance of the overall circuit. This careful consideration of component selection and configuration results in a highly effective LED driver and video modulation system.An alternate LED driver which uses the 74F3040 line driver was incorporated in this particular application example. The 74F3040.has a higher current rating, but not the variable threshold capabilities at the NE522. The LED diode is operated in the saturated on-off mode for best signal to noise. This receiver circuit consists of wideband differential amplifier NE592, VCO NE564 and LED driver NE522-tbe high-speed comparator.

The video signal is ac coupled into the modulator preamplifier and followed by a sync tip clamp to provide de restoration on the composite video signal and to prevent variation of modulation deviation with varying picture content. A video signal level of 250 to 300 m V peak is required to maintain optimnm picture modulation. Frequency compensation (preemphasis) is inserted in the form of a passive rc lead network at tbe input to the NE592 differential amplifier. The main FM modulator consists of an NE564 used only as a linear wideband VCO, but the other sections of the device are not used.

Differential de coupling to tbe VCO terminals is attained via tbe loop filter terminals, pins 4 and 5.